@c This file is part of the MIT/GNU Scheme Reference Manual.
@c $Id: io.texi,v 1.15 2006/01/31 06:30:48 cph Exp $

@c Copyright 1991,1992,1993,1994,1995 Massachusetts Institute of Technology
@c Copyright 1996,1997,1999,2000,2001 Massachusetts Institute of Technology
@c Copyright 2002,2003,2004,2005,2006 Massachusetts Institute of Technology
@c See file scheme.texinfo for copying conditions.

@node Input/Output, Operating-System Interface, Environments, Top
@chapter Input/Output

@cindex input
@cindex output
@cindex port
This chapter describes the procedures that are used for input and
output (@acronym{I/O}).  The chapter first describes @dfn{ports} and
how they are manipulated, then describes the @acronym{I/O} operations.
Finally, some low-level procedures are described that permit the
implementation of custom ports and high-performance @acronym{I/O}.

@menu
* Ports::                       
* File Ports::                  
* String Ports::                
* Input Procedures::            
* Output Procedures::           
* Format::                      
* Custom Output::               
* Prompting::                   
* Port Primitives::             
* Parser Buffers::              
* Parser Language::             
* XML Support::                 
@end menu

@node Ports, File Ports, Input/Output, Input/Output
@section Ports

@cindex port (defn)
@findex console-i/o-port
Scheme uses ports for @acronym{I/O}.  A @dfn{port}, which can be
treated like any other Scheme object, serves as a source or sink for
data.  A port must be open before it can be read from or written to.
The standard @acronym{I/O} port, @code{console-i/o-port}, is opened
automatically when you start Scheme.  When you use a file for input or
output, you need to explicitly open and close a port to the file (with
procedures described in this chapter).  Additional procedures let you
open ports to strings.

@cindex current input port (defn)
@cindex input port, current (defn)
@cindex port, current
@findex read-char
@findex read
Many input procedures, such as @code{read-char} and @code{read}, read
data from the current input port by default, or from a port that you
specify.  The current input port is initially @code{console-i/o-port},
but Scheme provides procedures that let you change the current input
port to be a file or string.

@cindex current output port (defn)
@cindex output port, current (defn)
@findex write-char
@findex display
Similarly, many output procedures, such as @code{write-char} and
@code{display}, write data to the current output port by default, or to
a port that you specify.  The current output port is initially
@code{console-i/o-port}, but Scheme provides procedures that let you
change the current output port to be a file or string.

All ports read or write only @acronym{ISO-8859-1} characters.

Every port is either an input port, an output port, or both.  The
following predicates distinguish all of the possible cases.

@deffn procedure port? object
@cindex type predicate, for port
Returns @code{#t} if @var{object} is a port, otherwise returns
@code{#f}.
@end deffn

@deffn procedure input-port? object
Returns @code{#t} if @var{object} is an input port, otherwise returns
@code{#f}.  Any object satisfying this predicate also satisfies
@code{port?}.
@end deffn

@deffn procedure output-port? object
Returns @code{#t} if @var{object} is an output port, otherwise returns
@code{#f}.  Any object satisfying this predicate also satisfies
@code{port?}.
@end deffn

@deffn procedure i/o-port? object
Returns @code{#t} if @var{object} is both an input port and an output
port, otherwise returns @code{#f}.  Any object satisfying this predicate
also satisfies @code{port?}, @code{input-port?}, and
@code{output-port?}.
@end deffn

@deffn procedure guarantee-port object
@deffnx procedure guarantee-input-port object
@deffnx procedure guarantee-output-port object
@deffnx procedure guarantee-i/o-port object
These procedures check the type of @var{object}, signalling an error
of type@* @code{condition-type:wrong-type-argument} if it is not a
port, input port, output port, or @acronym{I/O} port, respectively.
Otherwise they return @var{object}.
@findex condition-type:wrong-type-argument
@end deffn

@cindex standard ports
The next five procedures return the runtime system's @dfn{standard
ports}.  All of the standard ports are dynamically bound by the
@acronym{REP} loop; this means that when a new @acronym{REP} loop is
started, for example by an error, each of these ports is dynamically
bound to the @acronym{I/O} port of the @acronym{REP} loop.  When the
@acronym{REP} loop exits, the ports revert to their original values.

@deffn procedure current-input-port
@findex console-input-port
Returns the current input port.  This is the default port used by many
input procedures.  Initially, @code{current-input-port} returns the
value of @code{console-i/o-port}.
@end deffn

@deffn procedure current-output-port
@findex console-output-port
Returns the current output port.  This is the default port used by many
output procedures.  Initially, @code{current-output-port} returns the
value of @code{console-i/o-port}.
@end deffn

@deffn procedure notification-output-port
Returns an output port suitable for generating ``notifications'', that
is, messages to the user that supply interesting information about the
execution of a program.  For example, the @code{load} procedure writes
messages to this port informing the user that a file is being loaded.
Initially, @code{notification-output-port} returns the value of
@code{console-i/o-port}.
@end deffn

@deffn procedure trace-output-port
Returns an output port suitable for generating ``tracing'' information
about a program's execution.  The output generated by the @code{trace}
procedure is sent to this port.  Initially, @code{trace-output-port}
returns the value of @code{console-i/o-port}.
@end deffn

@deffn procedure interaction-i/o-port
Returns an @acronym{I/O} port suitable for querying or prompting the
user.  The standard prompting procedures use this port by default
(@pxref{Prompting}).  Initially, @code{interaction-i/o-port} returns
the value of @code{console-i/o-port}.
@end deffn

@deffn procedure with-input-from-port input-port thunk
@deffnx procedure with-output-to-port output-port thunk
@deffnx procedure with-notification-output-port output-port thunk
@deffnx procedure with-trace-output-port output-port thunk
@deffnx procedure with-interaction-i/o-port i/o-port thunk
@var{Thunk} must be a procedure of no arguments.  Each of these
procedures binds one of the standard ports to its first argument, calls
@var{thunk} with no arguments, restores the port to its original value,
and returns the result that was yielded by @var{thunk}.  This temporary
binding is performed the same way as dynamic binding of a variable,
including the behavior in the presence of continuations (@pxref{Dynamic
Binding}).

@code{with-input-from-port} binds the current input port,
@code{with-output-to-port} binds the current output port,
@code{with-notification-output-port} binds the ``notification'' output
port, @code{with-trace-output-port} binds the ``trace'' output port,
and @code{with-interaction-i/o-port} binds the ``interaction''
@acronym{I/O} port.
@end deffn

@deffn procedure set-current-input-port! input-port
@deffnx procedure set-current-output-port! output-port
@deffnx procedure set-notification-output-port! output-port
@deffnx procedure set-trace-output-port! output-port
@deffnx procedure set-interaction-i/o-port! i/o-port
Each of these procedures alters the binding of one of the standard ports
and returns an unspecified value.  The binding that is modified
corresponds to the name of the procedure.
@end deffn

@defvr variable console-i/o-port
@cindex port, console
@cindex console, port
@cindex input port, console
@cindex output port, console
@code{console-i/o-port} is an @acronym{I/O} port that communicates
with the ``console''.  Under unix, the console is the controlling
terminal of the Scheme process.  Under Windows and OS/2, the console
is the window that is created when Scheme starts up.

This variable is rarely used; instead programs should use one of the
standard ports defined above.  This variable should not be modified.
@end defvr

@deffn procedure close-port port
@cindex closing, of port
Closes @var{port} and returns an unspecified value.  If @var{port} is a
file port, the file is closed.
@end deffn

@deffn procedure close-input-port port
Closes @var{port} and returns an unspecified value.  @var{Port} must
be an input port or an @acronym{I/O} port; if it is an @acronym{I/O}
port, then only the input side of the port is closed.
@end deffn

@deffn procedure close-output-port port
Closes @var{port} and returns an unspecified value.  @var{Port} must
be an output port or an @acronym{I/O} port; if it is an @acronym{I/O}
port, then only the output side of the port is closed.
@end deffn

@node File Ports, String Ports, Ports, Input/Output
@section File Ports

@cindex file, input and output ports
@cindex port, file
@cindex input port, file
@cindex output port, file
@cindex I/O, to files
Before Scheme can access a file for reading or writing, it is necessary
to open a port to the file.  This section describes procedures used to
open ports to files.  Such ports are closed (like any other port) by
@code{close-port}.  File ports are automatically closed if and when they
are reclaimed by the garbage collector.

@findex merge-pathnames
Before opening a file for input or output, by whatever method, the
@var{filename} argument is converted to canonical form by calling the
procedure @code{merge-pathnames} with @var{filename} as its sole
argument.  Thus, @var{filename} can be either a string or a pathname,
and it is merged with the current pathname defaults to produce the
pathname that is then opened.

@cindex binary file ports
@cindex newline translation
Any file can be opened in one of two modes, @dfn{normal} or
@dfn{binary}.  Normal mode is for accessing text files, and binary mode
is for accessing other files.  Unix does not distinguish these modes,
but Windows and OS/2 do: in normal mode, their file ports perform
@dfn{newline translation}, mapping between the carriage-return/linefeed
sequence that terminates text lines in files, and the @code{#\newline}
that terminates lines in Scheme.  In binary mode, such ports do not
perform newline translation.  Unless otherwise mentioned, the procedures
in this section open files in normal mode.

@deffn procedure open-input-file filename
@cindex construction, of file input port
Takes a filename referring to an existing file and returns an input port
capable of delivering characters from the file.  If the file cannot be
opened, an error of type @code{condition-type:file-operation-error} is
signalled.
@findex condition-type:file-operation-error
@end deffn

@deffn procedure open-output-file filename [append?]
@cindex construction, of file output port
Takes a filename referring to an output file to be created and returns
an output port capable of writing characters to a new file by that name.
If the file cannot be opened, an error of type
@code{condition-type:file-operation-error} is signalled.
@findex condition-type:file-operation-error

@cindex appending, to output file
The optional argument @var{append?} is an MIT/GNU Scheme extension.  If
@var{append?} is given and not @code{#f}, the file is opened in
@dfn{append} mode.  In this mode, the contents of the file are not
overwritten; instead any characters written to the file are appended to
the end of the existing contents.  If the file does not exist, append
mode creates the file and writes to it in the normal way.
@end deffn

@deffn procedure open-i/o-file filename
@cindex construction, of file input port
Takes a filename referring to an existing file and returns an
@acronym{I/O} port capable of both reading and writing the file.  If
the file cannot be opened, an error of type
@code{condition-type:file-operation-error} is signalled.
@findex condition-type:file-operation-error

This procedure is often used to open special files.  For example, under
unix this procedure can be used to open terminal device files, @sc{pty}
device files, and named pipes.
@end deffn

@deffn procedure open-binary-input-file filename
@deffnx procedure open-binary-output-file filename [append?]
@deffnx procedure open-binary-i/o-file filename
These procedures open files in binary mode.  In all other respects they
are identical to @code{open-input-file}, @code{open-output-file}, and
@code{open-i/o-file}, respectively.
@end deffn

@deffn procedure close-all-open-files
@cindex closing, of file port
This procedure closes all file ports that are open at the time that it
is called, and returns an unspecified value.
@end deffn

@deffn procedure call-with-input-file filename procedure
@deffnx procedure call-with-output-file filename procedure
These procedures call @var{procedure} with one argument: the port
obtained by opening the named file for input or output, respectively.
If the file cannot be opened, an error of type
@code{condition-type:file-operation-error} is signalled.  If
@var{procedure} returns, then the port is closed automatically and the
value yielded by @var{procedure} is returned.  If @var{procedure} does
not return, then the port will not be closed automatically unless it is
reclaimed by the garbage collector.@footnote{Because Scheme's escape
procedures have unlimited extent, it is possible to escape from the
current continuation but later to escape back in.  If implementations
were permitted to close the port on any escape from the current
continuation, then it would be impossible to write portable code using
both @code{call-with-current-continuation} and
@code{call-with-input-file} or @code{call-with-output-file}.}
@end deffn

@deffn procedure call-with-binary-input-file filename procedure
@deffnx procedure call-with-binary-output-file filename procedure
These procedures open files in binary mode.  In all other respects they
are identical to @code{call-with-input-file} and
@code{call-with-output-file}, respectively.
@end deffn

@deffn procedure with-input-from-file filename thunk
@deffnx procedure with-output-to-file filename thunk
@cindex current input port, rebinding
@cindex current output port, rebinding
@findex current-input-port
@findex current-output-port
@var{Thunk} must be a procedure of no arguments.
The file is opened for input or output, an input or output port
connected to it is made the default value returned by
@code{current-input-port} or @code{current-output-port}, and the
@var{thunk} is called with no arguments.  When the @var{thunk} returns,
the port is closed and the previous default is restored.
@code{with-input-from-file} and @code{with-output-to-file} return the
value yielded by @var{thunk}.  If an escape procedure is used to escape
from the continuation of these procedures, their behavior is
implementation-dependent; in that situation MIT/GNU Scheme leaves the files
open.
@end deffn

@deffn procedure with-input-from-binary-file filename thunk
@deffnx procedure with-output-to-binary-file filename thunk
These procedures open files in binary mode.  In all other respects they
are identical to @code{with-input-from-file} and
@code{with-output-to-file}, respectively.
@end deffn

@node String Ports, Input Procedures, File Ports, Input/Output
@section String Ports

@cindex string, input and output ports
@cindex port, string
@cindex input port, string
@cindex output port, string
@cindex I/O, to strings
This section describes the simplest kinds of ports: input ports that
read their input from given strings, and output ports that accumulate
their output and return it as a string.  It also describes
``truncating'' output ports, which can limit the length of the resulting
string to a given value.

@deffn procedure open-input-string string [start [end]]
@cindex string, converting to input port
@cindex construction, of string input port
Returns a new string port that delivers characters from @var{string}.
The optional arguments @var{start} and @var{end} may be used to specify
that the string port delivers characters from a substring of
@var{string}; if not given, @var{start} defaults to @code{0} and
@var{end} defaults to @code{(string-length @var{string})}.
@end deffn

@deffn procedure with-input-from-string string thunk
@cindex current input port, rebinding
@var{Thunk} must be a procedure of no arguments.
@code{with-input-from-string} creates a new input port that reads from
@var{string}, makes that port the current input port, and calls
@var{thunk}.  When @var{thunk} returns, @code{with-input-from-string}
restores the previous current input port and returns the result yielded
by @var{thunk}.

@example
(with-input-from-string "(a b c) (d e f)" read)  @result{}  (a b c)
@end example

Note: this procedure is equivalent to:

@example
(with-input-from-port (open-input-string @var{string}) @var{thunk})
@end example
@end deffn

@deffn procedure open-output-string
@deffnx procedure get-output-string
@code{open-output-string} returns a new output port that accumulates in
a buffer everything that is written to it.  The accumulated output can
subsequently be obtained by calling @code{get-output-string} on the
port.
@end deffn

@deffn procedure call-with-output-string procedure
@var{Procedure} is called with one argument, an output port.  The value
yielded by @var{procedure} is ignored.  When @var{procedure} returns,
@code{call-with-output-string} returns the port's accumulated output as
a newly allocated string.  This is equivalent to:

@example
@group
(define (call-with-output-string procedure)
  (let ((port (open-output-string)))
    (procedure port)
    (get-output-string port)))
@end group
@end example
@end deffn

@deffn procedure with-output-to-string thunk
@cindex current output port, rebinding
@cindex construction, of string output port
@findex current-output-port
@var{Thunk} must be a procedure of no arguments.
@code{with-output-to-string} creates a new output port that accumulates
output, makes that port the default value returned by
@code{current-output-port}, and calls @var{thunk} with no arguments.
When @var{thunk} returns, @code{with-output-to-string} restores the
previous default and returns the accumulated output as a newly allocated
string.

@example
@group
(with-output-to-string
  (lambda ()
    (write 'abc)))                    @result{}  "abc"
@end group
@end example

Note: this procedure is equivalent to:

@example
@group
(call-with-output-string
 (lambda (port)
   (with-output-to-port port @var{thunk})))
@end group
@end example
@end deffn

@deffn procedure with-output-to-truncated-string k thunk
Similar to @code{with-output-to-string}, except that the output is
limited to @var{k} characters.  If @var{thunk} attempts to write more
than @var{k} characters, it will be aborted by invoking an escape
procedure that returns from @code{with-output-to-truncated-string}.

The value of this procedure is a pair; the car of the pair is @code{#t}
if @var{thunk} attempted to write more than @var{k} characters, and
@code{#f} otherwise.  The cdr of the pair is a newly allocated string
containing the accumulated output.

This procedure is helpful for displaying circular lists, as shown in this
example:

@example
@group
(define inf (list 'inf))
(with-output-to-truncated-string 40
  (lambda ()
    (write inf)))                       @result{}  (#f . "(inf)")
(set-cdr! inf inf)
(with-output-to-truncated-string 40
  (lambda ()
    (write inf)))
        @result{}  (#t . "(inf inf inf inf inf inf inf inf inf inf")
@end group
@end example
@end deffn

@deffn procedure write-to-string object [k]
Writes @var{object} to a string output port, and returns the resulting
newly allocated string.  If @var{k} is supplied and not @code{#f}, this
procedure is equivalent to

@example
@group
(with-output-to-truncated-string @var{k}
  (lambda ()
    (write @var{object})))
@end group
@end example

otherwise it is equivalent to

@example
@group
(with-output-to-string
 (lambda ()
   (write @var{object})))
@end group
@end example
@end deffn

@node Input Procedures, Output Procedures, String Ports, Input/Output
@section Input Procedures
@cindex input operations

This section describes the procedures that read input.  Input procedures
can read either from the current input port or from a given port.
Remember that to read from a file, you must first open a port to the
file.

@cindex interactive input ports (defn)
Input ports can be divided into two types, called @dfn{interactive} and
@dfn{non-interactive}.  Interactive input ports are ports that read
input from a source that is time-dependent; for example, a port that
reads input from a terminal or from another program.  Non-interactive
input ports read input from a time-independent source, such as an
ordinary file or a character string.

All optional arguments called @var{input-port}, if not supplied, default
to the current input port.

@deffn procedure read-char [input-port]
@cindex character, input from port
Returns the next character available from @var{input-port}, updating
@var{input-port} to point to the following character.  If no more
characters are available, an end-of-file object is returned.

In MIT/GNU Scheme, if @var{input-port} is an interactive input port and no
characters are immediately available, @code{read-char} will hang waiting
for input, even if the port is in non-blocking mode.
@end deffn

@deffn procedure peek-char [input-port]
Returns the next character available from @var{input-port},
@emph{without} updating @var{input-port} to point to the following
character.  If no more characters are available, an end-of-file object
is returned.@footnote{The value returned by a call to @code{peek-char}
is the same as the value that would have been returned by a call to
@code{read-char} on the same port.  The only difference is that the very
next call to @code{read-char} or @code{peek-char} on that
@var{input-port} will return the value returned by the preceding call to
@code{peek-char}.  In particular, a call to @code{peek-char} on an
interactive port will hang waiting for input whenever a call to
@code{read-char} would have hung.}

In MIT/GNU Scheme, if @var{input-port} is an interactive input port and no
characters are immediately available, @code{peek-char} will hang waiting
for input, even if the port is in non-blocking mode.
@end deffn

@deffn procedure char-ready? [input-port]
@findex read-char
Returns @code{#t} if a character is ready on @var{input-port} and
returns @code{#f} otherwise.  If @code{char-ready?} returns @code{#t}
then the next @code{read-char} operation on @var{input-port} is
guaranteed not to hang.  If @var{input-port} is a file port at end of
file then @code{char-ready?} returns
@code{#t}.@footnote{@code{char-ready?} exists to make it possible for a
program to accept characters from interactive ports without getting
stuck waiting for input.  Any input editors associated with such ports
must make sure that characters whose existence has been asserted by
@code{char-ready?} cannot be rubbed out.  If @code{char-ready?} were to
return @code{#f} at end of file, a port at end of file would be
indistinguishable from an interactive port that has no ready
characters.}
@end deffn

@deffn procedure read [input-port [environment]]
@cindex expression, input from port
@cindex external representation, parsing
@cindex parsing, of external representation
Converts external representations of Scheme objects into the objects
themselves.  @code{read} returns the next object parsable from
@var{input-port}, updating @var{input-port} to point to the first
character past the end of the written representation of the object.  If
an end of file is encountered in the input before any characters are
found that can begin an object, @code{read} returns an end-of-file
object.  The @var{input-port} remains open, and further attempts to read
will also return an end-of-file object.  If an end of file is
encountered after the beginning of an object's written representation,
but the written representation is incomplete and therefore not parsable,
an error is signalled.

@var{Environment} is used to look up the values of control variables
such as @samp{*parser-radix*}.  If not supplied, it defaults to the
@acronym{REP} environment.
@end deffn

@deffn procedure eof-object? object
@cindex type predicate, for EOF object
@cindex EOF object, predicate for
@cindex end of file object (see EOF object)
@cindex file, end-of-file marker (see EOF object)
Returns @code{#t} if @var{object} is an end-of-file object; otherwise
returns @code{#f}.
@end deffn

@deffn procedure read-char-no-hang [input-port]
If @var{input-port} can deliver a character without blocking, this
procedure acts exactly like @code{read-char}, immediately returning that
character.  Otherwise, @code{#f} is returned, unless @var{input-port} is
a file port at end of file, in which case an end-of-file object is
returned.  In no case will this procedure block waiting for input.
@end deffn

@deffn procedure read-string char-set [input-port]
@cindex string, input from port
Reads characters from @var{input-port} until it finds a terminating
character that is a member of @var{char-set} (@pxref{Character Sets}) or
encounters end of file.  The port is updated to point to the terminating
character, or to end of file if no terminating character was found.
@code{read-string} returns the characters, up to but excluding the
terminating character, as a newly allocated string.

This procedure ignores the blocking mode of the port, blocking
unconditionally until it sees either a delimiter or end of file.  If end
of file is encountered before any characters are read, an end-of-file
object is returned.

@findex read-char
On many input ports, this operation is significantly faster than the
following equivalent code using @code{peek-char} and @code{read-char}:

@example
@group
(define (read-string char-set input-port)
  (let ((char (peek-char input-port)))
    (if (eof-object? char)
        char
        (list->string
         (let loop ((char char))
           (if (or (eof-object? char)
                   (char-set-member? char-set char))
               '()
               (begin
                 (read-char input-port)
                 (cons char
                       (loop (peek-char input-port))))))))))
@end group
@end example
@end deffn

@deffn procedure read-line [input-port]
@code{read-line} reads a single line of text from @var{input-port}, and
returns that line as a newly allocated string.  The @code{#\newline}
terminating the line, if any, is discarded and does not appear in the
returned string.

This procedure ignores the blocking mode of the port, blocking
unconditionally until it has read an entire line.  If end of file is
encountered before any characters are read, an end-of-file object is
returned.
@end deffn

@deffn procedure read-string! string [input-port]
@deffnx procedure read-substring! string start end [input-port]
@code{read-string!} and @code{read-substring!} fill the specified region
of @var{string} with characters read from @var{input-port} until the
region is full or else there are no more characters available from the
port.  For @code{read-string!}, the region is all of @var{string}, and
for @code{read-substring!}, the region is that part of @var{string}
specified by @var{start} and @var{end}.

The returned value is the number of characters filled into the region.
However, there are several interesting cases to consider:

@itemize @bullet
@item
If @code{read-string!} (@code{read-substring!}) is called when
@var{input-port} is at ``end-of-file'', then the returned value is
@code{0}.  Note that ``end-of-file'' can mean a file port that is at the
file's end, a string port that is at the string's end, or any other port
that will never produce more characters.

@item
If @var{input-port} is an interactive port (e.g.@: a terminal), and one
or more characters are immediately available, the region is filled using
the available characters.  The procedure then returns immediately,
without waiting for further characters, even if the number of available
characters is less than the size of the region.  The returned value is
the number of characters actually filled in.

@item
If @var{input-port} is an interactive port and no characters are
immediately available, the result of the operation depends on the
blocking mode of the port.  If the port is in non-blocking mode,
@code{read-string!} (@code{read-substring!}) immediately returns the
value @code{#f}.  Otherwise, the operation blocks until a character is
available.  As soon as at least one character is available, the region
is filled using the available characters.  The procedure then returns
immediately, without waiting for further characters, even if the number
of available characters is less than the size of the region.  The
returned value is the number of characters actually filled in.
@end itemize

The importance of @code{read-string!} and @code{read-substring!} are
that they are both flexible and extremely fast, especially for large
amounts of data.
@end deffn

The following variables may be bound or assigned to change the behavior
of the @code{read} procedure.  They are looked up in the environment
that is passed to @code{read}, and so may have different values in
different environments.  It is recommended that the global bindings of
these variables be left unchanged; make local changes by shadowing the
global bindings in nested environments.

@defvr variable *parser-radix*
This variable defines the radix used by the reader when it parses
numbers.  This is similar to passing a radix argument to
@code{string->number}.  The value of this variable must be one of
@code{2}, @code{8}, @code{10}, or @code{16}; any other value is ignored,
and the reader uses radix @code{10}.

Note that much of the number syntax is invalid for radixes other than
@code{10}.  The reader detects cases where such invalid syntax is used
and signals an error.  However, problems can still occur when
@code{*parser-radix*} is set to @code{16}, because syntax that normally
denotes symbols can now denote numbers (e.g.@: @code{abc}).  Because of
this, it is usually undesirable to set this variable to anything other
than the default.

The default value of this variable is @code{10}.
@end defvr

@defvr variable *parser-canonicalize-symbols?*
This variable controls how the parser handles case-sensitivity of
symbols.  If it is bound to its default value of @code{#t}, symbols read
by the parser are converted to lower case before being interned.
Otherwise, symbols are interned without case conversion.

In general, it is a bad idea to use this feature, as it doesn't really
make Scheme case-sensitive, and therefore can break features of the
Scheme runtime that depend on case-insensitive symbols.
@end defvr

@node Output Procedures, Format, Input Procedures, Input/Output
@section Output Procedures
@cindex output procedures

@cindex buffering, of output
@cindex flushing, of buffered output
Output ports may or may not support @dfn{buffering} of output, in which
output characters are collected together in a buffer and then sent to
the output device all at once.  (Most of the output ports implemented by
the runtime system support buffering.)  Sending all of the characters in
the buffer to the output device is called @dfn{flushing} the buffer.  In
general, output procedures do not flush the buffer of an output port
unless the buffer is full.

@cindex discretionary flushing, of buffered output
@findex discretionary-flush-output
However, the standard output procedures described in this section
perform what is called @dfn{discretionary} flushing of the buffer.
Discretionary output flushing works as follows.  After a procedure
performs its output (writing characters to the output buffer), it checks
to see if the port implements an operation called
@code{discretionary-flush-output}.  If so, then that operation is
invoked to flush the buffer.  At present, only the console port defines
@code{discretionary-flush-output}; this is used to guarantee that output
to the console appears immediately after it is written, without
requiring calls to @code{flush-output}.

All optional arguments called @var{output-port}, if not supplied,
default to the current output port.

@deffn procedure write-char char [output-port]
@cindex character, output to port
Writes @var{char} (the character itself, not a written representation of
the character) to @var{output-port}, performs discretionary output
flushing, and returns an unspecified value.
@end deffn

@deffn procedure write-string string [output-port]
@cindex string, output to port
Writes @var{string} to @var{output-port}, performs discretionary output
flushing, and returns an unspecified value.  This is equivalent to
writing the contents of @var{string}, one character at a time using
@code{write-char}, except that it is usually much faster.
@end deffn

@deffn procedure write-substring string start end [output-port]
@cindex string, output to port
Writes the substring defined by @var{string}, @var{start}, and @var{end}
to @var{output-port}, performs discretionary output flushing, and
returns an unspecified value.  This is equivalent to writing the
contents of the substring, one character at a time using
@code{write-char}, except that it is usually much faster.
@end deffn

@deffn procedure write object [output-port]
@cindex expression, output to port
Writes a written representation of @var{object} to @var{output-port},
and returns an unspecified value.  If @var{object} has a standard
external representation, then the written representation generated by
@code{write} shall be parsable by @code{read} into an equivalent object.
Thus strings that appear in the written representation are enclosed in
doublequotes, and within those strings backslash and doublequote are
escaped by backslashes.  @code{write} performs discretionary output
flushing and returns an unspecified value.
@end deffn

@deffn procedure display object [output-port]
@cindex external representation, generating
@cindex generating, external representation
Writes a representation of @var{object} to @var{output-port}.  Strings
appear in the written representation as if written by
@code{write-string} instead of by @code{write}.  Character objects
appear in the representation as if written by @code{write-char} instead
of by @code{write}.  @code{display} performs discretionary output
flushing and returns an unspecified value.@footnote{@code{write} is
intended for producing machine-readable output and @code{display} is for
producing human-readable output.}
@end deffn

@deffn procedure newline [output-port]
@cindex newline character, output to port
Writes an end-of-line to @var{output-port}, performs discretionary
output flushing, and returns an unspecified value.  Equivalent to
@code{(write-char #\newline @var{output-port})}.
@end deffn

@deffn procedure fresh-line [output-port]
Most output ports are able to tell whether or not they are at the
beginning of a line of output.  If @var{output-port} is such a port,
this procedure writes an end-of-line to the port only if the port is not
already at the beginning of a line.  If @var{output-port} is not such a
port, this procedure is identical to @code{newline}.  In either case,
@code{fresh-line} performs discretionary output flushing and returns an
unspecified value.
@end deffn

@deffn procedure write-line object [output-port]
Like @code{write}, except that it writes an end-of-line to
@var{output-port} after writing @var{object}'s representation.  This
procedure performs discretionary output flushing and returns an
unspecified value.
@end deffn

@deffn procedure flush-output [output-port]
If @var{output-port} is buffered, this causes the contents of its buffer
to be written to the output device.  Otherwise it has no effect.
Returns an unspecified value.
@end deffn

@deffn procedure beep [output-port]
@cindex console, ringing the bell
@cindex ringing the console bell
@cindex bell, ringing on console
Performs a ``beep'' operation on @var{output-port}, performs
discretionary output flushing, and returns an unspecified value.  On the
console port, this usually causes the console bell to beep, but more
sophisticated interactive ports may take other actions, such as flashing
the screen.  On most output ports, e.g.@: file and string output ports,
this does nothing.
@end deffn

@deffn procedure clear [output-port]
@cindex console, clearing
@cindex display, clearing
@cindex screen, clearing
@cindex terminal screen, clearing
@cindex clearing the console screen
``Clears the screen'' of @var{output-port}, performs discretionary
output flushing, and returns an unspecified value.  On a terminal or
window, this has a well-defined effect.  On other output ports, e.g.@:
file and string output ports, this does nothing.
@end deffn

@deffn procedure pp object [output-port [as-code?]]
@cindex pretty printer
@code{pp} prints @var{object} in a visually appealing and structurally
revealing manner on @var{output-port}.  If object is a procedure,
@code{pp} attempts to print the source text.  If the optional argument
@var{as-code?} is true, @code{pp} prints lists as Scheme code, providing
appropriate indentation; by default this argument is false.  @code{pp}
performs discretionary output flushing and returns an unspecified value.
@end deffn

The following variables may be dynamically bound to change the behavior
of the @code{write} and @code{display} procedures.

@defvr variable *unparser-radix*
This variable specifies the default radix used to print numbers.  Its
value must be one of the exact integers @code{2}, @code{8}, @code{10},
or @code{16}; the default is @code{10}.  If @code{*unparser-radix*} is
not @code{10}, numbers are prefixed to indicate their radix.
@end defvr

@defvr variable *unparser-list-breadth-limit*
This variable specifies a limit on the length of the printed
representation of a list or vector; for example, if the limit is
@code{4}, only the first four elements of any list are printed, followed
by ellipses to indicate any additional elements.  The value of this
variable must be an exact non-negative integer, or @code{#f} meaning no
limit; the default is @code{#f}.

@example
@group
(fluid-let ((*unparser-list-breadth-limit* 4))
  (write-to-string '(a b c d)))
                                @result{} "(a b c d)"
(fluid-let ((*unparser-list-breadth-limit* 4))
  (write-to-string '(a b c d e)))
                                @result{} "(a b c d ...)"
@end group
@end example
@end defvr

@defvr variable *unparser-list-depth-limit*
This variable specifies a limit on the nesting of lists and vectors in
the printed representation.  If lists (or vectors) are more deeply
nested than the limit, the part of the representation that exceeds the
limit is replaced by ellipses.  The value of this variable must be an
exact non-negative integer, or @code{#f} meaning no limit; the default
is @code{#f}.

@example
@group
(fluid-let ((*unparser-list-depth-limit* 4))
  (write-to-string '((((a))) b c d)))
                                @result{} "((((a))) b c d)"
(fluid-let ((*unparser-list-depth-limit* 4))
  (write-to-string '(((((a)))) b c d)))
                                @result{} "((((...))) b c d)"
@end group
@end example
@end defvr

@defvr variable *unparser-string-length-limit*
This variable specifies a limit on the length of the printed
representation of strings.  If a string's length exceeds this limit, the
part of the printed representation for the characters exceeding the
limit is replaced by ellipses.  The value of this variable must be an
exact non-negative integer, or @code{#f} meaning no limit; the default
is @code{#f}.

@example
@group
(fluid-let ((*unparser-string-length-limit* 4))
  (write-to-string "abcd"))
                                @result{} "\"abcd\""
(fluid-let ((*unparser-string-length-limit* 4))
  (write-to-string "abcde"))
                                @result{} "\"abcd...\""
@end group
@end example
@end defvr

@defvr variable *unparse-with-maximum-readability?*
This variable, which takes a boolean value, tells the printer to use a
special printed representation for objects that normally print in a form
that cannot be recognized by @code{read}.  These objects are printed
using the representation @code{#@@@var{n}}, where @var{n} is the result
of calling @code{hash} on the object to be printed.  The reader
recognizes this syntax, calling @code{unhash} on @var{n} to get back the
original object.  Note that this printed representation can only be
recognized by the Scheme program in which it was generated, because
these hash numbers are different for each invocation of Scheme.
@end defvr

@node Format, Custom Output, Output Procedures, Input/Output
@section Format

@comment **** begin CLTL ****

The procedure @code{format} is very useful for producing nicely
formatted text, producing good-looking messages, and so on.  MIT/GNU
Scheme's implementation of @code{format} is similar to that of Common
Lisp, except that Common Lisp defines many more
directives.@footnote{This description of @code{format} is adapted from
@cite{Common Lisp, The Language}, second edition, section 22.3.3.}

@cindex run-time-loadable option
@cindex option, run-time-loadable
@code{format} is a run-time-loadable option.  To use it, execute

@example
(load-option 'format)
@end example
@findex load-option

@noindent
once before calling it.

@deffn procedure format destination control-string argument @dots{}
@findex write-string
@cindex format directive (defn)
@cindex directive, format (defn)
Writes the characters of @var{control-string} to @var{destination},
except that a tilde (@code{~}) introduces a @dfn{format directive}.  The
character after the tilde, possibly preceded by prefix parameters and
modifiers, specifies what kind of formatting is desired.  Most
directives use one or more @var{argument}s to create their output; the
typical directive puts the next @var{argument} into the output,
formatted in some special way.  It is an error if no argument remains
for a directive requiring an argument, but it is not an error if one or
more arguments remain unprocessed by a directive.

The output is sent to @var{destination}.  If @var{destination} is
@code{#f}, a string is created that contains the output; this string is
returned as the value of the call to @code{format}.  In all other cases
@code{format} returns an unspecified value.  If @var{destination} is
@code{#t}, the output is sent to the current output port.  Otherwise,
@var{destination} must be an output port, and the output is sent there.

This procedure performs discretionary output flushing (@pxref{Output
Procedures}).

A @code{format} directive consists of a tilde (@code{~}), optional
prefix parameters separated by commas, optional colon (@code{:}) and
at-sign (@code{@@}) modifiers, and a single character indicating what
kind of directive this is.  The alphabetic case of the directive
character is ignored.  The prefix parameters are generally integers,
notated as optionally signed decimal numbers.  If both the colon and
at-sign modifiers are given, they may appear in either order.

@cindex V as format parameter
@cindex # as format parameter
In place of a prefix parameter to a directive, you can put the letter
@samp{V} (or @samp{v}), which takes an @var{argument} for use as a
parameter to the directive.  Normally this should be an exact integer.
This feature allows variable-width fields and the like.  You can also
use the character @samp{#} in place of a parameter; it represents the
number of arguments remaining to be processed.

It is an error to give a format directive more parameters than it is
described here as accepting.  It is also an error to give colon or
at-sign modifiers to a directive in a combination not specifically
described here as being meaningful.

@table @code
@item ~A
The next @var{argument}, which may be any object, is printed as if by
@code{display}.  @code{~@var{mincol}A} inserts spaces on the right, if
necessary, to make the width at least @var{mincol} columns.  The
@code{@@} modifier causes the spaces to be inserted on the left rather
than the right.

@item ~S
The next @var{argument}, which may be any object, is printed as if by
@code{write}.  @code{~@var{mincol}S} inserts spaces on the right, if
necessary, to make the width at least @var{mincol} columns.  The
@code{@@} modifier causes the spaces to be inserted on the left rather
than the right.

@item ~%
This outputs a @code{#\newline} character.  @code{~@var{n}%} outputs
@var{n} newlines.  No @var{argument} is used.  Simply putting a newline
in @var{control-string} would work, but @code{~%} is often used because
it makes the control string look nicer in the middle of a program.

@item ~~
This outputs a tilde.  @code{~@var{n}~} outputs @var{n} tildes.

@item ~@var{newline}
Tilde immediately followed by a newline ignores the newline and any
following non-newline whitespace characters.  With an @code{@@}, the
newline is left in place, but any following whitespace is ignored.  This
directive is typically used when @var{control-string} is too long to fit
nicely into one line of the program:

@example
@group
(define (type-clash-error procedure arg spec actual)
  (format
   #t
   "~%Procedure ~S~%requires its %A argument ~
    to be of type ~S,~%but it was called with ~
    an argument of type ~S.~%"
   procedure arg spec actual))
@end group
@end example

@example
@group
(type-clash-error 'vector-ref
                  "first"
                  'integer
                  'vector)

@r{prints}

Procedure vector-ref
requires its first argument to be of type integer,
but it was called with an argument of type vector.
@end group
@end example

@noindent
Note that in this example newlines appear in the output only as
specified by the @code{~%} directives; the actual newline characters in
the control string are suppressed because each is preceded by a tilde.
@end table
@end deffn

@comment **** end CLTL ****

@node Custom Output, Prompting, Format, Input/Output
@section Custom Output

MIT/GNU Scheme provides hooks for specifying that certain kinds of objects
have special written representations.  There are no restrictions on the
written representations, but only a few kinds of objects may have custom
representation specified for them, specifically: records
(@pxref{Records}), vectors that have special tags in their zero-th
elements (@pxref{Vectors}), and pairs that have special tags in their
car fields (@pxref{Lists}).  There is a different procedure for
specifying the written representation of each of these types.

@deffn procedure set-record-type-unparser-method! record-type unparser-method
Changes the unparser method of the type represented by @var{record-type}
to be @var{unparser-method}, and returns an unspecified value.
Subsequently, when the unparser encounters a record of this type, it
will invoke @var{unparser-method} to generate the written
representation.
@end deffn

@deffn procedure unparser/set-tagged-vector-method! tag unparser-method
Changes the unparser method of the vector type represented by @var{tag}
to be @var{unparser-method}, and returns an unspecified value.
Subsequently, when the unparser encounters a vector with @var{tag} as
its zero-th element, it will invoke @var{unparser-method} to generate
the written representation.
@end deffn

@deffn procedure unparser/set-tagged-pair-method! tag unparser-method
Changes the unparser method of the pair type represented by @var{tag} to
be @var{unparser-method}, and returns an unspecified value.
Subsequently, when the unparser encounters a pair with @var{tag} in its
car field, it will invoke @var{unparser-method} to generate the written
representation.
@end deffn

@cindex unparser method (defn)
@cindex method, unparser (defn)
An @dfn{unparser method} is a procedure that is invoked with two
arguments: an unparser state and an object.  An unparser method
generates a written representation for the object, writing it to the
output port specified by the unparser state.  The value yielded by an
unparser method is ignored.  Note that an unparser state is not an
output port, rather it is an object that contains an output port as one
of its components.  Application programs generally do not construct or
examine unparser state objects, but just pass them along.

There are two ways to create an unparser method (which is then
registered by one of the above procedures).  The first, and easiest, is
to use @code{standard-unparser-method}.  The second is to define your
own method using the procedure @code{with-current-unparser-state}.  We
encourage the use of the first method, as it results in a more uniform
appearance for objects.  Many predefined datatypes, for example
procedures and environments, already have this appearance.

@deffn procedure standard-unparser-method name procedure
Returns a standard unparser method.  @var{Name} may be any object, and
is used as the name of the type with which the unparser method is
associated; @var{name} is usually a symbol.  @var{Procedure} must be
@code{#f} or a procedure of two arguments.

@cindex #[ as external representation
If @var{procedure} is @code{#f}, the returned method generates an
external representation of this form:

@example
#[@var{name} @var{hash}]
@end example

@noindent
@findex write
@findex write-string
@findex hash
Here @var{name} is the external representation of the argument
@var{name}, as generated by @code{write},@footnote{Except that if the
argument @var{name} is a string, its external representation is
generated by @code{write-string}.} and @var{hash} is the external
representation of an exact non-negative integer unique to the object
being printed (specifically, it is the result of calling @code{hash} on
the object).  Subsequently, the expression

@example
#@@@var{hash}
@end example

@noindent
is notation for the object.

If @var{procedure} is supplied, the returned method generates a slightly
different external representation:

@example
#[@var{name} @var{hash} @var{output}]
@end example

@noindent
Here @var{name} and @var{hash} are as above, and @var{output} is the
output generated by @var{procedure}.  The representation is constructed
in three stages:

@enumerate
@item
The first part of the format (up to @var{output}) is written to the
output port specified by the unparser state.  This is @code{"#["},
@var{name}, @code{" "}, and @var{hash}.

@item
@var{Procedure} is invoked on two arguments: the object and an output
port.

@item
The closing bracket is written to the output port.
@end enumerate
@end deffn

The following procedure is useful for writing more general kinds of
unparser methods.

@deffn procedure with-current-unparser-state unparser-state procedure
This procedure calls @var{procedure} with one argument, the output port
from @var{unparser-state}.  Additionally, it arranges for the remaining
components of @var{unparser-state} to be given to the printer when they
are needed.  The @var{procedure} generates some output by writing to the
output port using the usual output operations, and the value yielded by
@var{procedure} is returned from @code{with-current-unparser-state}.

The port passed to @var{procedure} should only be used within the
dynamic extent of @var{procedure}.
@end deffn

@node Prompting, Port Primitives, Custom Output, Input/Output
@section Prompting
@cindex prompting

This section describes procedures that prompt the user for input.  Why
should the programmer use these procedures when it is possible to do
prompting using ordinary input and output procedures?  One reason is
that the prompting procedures are more succinct.  However, a second and
better reason is that the prompting procedures can be separately
customized for each user interface, providing more natural interaction.
The interfaces for Edwin and for GNU Emacs have already been customized
in this fashion; because Edwin and Emacs are very similar editors, their
customizations provide very similar behavior.

@findex interaction-i/o-port
Each of these procedure accepts an optional argument called
@var{port}, which if given must be an @acronym{I/O} port.  If not
given, this port defaults to the value of
@code{(interaction-i/o-port)}; this is initially the console
@acronym{I/O} port.

@deffn procedure prompt-for-command-expression prompt [port [environment]]
Prompts the user for an expression that is to be executed as a command.
This is the procedure called by the @acronym{REP} loop to read the
user's expressions.

If @var{prompt} is a string, it is used verbatim as the prompt string.
Otherwise, it must be a pair whose car is the symbol @samp{standard} and
whose cdr is a string; in this case the prompt string is formed by
prepending to the string the current @acronym{REP} loop ``level number''
and a space.  Also, a space is appended to the string, unless it already
ends in a space or is an empty string.

If @var{environment} is given, it is passed as the second argument to
@code{read}.

The default behavior of this procedure is to print a fresh line, a
newline, and the prompt string; flush the output buffer; then read an
object and return it.

Under Edwin and Emacs, before the object is read, the interaction buffer
is put into a mode that allows expressions to be edited and submitted
for input using specific editor commands.  The first expression that is
submitted is returned as the value of this procedure.
@end deffn

@deffn procedure prompt-for-command-char prompt [port]
@findex char-graphic?
Prompts the user for a single character that is to be executed as a
command; the returned character is guaranteed to satisfy
@code{char-graphic?}.  If at all possible, the character is read from
the user interface using a mode that reads the character as a single
keystroke; in other words, it should not be necessary for the user to
follow the character with a carriage return or something similar.

@findex debug
@findex where
This is the procedure called by @code{debug} and @code{where} to read
the user's commands.

If @var{prompt} is a string, it is used verbatim as the prompt string.
Otherwise, it must be a pair whose car is @code{standard} and whose cdr
is a string; in this case the prompt string is formed by prepending to
the string the current @acronym{REP} loop ``level number'' and a space.
Also, a space is appended to the string, unless it already ends in a
space or is an empty string.

The default behavior of this procedure is to print a fresh line, a
newline, and the prompt string; flush the output buffer; read a
character in raw mode, echo that character, and return it.

Under Edwin and Emacs, instead of reading a character, the interaction
buffer is put into a mode in which graphic characters submit themselves
as input.  After this mode change, the first such character submitted is
returned as the value of this procedure.
@end deffn

@deffn procedure prompt-for-expression prompt [port [environment]]
Prompts the user for an expression.

The prompt string is formed by appending a colon and a space to
@var{prompt}, unless @var{prompt} already ends in a space or is the null
string.

If @var{environment} is given, it is passed as the second argument to
@code{read}.

The default behavior of this procedure is to print a fresh line, a
newline, and the prompt string; flush the output buffer; then read an
object and return it.

Under Edwin and Emacs, the expression is read in the minibuffer.
@end deffn

@deffn procedure prompt-for-evaluated-expression prompt [environment [port]]
Prompts the user for an evaluated expression.  Calls
@code{prompt-for-expression} to read an expression, then evaluates the
expression using @var{environment}; if @var{environment} is not given,
the @acronym{REP} loop environment is used.
@end deffn

@deffn procedure prompt-for-confirmation prompt [port]
Prompts the user for confirmation.  The result yielded by this procedure
is a boolean.

The prompt string is formed by appending the string @code{" (y or n)? "}
to @var{prompt}, unless @var{prompt} already ends in a space or is the
null string.

The default behavior of this procedure is to print a fresh line, a
newline, and the prompt string; flush the output buffer; then read a
character in raw mode.  If the character is @code{#\y}, @code{#\Y}, or
@code{#\space}, the procedure returns @code{#t}; If the character is
@code{#\n}, @code{#\N}, or @code{#\rubout}, the procedure returns
@code{#f}.  Otherwise the prompt is repeated.

Under Edwin or Emacs, the confirmation is read in the minibuffer.
@end deffn

@node Port Primitives, Parser Buffers, Prompting, Input/Output
@section Port Primitives
@cindex port primitives

This section describes the low-level operations that can be used to
build and manipulate @acronym{I/O} ports.

The purpose of these operations is twofold: to allow programmers to
construct new kinds of @acronym{I/O} ports, and to provide faster
@acronym{I/O} operations than those supplied by the standard high
level procedures.  The latter is useful because the standard
@acronym{I/O} operations provide defaulting and error checking, and
sometimes other features, which are often unnecessary.  This interface
provides the means to bypass such features, thus improving
performance.

The abstract model of an @acronym{I/O} port, as implemented here, is a
combination of a set of named operations and a state.  The state is an
arbitrary object, the meaning of which is determined by the
operations.  The operations are defined by a mapping from names to
procedures.

@cindex port type
The set of named operations is represented by an object called a
@dfn{port type}.  A port type is constructed from a set of named
operations, and is subsequently used to construct a port.  The port type
completely specifies the behavior of the port.  Port types also support
a simple form of inheritance, allowing you to create new ports that are
similar to existing ports.

The port operations are divided into two classes:

@table @asis
@item Standard operations
There is a specific set of standard operations for input ports, and a
different set for output ports.  Applications can assume that the
standard input operations are implemented for all input ports, and
likewise the standard output operations are implemented for all output
ports.
@cindex standard operations, on port

@item Custom operations
Some ports support additional operations.  For example, ports that
implement output to terminals (or windows) may define an operation named
@code{y-size} that returns the height of the terminal in characters.
Because only some ports will implement these operations, programs that
use custom operations must test each port for their existence, and be
prepared to deal with ports that do not implement them.
@cindex custom operations, on port
@findex y-size
@end table

@menu
* Port Types::                  
* Constructors and Accessors for Ports::  
* Input Port Operations::       
* Output Port Operations::      
* Blocking Mode::               
* Terminal Mode::               
@end menu

@node Port Types, Constructors and Accessors for Ports, Port Primitives, Port Primitives
@subsection Port Types

The procedures in this section provide means for constructing port types
with standard and custom operations, and accessing their operations.

@deffn procedure make-port-type operations port-type
@cindex construction, of port type
Creates and returns a new port type.
@var{Operations} must be a list; each element is a list of two elements,
the name of the operation (a symbol) and the procedure that implements
it.  @var{Port-type} is either @code{#f} or a port type; if it is a port
type, any operations implemented by @var{port-type} but not specified in
@var{operations} will be implemented by the resulting port type.

@var{Operations} need not contain definitions for all of the standard
operations; the procedure will provide defaults for any standard
operations that are not defined.  At a minimum, the following operations
must be defined: for input ports, @code{read-char} and @code{peek-char};
for output ports, either @code{write-char} or @code{write-substring}.
@acronym{I/O} ports must supply the minimum operations for both input and
output.

If an operation in @var{operations} is defined to be @code{#f}, then the
corresponding operation in @var{port-type} is @emph{not} inherited.

If @code{read-char} is defined in @var{operations}, then any standard
input operations defined in @var{port-type} are ignored.  Likewise, if
@code{write-char} or @code{write-substring} is defined in
@var{operations}, then any standard output operations defined in
@var{port-type} are ignored.  This feature allows overriding the
standard operations without having to enumerate them.
@end deffn

@deffn procedure port-type? object
@deffnx procedure input-port-type? object
@deffnx procedure output-port-type? object
@deffnx procedure i/o-port-type? object
These predicates return @code{#t} if @var{object} is a port type,
input-port type, output-port type, or @acronym{I/O}-port type,
respectively.  Otherwise, they return @code{#f}.
@end deffn

@deffn procedure port-type/operations port-type
Returns a newly allocated list containing all of the operations
implemented by @var{port-type}.  Each element of the list is a list of
two elements --- the name and its associated operation.
@end deffn

@deffn procedure port-type/operation-names port-type
Returns a newly allocated list whose elements are the names of the
operations implemented by @var{port-type}.
@end deffn

@deffn procedure port-type/operation port-type symbol
Returns the operation named @var{symbol} in @var{port-type}.  If
@var{port-type} has no such operation, returns @code{#f}.
@end deffn

@node Constructors and Accessors for Ports, Input Port Operations, Port Types, Port Primitives
@subsection Constructors and Accessors for Ports

The procedures in this section provide means for constructing ports,
accessing the type of a port, and manipulating the state of a port.

@deffn procedure make-port port-type state
Returns a new port with type @var{port-type} and the given
@var{state}.  The port will be an input, output, or @acronym{I/O} port
according to @var{port-type}.
@end deffn

@deffn procedure port/type port
Returns the port type of @var{port}.
@end deffn

@deffn procedure port/state port
Returns the state component of @var{port}.
@end deffn

@deffn procedure set-port/state! port object
Changes the state component of @var{port} to be @var{object}.
Returns an unspecified value.
@end deffn

@deffn procedure port/operation port symbol
Equivalent to

@example
(port-type/operation (port/type @var{port}) @var{symbol})
@end example
@end deffn

@deffn procedure port/operation-names port
Equivalent to

@example
(port-type/operation-names (port/type @var{port}))
@end example
@end deffn

@deffn procedure make-eof-object input-port
@cindex EOF object, construction
@cindex construction, of EOF object
@findex eof-object?
Returns an object that satisfies the predicate @code{eof-object?}.  This
is sometimes useful when building input ports.
@end deffn

@node Input Port Operations, Output Port Operations, Constructors and Accessors for Ports, Port Primitives
@subsection Input Port Operations
@cindex input port operations

This section describes the standard operations on input ports.
Following that, some useful custom operations are described.

@defop operation {input port} read-char input-port
@cindex character, input from port
Removes the next character available from @var{input-port} and returns
it.  If @var{input-port} has no more characters and will never have any
(e.g.@: at the end of an input file), this operation returns an
end-of-file object.  If @var{input-port} has no more characters but will
eventually have some more (e.g.@: a terminal where nothing has been
typed recently), and it is in non-blocking mode, @code{#f} is returned;
otherwise the operation hangs until input is available.
@end defop

@defop operation {input port} peek-char input-port
Reads the next character available from @var{input-port} and returns it.
The character is @emph{not} removed from @var{input-port}, and a
subsequent attempt to read from the port will get that character again.
In other respects this operation behaves like @code{read-char}.
@end defop

@defop operation {input port} discard-char input-port
Discards the next character available from @var{input-port} and returns
an unspecified value.  In other respects this operation behaves like
@code{read-char}.
@end defop

@defop operation {input port} char-ready? input-port k
@code{char-ready?} returns @code{#t} if at least one character is
available to be read from @var{input-port}.  If no characters are
available, the operation waits up to @var{k} milliseconds before
returning @code{#f}, returning immediately if any characters become
available while it is waiting.
@end defop

@defop operation {input port} read-string input-port char-set
@defopx operation {input port} discard-chars input-port char-set
@cindex string, input from port
These operations are like @code{read-char} and @code{discard-char},
except that they read or discard multiple characters at once.  This can
have a marked performance improvement on buffered input ports.  All
characters up to, but excluding, the first character in @var{char-set}
(or end of file) are read from @var{input-port}.  @code{read-string}
returns these characters as a newly allocated string, while
@code{discard-chars} discards them and returns an unspecified value.
These operations hang until sufficient input is available, even if
@var{input-port} is in non-blocking mode.  If end of file is encountered
before any input characters, @code{read-string} returns an end-of-file
object.
@end defop

@defop operation {input port} read-substring input-port string start end
Reads characters from @var{input-port} into the substring defined by
@var{string}, @var{start}, and @var{end} until either the substring has
been filled or there are no more characters available.  Returns the
number of characters written to the substring.

If @var{input-port} is an interactive port, and at least one character
is immediately available, the available characters are written to the
substring and this operation returns immediately.  If no characters are
available, and @var{input-port} is in blocking mode, the operation
blocks until at least one character is available.  Otherwise, the
operation returns @code{#f} immediately.

This is an extremely fast way to read characters from a port.
@end defop

@deffn procedure input-port/read-char input-port
@deffnx procedure input-port/peek-char input-port
@deffnx procedure input-port/discard-char input-port
@deffnx procedure input-port/char-ready? input-port k
@deffnx procedure input-port/read-string input-port char-set
@deffnx procedure input-port/discard-chars input-port char-set
@deffnx procedure input-port/read-substring input-port string start end
Each of these procedures invokes the respective operation on
@var{input-port}.  For example, the following are equivalent:

@example
@group
(input-port/read-char @var{input-port})
((port/operation @var{input-port} 'read-char) @var{input-port})
@end group
@end example
@end deffn

The following custom operations are implemented for input ports to
files, and will also work with some other kinds of input ports:

@defop operation {input port} eof? input-port
Returns @code{#t} if @var{input-port} is known to be at end of file,
otherwise it returns @code{#f}.
@end defop

@defop operation {input port} chars-remaining input-port
Returns an estimate of the number of characters remaining to be read
from @var{input-port}.  This is useful only when @var{input-port} is a
file port in binary mode; in other cases, it returns @code{#f}.
@end defop

@defop operation {input port} buffered-input-chars input-port
Returns the number of unread characters that are stored in
@var{input-port}'s buffer.  This will always be less than or equal to
the buffer's size.
@end defop

@defop operation {input port} input-buffer-size input-port
Returns the maximum number of characters that @var{input-port}'s buffer
can hold.
@end defop

@defop operation {input port} set-input-buffer-size input-port size
Resizes @var{input-port}'s buffer so that it can hold at most @var{size}
characters.  Characters in the buffer are discarded.  @var{Size} must be
an exact non-negative integer.
@end defop

@node Output Port Operations, Blocking Mode, Input Port Operations, Port Primitives
@subsection Output Port Operations
@cindex output port operations

This section describes the standard operations on output ports.
Following that, some useful custom operations are described.

@defop operation {output port} write-char output-port char
@cindex character, output to port
Writes @var{char} to @var{output-port} and returns an unspecified value.
@end defop

@defop operation {output port} write-substring output-port string start end
@cindex substring, output to port
Writes the substring specified by @var{string}, @var{start}, and
@var{end} to @var{output-port} and returns an unspecified value.
Equivalent to writing the characters of the substring, one by one, to
@var{output-port}, but is implemented very efficiently.
@end defop

@defop operation {output port} fresh-line output-port
Most output ports are able to tell whether or not they are at the
beginning of a line of output.  If @var{output-port} is such a port,
end-of-line is written to the port only if the port is not already at
the beginning of a line.  If @var{output-port} is not such a port, an
end-of-line is unconditionally written to the port.  Returns an
unspecified value.
@end defop

@defop operation {output port} flush-output output-port
If @var{output-port} is buffered, this causes its buffer to be written
out.  Otherwise it has no effect.  Returns an unspecified value.
@end defop

@defop operation {output port} discretionary-flush-output output-port
Normally, this operation does nothing.  However, ports that support
discretionary output flushing implement this operation identically to @code{flush-output}.
@end defop

@deffn procedure output-port/write-char output-port char
@deffnx procedure output-port/write-substring output-port string start end
@deffnx procedure output-port/fresh-line output-port
@deffnx procedure output-port/flush-output output-port
@deffnx procedure output-port/discretionary-flush-output output-port
Each of these procedures invokes the respective operation on
@var{output-port}.  For example, the following are equivalent:

@example
@group
(output-port/write-char @var{output-port} @var{char})
((port/operation @var{output-port} 'write-char)
 @var{output-port} @var{char})
@end group
@end example
@end deffn

@deffn procedure output-port/write-string output-port string
Writes @var{string} to @var{output-port}.  Equivalent to

@example
@group
(output-port/write-substring @var{output-port}
                             @var{string}
                             0
                             (string-length @var{string}))
@end group
@end example
@end deffn

The following custom operations are generally useful.

@defop operation {output port} buffered-output-chars output-port
Returns the number of unwritten characters that are stored in
@var{output-port}'s buffer.  This will always be less than or equal to
the buffer's size.
@end defop

@defop operation {output port} output-buffer-size output-port
Returns the maximum number of characters that @var{output-port}'s buffer
can hold.
@end defop

@defop operation {output port} set-output-buffer-size output-port size
Resizes @var{output-port}'s buffer so that it can hold at most @var{size}
characters.  Characters in the buffer are discarded.  @var{Size} must be
an exact non-negative integer.
@end defop

@defop operation {output port} x-size output-port
Returns an exact positive integer that is the width of @var{output-port}
in characters.  If @var{output-port} has no natural width, e.g.@: if it is
a file port, @code{#f} is returned.
@end defop

@defop operation {output port} y-size output-port
Returns an exact positive integer that is the height of
@var{output-port} in characters.  If @var{output-port} has no natural
height, e.g.@: if it is a file port, @code{#f} is returned.
@end defop

@deffn procedure output-port/x-size output-port
This procedure invokes the custom operation whose name is the symbol
@code{x-size}, if it exists.  If the @code{x-size} operation is both
defined and returns a value other than @code{#f}, that value is returned
as the result of this procedure.  Otherwise, @code{output-port/x-size}
returns a default value (currently @code{80}).

@code{output-port/x-size} is useful for programs that tailor their
output to the width of the display (a fairly common practice).  If the
output device is not a display, such programs normally want some
reasonable default width to work with, and this procedure provides
exactly that.
@end deffn

@deffn procedure output-port/y-size output-port
This procedure invokes the custom operation whose name is the symbol
@code{y-size}, if it exists.  If the @code{y-size} operation is defined,
the value it returns is returned as the result of this procedure;
otherwise, @code{#f} is returned.
@end deffn

@node Blocking Mode, Terminal Mode, Output Port Operations, Port Primitives
@subsection Blocking Mode

@cindex blocking mode, of port
An interactive port is always in one of two modes: @dfn{blocking} or
@dfn{non-blocking}.  This mode is independent of the terminal mode:
each can be changed independent of the other.  Furthermore, if it is
an interactive @acronym{I/O} port, there are separate blocking modes
for input and for output.

If an input port is in blocking mode, attempting to read from it when no
input is available will cause Scheme to ``block'', i.e.@: suspend
itself, until input is available.  If an input port is in non-blocking
mode, attempting to read from it when no input is available will cause
the reading procedure to return immediately, indicating the lack of
input in some way (exactly how this situation is indicated is separately
specified for each procedure or operation).

An output port in blocking mode will block if the output device is not
ready to accept output.  In non-blocking mode it will return immediately
after performing as much output as the device will allow (again, each
procedure or operation reports this situation in its own way).

Interactive ports are initially in blocking mode; this can be changed at
any time with the procedures defined in this section.

These procedures represent blocking mode by the symbol @code{blocking},
and non-blocking mode by the symbol @code{nonblocking}.  An argument
called @var{mode} must be one of these symbols.  A @var{port} argument
to any of these procedures may be any port, even if that port does not
support blocking mode; in that case, the port is not modified in any
way.

@deffn procedure port/input-blocking-mode port
Returns the input blocking mode of @var{port}.
@end deffn

@deffn procedure port/set-input-blocking-mode port mode
Changes the input blocking mode of @var{port} to be @var{mode}.  Returns
an unspecified value.
@end deffn

@deffn procedure port/with-input-blocking-mode port mode thunk
@var{Thunk} must be a procedure of no arguments.
@code{port/with-input-blocking-mode}
binds the input blocking mode of @var{port} to be @var{mode}, executes
@var{thunk}, restores the input blocking mode of @var{port} to what it
was when @code{port/with-input-blocking-mode} was called, and returns
the value that was yielded by @var{thunk}.  This binding is performed
by @code{dynamic-wind}, which guarantees that the input blocking mode is
restored if @var{thunk} escapes from its continuation.
@end deffn

@deffn procedure port/output-blocking-mode port
Returns the output blocking mode of @var{port}.
@end deffn

@deffn procedure port/set-output-blocking-mode port mode
Changes the output blocking mode of @var{port} to be @var{mode}.
Returns an unspecified value.
@end deffn

@deffn procedure port/with-output-blocking-mode port mode thunk
@var{Thunk} must be a procedure of no arguments.
@code{port/with-output-blocking-mode}
binds the output blocking mode of @var{port} to be @var{mode}, executes
@var{thunk}, restores the output blocking mode of @var{port} to what it
was when @code{port/with-output-blocking-mode} was called, and returns
the value that was yielded by @var{thunk}.  This binding is performed
by @code{dynamic-wind}, which guarantees that the output blocking mode
is restored if @var{thunk} escapes from its continuation.
@end deffn

@node Terminal Mode,  , Blocking Mode, Port Primitives
@subsection Terminal Mode

@cindex terminal mode, of port
A port that reads from or writes to a terminal has a @dfn{terminal
mode}; this is either @dfn{cooked} or @dfn{raw}.  This mode is
independent of the blocking mode: each can be changed independent of
the other.  Furthermore, a terminal @acronym{I/O} port has independent
terminal modes both for input and for output.

@cindex cooked mode, of terminal port
A terminal port in cooked mode provides some standard processing to make
the terminal easy to communicate with.  For example, under unix, cooked
mode on input reads from the terminal a line at a time and provides
rubout processing within the line, while cooked mode on output might
translate linefeeds to carriage-return/linefeed pairs.  In general, the
precise meaning of cooked mode is operating-system dependent, and
furthermore might be customizable by means of operating system
utilities.  The basic idea is that cooked mode does whatever is
necessary to make the terminal handle all of the usual user-interface
conventions for the operating system, while keeping the program's
interaction with the port as normal as possible.

@cindex raw mode, of terminal port
A terminal port in raw mode disables all of that processing.  In raw
mode, characters are directly read from and written to the device
without any translation or interpretation by the operating system.  On
input, characters are available as soon as they are typed, and are not
echoed on the terminal by the operating system.  In general, programs
that put ports in raw mode have to know the details of interacting with
the terminal.  In particular, raw mode is used for writing programs such
as text editors.

Terminal ports are initially in cooked mode; this can be changed at any
time with the procedures defined in this section.

These procedures represent cooked mode by the symbol @code{cooked}, and
raw mode by the symbol @code{raw}.  Additionally, the value @code{#f}
represents ``no mode''; it is the terminal mode of a port that is not a
terminal.  An argument called @var{mode} must be one of these three
values.  A @var{port} argument to any of these procedures may be any
port, even if that port does not support terminal mode; in that case,
the port is not modified in any way.

@deffn procedure port/input-terminal-mode port
Returns the input terminal mode of @var{port}.
@end deffn

@deffn procedure port/set-input-terminal-mode port mode
Changes the input terminal mode of @var{port} to be @var{mode}.
Returns an unspecified value.
@end deffn

@deffn procedure port/with-input-terminal-mode port mode thunk
@var{Thunk} must be a procedure of no arguments.
@code{port/with-input-terminal-mode}
binds the input terminal mode of @var{port} to be @var{mode}, executes
@var{thunk}, restores the input terminal mode of @var{port} to what it
was when @code{port/with-input-terminal-mode} was called, and returns
the value that was yielded by @var{thunk}.  This binding is performed
by @code{dynamic-wind}, which guarantees that the input terminal mode is
restored if @var{thunk} escapes from its continuation.
@end deffn

@deffn procedure port/output-terminal-mode port
Returns the output terminal mode of @var{port}.
@end deffn

@deffn procedure port/set-output-terminal-mode port mode
Changes the output terminal mode of @var{port} to be @var{mode}.
Returns an unspecified value.
@end deffn

@deffn procedure port/with-output-terminal-mode port mode thunk
@var{Thunk} must be a procedure of no arguments.
@code{port/with-output-terminal-mode}
binds the output terminal mode of @var{port} to be @var{mode}, executes
@var{thunk}, restores the output terminal mode of @var{port} to what it
was when @code{port/with-output-terminal-mode} was called, and returns
the value that was yielded by @var{thunk}.  This binding is performed
by @code{dynamic-wind}, which guarantees that the output terminal mode is
restored if @var{thunk} escapes from its continuation.
@end deffn

@node Parser Buffers, Parser Language, Port Primitives, Input/Output
@section Parser Buffers

@cindex Parser buffer
The @dfn{parser buffer} mechanism facilitates construction of parsers
for complex grammars.  It does this by providing an input stream with
unbounded buffering and backtracking.  The amount of buffering is
under program control.  The stream can backtrack to any position in
the buffer.

@cindex Parser-buffer pointer
The mechanism defines two data types: the @dfn{parser buffer} and the
@dfn{parser-buffer pointer}.  A parser buffer is like an input port
with buffering and backtracking.  A parser-buffer pointer is a pointer
into the stream of characters provided by a parser buffer.

Note that all of the procedures defined here consider a parser buffer
to contain a stream of 8-bit characters in the @acronym{ISO-8859-1}
character set, except for @code{match-utf8-char-in-alphabet} which
treats it as a stream of Unicode characters encoded as 8-bit bytes in
the @acronym{UTF-8} encoding.

There are several constructors for parser buffers:

@deffn procedure input-port->parser-buffer port
Returns a parser buffer that buffers characters read from @var{port}.
@end deffn

@deffn procedure substring->parser-buffer string start end
Returns a parser buffer that buffers the characters in the argument
substring.  This is equivalent to creating a string input port and
calling @code{input-port->parser-buffer}, but it runs faster and uses
less memory.
@end deffn

@deffn procedure string->parser-buffer string
Like @code{substring->parser-buffer} but buffers the entire string.
@end deffn

@deffn procedure source->parser-buffer source
Returns a parser buffer that buffers the characters returned by
calling @var{source}.  @var{Source} is a procedure of three arguments:
a string, a start index, and an end index (in other words, a substring
specifier).  Each time @var{source} is called, it writes some
characters in the substring, and returns the number of characters
written.  When there are no more characters available, it returns
zero.  It must not return zero in any other circumstance.
@end deffn

Parser buffers and parser-buffer pointers may be distinguished from
other objects:

@deffn procedure parser-buffer? object
Returns @code{#t} if @var{object} is a parser buffer, otherwise
returns @code{#f}.
@end deffn

@deffn procedure parser-buffer-pointer? object
Returns @code{#t} if @var{object} is a parser-buffer pointer,
otherwise returns @code{#f}.
@end deffn

Characters can be read from a parser buffer much as they can be read
from an input port.  The parser buffer maintains an internal pointer
indicating its current position in the input stream.  Additionally,
the buffer remembers all characters that were previously read, and can
look at characters arbitrarily far ahead in the stream.  It is this
buffering capability that facilitates complex matching and
backtracking.

@deffn procedure read-parser-buffer-char buffer
Returns the next character in @var{buffer}, advancing the internal
pointer past that character.  If there are no more characters
available, returns @code{#f} and leaves the internal pointer
unchanged.
@end deffn

@deffn procedure peek-parser-buffer-char buffer
Returns the next character in @var{buffer}, or @code{#f} if no
characters are available.  Leaves the internal pointer unchanged.
@end deffn

@deffn procedure parser-buffer-ref buffer index
Returns a character in @var{buffer}.  @var{Index} is a non-negative
integer specifying the character to be returned.  If @var{index} is
zero, returns the next available character; if it is one, returns the
character after that, and so on.  If @var{index} specifies a position
after the last character in @var{buffer}, returns @code{#f}.  Leaves
the internal pointer unchanged.
@end deffn

The internal pointer of a parser buffer can be read or written:

@deffn procedure get-parser-buffer-pointer buffer
Returns a parser-buffer pointer object corresponding to the internal
pointer of @var{buffer}.
@end deffn

@deffn procedure set-parser-buffer-pointer! buffer pointer
Sets the internal pointer of @var{buffer} to the position specified by
@var{pointer}.  @var{Pointer} must have been returned from a previous
call of @code{get-parser-buffer-pointer} on @var{buffer}.
Additionally, if some of @var{buffer}'s characters have been discarded
by @code{discard-parser-buffer-head!}, @var{pointer} must be outside
the range that was discarded.
@end deffn

@deffn procedure get-parser-buffer-tail buffer pointer
Returns a newly-allocated string consisting of all of the characters
in @var{buffer} that fall between @var{pointer} and @var{buffer}'s
internal pointer.  @var{Pointer} must have been returned from a
previous call of @code{get-parser-buffer-pointer} on @var{buffer}.
Additionally, if some of @var{buffer}'s characters have been discarded
by @code{discard-parser-buffer-head!}, @var{pointer} must be outside
the range that was discarded.
@end deffn

@deffn procedure discard-parser-buffer-head! buffer
Discards all characters in @var{buffer} that have already been read;
in other words, all characters prior to the internal pointer.  After
this operation has completed, it is no longer possible to move the
internal pointer backwards past the current position by calling
@code{set-parser-buffer-pointer!}.
@end deffn

The next rather large set of procedures does conditional matching
against the contents of a parser buffer.  All matching is performed
relative to the buffer's internal pointer, so the first character to
be matched against is the next character that would be returned by
@code{peek-parser-buffer-char}.  The returned value is always
@code{#t} for a successful match, and @code{#f} otherwise.  For
procedures whose names do not end in @samp{-no-advance}, a successful
match also moves the internal pointer of the buffer forward to the end
of the matched text; otherwise the internal pointer is unchanged.

@deffn procedure match-parser-buffer-char buffer char
@deffnx procedure match-parser-buffer-char-ci buffer char
@deffnx procedure match-parser-buffer-not-char buffer char
@deffnx procedure match-parser-buffer-not-char-ci buffer char
@deffnx procedure match-parser-buffer-char-no-advance buffer char
@deffnx procedure match-parser-buffer-char-ci-no-advance buffer char
@deffnx procedure match-parser-buffer-not-char-no-advance buffer char
@deffnx procedure match-parser-buffer-not-char-ci-no-advance buffer char
Each of these procedures compares a single character in @var{buffer}
to @var{char}.  The basic comparison @code{match-parser-buffer-char}
compares the character to @var{char} using @code{char=?}.  The
procedures whose names contain the @samp{-ci} modifier do
case-insensitive comparison (i.e.@: they use @code{char-ci=?}).  The
procedures whose names contain the @samp{not-} modifier are successful
if the character @emph{doesn't} match @var{char}.
@end deffn

@deffn procedure match-parser-buffer-char-in-set buffer char-set
@deffnx procedure match-parser-buffer-char-in-set-no-advance buffer char-set
These procedures compare the next character in @var{buffer} against
@var{char-set} using @code{char-set-member?}.
@end deffn

@deffn procedure match-parser-buffer-string buffer string
@deffnx procedure match-parser-buffer-string-ci buffer string
@deffnx procedure match-parser-buffer-string-no-advance buffer string
@deffnx procedure match-parser-buffer-string-ci-no-advance buffer string
These procedures match @var{string} against @var{buffer}'s contents.
The @samp{-ci} procedures do case-insensitive matching.
@end deffn

@deffn procedure match-parser-buffer-substring buffer string start end
@deffnx procedure match-parser-buffer-substring-ci buffer string start end
@deffnx procedure match-parser-buffer-substring-no-advance buffer string start end
@deffnx procedure match-parser-buffer-substring-ci-no-advance buffer string start end
These procedures match the specified substring against @var{buffer}'s
contents.  The @samp{-ci} procedures do case-insensitive matching.
@end deffn

@deffn procedure match-utf8-char-in-alphabet buffer alphabet
This procedure treats @var{buffer}'s contents as @acronym{UTF-8}
encoded Unicode characters and matches the next such character against
@var{alphabet}, which must be a Unicode alphabet (@pxref{Unicode}).
@acronym{UTF-8} represents characters with 1 to 6 bytes, so a
successful match can move the internal pointer forward by as many as 6
bytes.
@end deffn

The remaining procedures provide information that can be used to
identify locations in a parser buffer's stream.

@deffn procedure parser-buffer-position-string pointer
Returns a string describing the location of @var{pointer} in terms of
its character and line indexes.  This resulting string is meant to be
presented to an end user in order to direct their attention to a
feature in the input stream.  In this string, the indexes are
presented as one-based numbers.

@var{Pointer} may alternatively be a parser buffer, in which case it
is equivalent to having specified the buffer's internal pointer.
@end deffn

@deffn procedure parser-buffer-pointer-index pointer
@deffnx procedure parser-buffer-pointer-line pointer
Returns the character or line index, respectively, of @var{pointer}.
Both indexes are zero-based.
@end deffn

@node Parser Language, XML Support, Parser Buffers, Input/Output
@section Parser Language

@cindex Parser language
Although it is possible to write parsers using the parser-buffer
abstraction (@pxref{Parser Buffers}), it is tedious.  The problem is
that the abstraction isn't closely matched to the way that people
think about syntactic structures.  In this section, we introduce a
higher-level mechanism that greatly simplifies the implementation of a
parser.

The @dfn{parser language} described here allows the programmer to
write @acronym{BNF}-like specifications that are translated into
efficient Scheme code at compile time.  The language is declarative,
but it can be freely mixed with Scheme code; this allows the parsing
of grammars that aren't conveniently described in the language.

@cindex Backtracking, in parser language
The language also provides backtracking.  For example, this expression
matches any sequence of alphanumeric characters followed by a single
alphabetic character:

@example
@group
(*matcher
 (seq (* (char-set char-set:alphanumeric))
      (char-set char-set:alphabetic)))
@end group
@end example

@noindent
The way that this works is that the matcher matches alphanumeric
characters in the input stream until it finds a non-alphanumeric
character.  It then tries to match an alphabetic character, which of
course fails.  At this point, if it matched at least one alphanumeric
character, it @emph{backtracks}: the last matched alphanumeric is
``unmatched'', and it again attempts to match an alphabetic
character.  The backtracking can be arbitrarily deep; the matcher will
continue to back up until it finds a way to match the remainder of the
expression.

So far, this sounds a lot like regular-expression matching
(@pxref{Regular Expressions}).  However, there are some important
differences.

@itemize @bullet
@item
The parser language uses a Scheme-like syntax that is easier to read
and write than regular-expression notation.

@item
The language provides macros so that common syntactic constructs can
be abstracted.

@item
The language mixes easily with Scheme code, allowing the full power of
Scheme to be applied to program around limitations in the parser
language.

@item
The language provides expressive facilities for converting syntax into
parsed structure.  It also makes it easy to convert parsed strings
into meaningful objects (e.g.@: numbers).

@item
The language is compiled into machine language; regular expressions
are usually interpreted.
@end itemize

Here is an example that shows off several of the features of the
parser language.  The example is a parser for @acronym{XML} start
tags:

@anchor{with-pointer example}
@example
@group
(*parser
 (with-pointer p
   (seq "<"
        parse-name
        parse-attribute-list
        (alt (match ">")
             (match "/>")
             (sexp
              (lambda (b)
                (error
                 (string-append
                  "Unterminated start tag at "
                  (parser-buffer-position-string p)))))))))
@end group
@end example

@noindent
This shows that the basic description of a start tag is very similar
to its @acronym{BNF}.  Non-terminal symbols @code{parse-name} and
@code{parse-attribute-list} do most of the work, and the noise strings
@code{"<"} and @code{">"} are the syntactic markers delimiting the
form.  There are two alternate endings for start tags, and if the
parser doesn't find either of the endings, the Scheme code (wrapped in
@code{sexp}) is run to signal an error.  The error procedure
@code{perror} takes a pointer @code{p}, which it uses to indicate the
position in the input stream at which the error occurred.  In this
case, that is the beginning of the start tag, i.e.@: the position of
the leading @code{"<"} marker.

This example still looks pretty complicated, mostly due to the
error-signalling code.  In practice, this is abstracted into a macro,
after which the expression is quite succinct:

@example
@group
(*parser
 (bracket "start tag"
     (seq (noise (string "<")) parse-name)
     (match (alt (string ">") (string "/>")))
   parse-attribute-list))
@end group
@end example

@noindent
The @code{bracket} macro captures the pattern of a bracketed item, and
hides much of the detail.

The parser language actually consists of two languages: one for
defining matchers, and one for defining parsers.  The languages are
intentionally very similar, and are meant to be used together.  Each
sub-language is described below in its own section.

@cindex run-time-loadable option
@cindex option, run-time-loadable
The parser language is a run-time-loadable option; to use it, execute

@example
(load-option '*parser)
@end example
@findex load-option

@noindent
once before compiling any code that uses the language.

@menu
* *Matcher::                    
* *Parser::                     
* Parser-language Macros::      
@end menu

@node *Matcher, *Parser, Parser Language, Parser Language
@subsection *Matcher

@cindex Matcher language
@cindex Matcher procedure
The @dfn{matcher language} is a declarative language for specifying a
@dfn{matcher procedure}.  A matcher procedure is a procedure that
accepts a single parser-buffer argument and returns a boolean value
indicating whether the match it performs was successful.  If the match
succeeds, the internal pointer of the parser buffer is moved forward
over the matched text.  If the match fails, the internal pointer is
unchanged.

For example, here is a matcher procedure that matches the character
@samp{a}:

@example
(lambda (b) (match-parser-buffer-char b #\a))
@end example

@noindent
Here is another example that matches two given characters, @var{c1}
and @var{c2}, in sequence:

@example
@group
(lambda (b)
  (let ((p (get-parser-buffer-pointer b)))
    (if (match-parser-buffer-char b @var{c1})
        (if (match-parser-buffer-char b @var{c2})
            #t
            (begin
              (set-parser-buffer-pointer! b p)
              #f))
        #f)))
@end group
@end example

@noindent
This is code is clear, but has lots of details that get in the way of
understanding what it is doing.  Here is the same example in the
matcher language:

@example
(*matcher (seq (char @var{c1}) (char @var{c2})))
@end example

@noindent
This is much simpler and more intuitive.  And it generates virtually
the same code:

@example
@group
(pp (*matcher (seq (char c1) (char c2))))
@print{} (lambda (#[b1])
@print{}   (let ((#[p1] (get-parser-buffer-pointer #[b1])))
@print{}     (and (match-parser-buffer-char #[b1] c1)
@print{}          (if (match-parser-buffer-char #[b1] c2)
@print{}              #t
@print{}              (begin
@print{}                (set-parser-buffer-pointer! #[b1] #[p1])
@print{}                #f)))))
@end group
@end example

Now that we have seen an example of the language, it's time to look at
the detail.  The @code{*matcher} special form is the interface between
the matcher language and Scheme.

@deffn {special form} *matcher mexp
The operand @var{mexp} is an expression in the matcher language.  The
@code{*matcher} expression expands into Scheme code that implements a
matcher procedure.
@end deffn

Here are the predefined matcher expressions.  New matcher expressions
can be defined using the macro facility (@pxref{Parser-language
Macros}).  We will start with the primitive expressions.

@deffn {matcher expression} char expression
@deffnx {matcher expression} char-ci expression
@deffnx {matcher expression} not-char expression
@deffnx {matcher expression} not-char-ci expression
These expressions match a given character.  In each case, the
@var{expression} operand is a Scheme expression that must evaluate to
a character at run time.  The @samp{-ci} expressions do
case-insensitive matching.  The @samp{not-} expressions match any
character other than the given one.
@end deffn

@deffn {matcher expression} string expression
@deffnx {matcher expression} string-ci expression
These expressions match a given string.  The @var{expression} operand
is a Scheme expression that must evaluate to a string at run time.
The @code{string-ci} expression does case-insensitive matching.
@end deffn

@deffn {matcher expression} char-set expression
These expressions match a single character that is a member of a given
character set.  The @var{expression} operand is a Scheme expression
that must evaluate to a character set at run time.
@end deffn

@deffn {matcher expression} alphabet expression
These expressions match a single character that is a member of a given
Unicode alphabet (@pxref{Unicode}).  The @var{expression} operand is a
Scheme expression that must evaluate to an alphabet at run time.
@end deffn

@deffn {matcher expression} end-of-input
The @code{end-of-input} expression is successful only when there are
no more characters available to be matched.
@end deffn

@deffn {matcher expression} discard-matched
The @code{discard-matched} expression always successfully matches the
null string.  However, it isn't meant to be used as a matching
expression; it is used for its effect.  @code{discard-matched} causes
all of the buffered text prior to this point to be discarded (i.e.@:
it calls @code{discard-parser-buffer-head!} on the parser buffer).

Note that @code{discard-matched} may not be used in certain places in
a matcher expression.  The reason for this is that it deliberately
discards information needed for backtracking, so it may not be used in
a place where subsequent backtracking will need to back over it.  As a
rule of thumb, use @code{discard-matched} only in the last operand of
a @code{seq} or @code{alt} expression (including any @code{seq} or
@code{alt} expressions in which it is indirectly contained).
@end deffn

In addition to the above primitive expressions, there are two
convenient abbreviations.  A character literal (e.g.@: @samp{#\A}) is
a legal primitive expression, and is equivalent to a @code{char}
expression with that literal as its operand (e.g.@: @samp{(char
#\A)}).  Likewise, a string literal is equivalent to a @code{string}
expression (e.g.@: @samp{(string "abc")}).

Next there are several combinator expressions.  These closely
correspond to similar combinators in regular expressions.  Parameters
named @var{mexp} are arbitrary expressions in the matcher language.

@deffn {matcher expression} seq mexp @dots{}
This matches each @var{mexp} operand in sequence.  For example,

@example
@group
(seq (char-set char-set:alphabetic)
     (char-set char-set:numeric))
@end group
@end example

@noindent
matches an alphabetic character followed by a numeric character, such
as @samp{H4}.

Note that if there are no @var{mexp} operands, the @code{seq}
expression successfully matches the null string.
@end deffn

@deffn {matcher expression} alt mexp @dots{}
This attempts to match each @var{mexp} operand in order from left to
right.  The first one that successfully matches becomes the match for
the entire @code{alt} expression.

The @code{alt} expression participates in backtracking.  If one of the
@var{mexp} operands matches, but the overall match in which this
expression is embedded fails, the backtracking mechanism will cause
the @code{alt} expression to try the remaining @var{mexp} operands.
For example, if the expression

@example
(seq (alt "ab" "a") "b")
@end example

@noindent
is matched against the text @samp{abc}, the @code{alt} expression will
initially match its first operand.  But it will then fail to match the
second operand of the @code{seq} expression.  This will cause the
@code{alt} to be restarted, at which time it will match @samp{a}, and
the overall match will succeed.

Note that if there are no @var{mexp} operands, the @code{alt} match
will always fail.
@end deffn

@deffn {matcher expression} * mexp
This matches zero or more occurrences of the @var{mexp} operand.
(Consequently this match always succeeds.)

The @code{*} expression participates in backtracking; if it matches
@var{N} occurrences of @var{mexp}, but the overall match fails, it
will backtrack to @var{N-1} occurrences and continue.  If the overall
match continues to fail, the @code{*} expression will continue to
backtrack until there are no occurrences left.
@end deffn

@deffn {matcher expression} + mexp
This matches one or more occurrences of the @var{mexp} operand.  It is
equivalent to

@example
(seq @var{mexp} (* @var{mexp}))
@end example
@end deffn

@deffn {matcher expression} ? mexp
This matches zero or one occurrences of the @var{mexp} operand.  It is
equivalent to

@example
(alt @var{mexp} (seq))
@end example
@end deffn

@deffn {matcher expression} sexp expression
The @code{sexp} expression allows arbitrary Scheme code to be embedded
inside a matcher.  The @var{expression} operand must evaluate to a
matcher procedure at run time; the procedure is called to match the
parser buffer.  For example,

@example
@group
(*matcher
 (seq "a"
      (sexp parse-foo)
      "b"))
@end group
@end example

@noindent
expands to

@example
@group
(lambda (#[b1])
  (let ((#[p1] (get-parser-buffer-pointer #[b1])))
    (and (match-parser-buffer-char #[b1] #\a)
         (if (parse-foo #[b1])
             (if (match-parser-buffer-char #[b1] #\b)
                 #t
                 (begin
                   (set-parser-buffer-pointer! #[b1] #[p1])
                   #f))
             (begin
               (set-parser-buffer-pointer! #[b1] #[p1])
               #f)))))
@end group
@end example

The case in which @var{expression} is a symbol is so common that it
has an abbreviation: @samp{(sexp @var{symbol})} may be abbreviated as
just @var{symbol}.
@end deffn

@deffn {matcher expression} with-pointer identifier mexp
The @code{with-pointer} expression fetches the parser buffer's
internal pointer (using @code{get-parser-buffer-pointer}), binds it to
@var{identifier}, and then matches the pattern specified by
@var{mexp}.  @var{Identifier} must be a symbol.

This is meant to be used on conjunction with @code{sexp}, as a way to
capture a pointer to a part of the input stream that is outside the
@code{sexp} expression.  An example of the use of @code{with-pointer}
appears above (@pxref{with-pointer example}).
@end deffn

@node *Parser, Parser-language Macros, *Matcher, Parser Language
@subsection *Parser

@cindex Parser language
@cindex Parser procedure
The @dfn{parser language} is a declarative language for specifying a
@dfn{parser procedure}.  A parser procedure is a procedure that
accepts a single parser-buffer argument and parses some of the input
from the buffer.  If the parse is successful, the procedure returns a
vector of objects that are the result of the parse, and the internal
pointer of the parser buffer is advanced past the input that was
parsed.  If the parse fails, the procedure returns @code{#f} and the
internal pointer is unchanged.  This interface is much like that of a
matcher procedure, except that on success the parser procedure returns
a vector of values rather than @code{#t}.

The @code{*parser} special form is the interface between the parser
language and Scheme.

@deffn {special form} *parser pexp
The operand @var{pexp} is an expression in the parser language.  The
@code{*parser} expression expands into Scheme code that implements a
parser procedure.
@end deffn

There are several primitive expressions in the parser language.  The
first two provide a bridge to the matcher language (@pxref{*Matcher}):

@deffn {parser expression} match mexp
The @code{match} expression performs a match on the parser buffer.
The match to be performed is specified by @var{mexp}, which is an
expression in the matcher language.  If the match is successful, the
result of the @code{match} expression is a vector of one element: a
string containing that text.
@end deffn

@deffn {parser expression} noise mexp
The @code{noise} expression performs a match on the parser buffer.
The match to be performed is specified by @var{mexp}, which is an
expression in the matcher language.  If the match is successful, the
result of the @code{noise} expression is a vector of zero elements.
(In other words, the text is matched and then thrown away.)

The @var{mexp} operand is often a known character or string, so in the
case that @var{mexp} is a character or string literal, the
@code{noise} expression can be abbreviated as the literal.  In other
words, @samp{(noise "foo")} can be abbreviated just @samp{"foo"}.
@end deffn

@deffn {parser expression} values expression @dots{}
Sometimes it is useful to be able to insert arbitrary values into the
parser result.  The @code{values} expression supports this.  The
@var{expression} arguments are arbitrary Scheme expressions that are
evaluated at run time and returned in a vector.  The @code{values}
expression always succeeds and never modifies the internal pointer of
the parser buffer.
@end deffn

@deffn {parser expression} discard-matched
The @code{discard-matched} expression always succeeds, returning a
vector of zero elements.  In all other respects it is identical to the
@code{discard-matched} expression in the matcher language.
@end deffn

Next there are several combinator expressions.  Parameters named
@var{pexp} are arbitrary expressions in the parser language.  The
first few combinators are direct equivalents of those in the matcher
language.

@deffn {parser expression} seq pexp @dots{}
The @code{seq} expression parses each of the @var{pexp} operands in
order.  If all of the @var{pexp} operands successfully match, the
result is the concatenation of their values (by @code{vector-append}).
@end deffn

@deffn {parser expression} alt pexp @dots{}
The @code{alt} expression attempts to parse each @var{pexp} operand in
order from left to right.  The first one that successfully parses
produces the result for the entire @code{alt} expression.

Like the @code{alt} expression in the matcher language, this
expression participates in backtracking.
@end deffn

@deffn {parser expression} * pexp
The @code{*} expression parses zero or more occurrences of @var{pexp}.
The results of the parsed occurrences are concatenated together (by
@code{vector-append}) to produce the expression's result.

Like the @code{*} expression in the matcher language, this expression
participates in backtracking.
@end deffn

@deffn {parser expression} + pexp
The @code{*} expression parses one or more occurrences of @var{pexp}.
It is equivalent to

@example
(seq @var{pexp} (* @var{pexp}))
@end example
@end deffn

@deffn {parser expression} ? pexp
The @code{*} expression parses zero or one occurrences of @var{pexp}.
It is equivalent to

@example
(alt @var{pexp} (seq))
@end example
@end deffn

The next three expressions do not have equivalents in the matcher
language.  Each accepts a single @var{pexp} argument, which is parsed
in the usual way.  These expressions perform transformations on the
returned values of a successful match.

@deffn {parser expression} transform expression pexp
The @code{transform} expression performs an arbitrary transformation
of the values returned by parsing @var{pexp}.  @var{Expression} is a
Scheme expression that must evaluate to a procedure at run time.  If
@var{pexp} is successfully parsed, the procedure is called with the
vector of values as its argument, and must return a vector or
@code{#f}.  If it returns a vector, the parse is successful, and those
are the resulting values.  If it returns @code{#f}, the parse fails
and the internal pointer of the parser buffer is returned to what it
was before @var{pexp} was parsed.

For example:

@example
(transform (lambda (v) (if (= 0 (vector-length v)) #f v)) @dots{})
@end example
@end deffn

@deffn {parser expression} encapsulate expression pexp
The @code{encapsulate} expression transforms the values returned by
parsing @var{pexp} into a single value.  @var{Expression} is a Scheme
expression that must evaluate to a procedure at run time.  If
@var{pexp} is successfully parsed, the procedure is called with the
vector of values as its argument, and may return any Scheme object.
The result of the @code{encapsulate} expression is a vector of length
one containing that object.  (And consequently @code{encapsulate}
doesn't change the success or failure of @var{pexp}, only its value.)

For example:

@example
(encapsulate vector->list @dots{})
@end example
@end deffn

@deffn {parser expression} map expression pexp
The @code{map} expression performs a per-element transform on the
values returned by parsing @var{pexp}.  @var{Expression} is a Scheme
expression that must evaluate to a procedure at run time.  If
@var{pexp} is successfully parsed, the procedure is mapped (by
@code{vector-map}) over the values returned from the parse.  The
mapped values are returned as the result of the @code{map} expression.
(And consequently @code{map} doesn't change the success or failure of
@var{pexp}, nor the number of values returned.)

For example:

@example
(map string->symbol @dots{})
@end example
@end deffn

Finally, as in the matcher language, we have @code{sexp} and
@code{with-pointer} to support embedding Scheme code in the parser.

@deffn {parser expression} sexp expression
The @code{sexp} expression allows arbitrary Scheme code to be embedded
inside a parser.  The @var{expression} operand must evaluate to a
parser procedure at run time; the procedure is called to parse the
parser buffer.  This is the parser-language equivalent of the
@code{sexp} expression in the matcher language.

The case in which @var{expression} is a symbol is so common that it
has an abbreviation: @samp{(sexp @var{symbol})} may be abbreviated as
just @var{symbol}.
@end deffn

@deffn {parser expression} with-pointer identifier pexp
The @code{with-pointer} expression fetches the parser buffer's
internal pointer (using @code{get-parser-buffer-pointer}), binds it to
@var{identifier}, and then parses the pattern specified by @var{pexp}.
@var{Identifier} must be a symbol.  This is the parser-language
equivalent of the @code{with-pointer} expression in the matcher
language.
@end deffn

@node Parser-language Macros,  , *Parser, Parser Language
@subsection Parser-language Macros

The parser and matcher languages provide a macro facility so that
common patterns can be abstracted.  The macro facility allows new
expression types to be independently defined in the two languages.
The macros are defined in heirarchically organized tables, so that
different applications can have private macro bindings.

@deffn {special form} define-*matcher-macro formals expression
@deffnx {special form} define-*parser-macro formals expression
These special forms are used to define macros in the matcher and
parser language, respectively.  @var{Formals} is like the
@var{formals} list of a @code{define} special form, and
@var{expression} is a Scheme expression.

If @var{formals} is a list (or improper list) of symbols, the first
symbol in the list is the name of the macro, and the remaining symbols
are interpreted as the @var{formals} of a lambda expression.  A lambda
expression is formed by combining the latter @var{formals} with the
@var{expression}, and this lambda expression, when evaluated, becomes
the @dfn{expander}.  The defined macro accepts the same number of
operands as the expander.  A macro instance is expanded by applying
the expander to the list of operands; the result of the application is
interpreted as a replacement expression for the macro instance.

If @var{formals} is a symbol, it is the name of the macro.  In this
case, the expander is a procedure of no arguments whose body is
@var{expression}.  When the @var{formals} symbol appears by itself as
an expression in the language, the expander is called with no
arguments, and the result is interpreted as a replacement expression
for the symbol.
@end deffn

@deffn procedure define-*matcher-expander identifier expander
@deffnx procedure define-*parser-expander identifier expander
These procedures provide a procedural interface to the
macro-definition mechanism.  @var{Identifier} must be a symbol, and
@var{expander} must be an expander procedure, as defined above.
Instances of the @code{define-*matcher-macro} and
@code{define-*parser-macro} special forms expand into calls to these
procedures.
@end deffn

The remaining procedures define the interface to the parser-macros
table abstraction.  Each parser-macro table has a separate binding
space for macros in the matcher and parser languages.  However, the
table inherits bindings from one specified table; it's not possible to
inherit matcher-language bindings from one table and parser-language
bindings from another.

@deffn procedure make-parser-macros parent-table
Create and return a new parser-macro table that inherits from
@var{parent-table}.  @var{Parent-table} must be either a parser-macro
table, or @code{#f}; usually it is specified as the value of
@code{global-parser-macros}.
@end deffn

@deffn procedure parser-macros? object
This is a predicate for parser-macro tables.
@end deffn

@deffn procedure global-parser-macros
Return the global parser-macro table.  This table is predefined and
contains all of the bindings documented here.
@end deffn

There is a ``current'' table at all times, and macro definitions are
always placed in this table.  By default, the current table is the
global macro table, but the following procedures allow this to be
changed.

@deffn procedure current-parser-macros
Return the current parser-macro table.
@end deffn

@deffn procedure set-current-parser-macros! table
Change the current parser-macro table to @var{table}, which must
satisfy @code{parser-macros?}.
@end deffn

@deffn procedure with-current-parser-macros table thunk
Bind the current parser-macro table to @var{table}, call @var{thunk}
with no arguments, then restore the original table binding.  The value
returned by @var{thunk} is the returned as the value of this
procedure.  @var{Table} must satisfy @code{parser-macros?}, and
@var{thunk} must be a procedure of no arguments.
@end deffn


@node XML Support,  , Parser Language, Input/Output
@section XML Support

MIT/GNU Scheme provides a simple non-validating @acronym{XML} parser.
This parser is believed to be conformant with @acronym{XML} 1.0.  It
passes all of the tests in the "xmltest" directory of the @acronym{XML}
conformance tests (dated 2001-03-15).  The parser supports @acronym{XML}
namespaces; it doesn't support external document type declarations
(@acronym{DTD}s), and it doesn't yet support @acronym{XML} 1.1.  The
output of the parser is a record tree that closely reflects the
structure of the @acronym{XML} document.

MIT/GNU Scheme also provides support for writing an @acronym{XML} record
tree to an output port.  There is no guarantee that parsing an
@acronym{XML} document and writing it back out will make a verbatim copy
of the document.  The output will be semantically identical but may have
small syntactic differences.  For example, entities are substituted
during the parsing process.

The purpose of the @acronym{XML} support is to provide a mechanism for
reading and writing simple @acronym{XML} documents.  In the future
this support may be further developed to support a standard interface
such as @acronym{DOM} or @acronym{SAX}.

@cindex run-time-loadable option
@cindex option, run-time-loadable
The @acronym{XML} support is a run-time-loadable option; to use it,
execute

@example
(load-option 'xml)
@end example
@findex load-option

@noindent
once before running any code that uses it.

@menu
* XML Input::                   
* XML Output::                  
* XML Names::                   
* XML Structure::               
@end menu


@node XML Input, XML Output, XML Support, XML Support
@subsection XML Input

@cindex XML parser
@cindex parser, XML
@cindex XML input
@cindex input, XML
The primary entry point for the @acronym{XML} parser is @code{read-xml},
which reads characters from a port and returns an @acronym{XML} document
record.  The character coding of the input is determined by reading some
of the input stream and looking for a byte order mark and/or an encoding
in the @acronym{XML} declaration.  We support all @acronym{ISO} 8859
codings, as well as @acronym{UTF-8}, @acronym{UTF-16}, and
@acronym{UTF-32}.

When an @acronym{XHTML} document is read, the parser provides entity
definitions for all of the named @acronym{XHTML} characters; for
example, it defines @samp{&nbsp;} and @samp{&copy;}.  In order for a
document to be recognized as @acronym{XHTML}, it must contain an
@acronym{XHTML} @acronym{DTD}, such as this:

@example
@group
<!DOCTYPE html
          PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
          "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
@end group
@end example

@noindent
At present the parser recognizes @acronym{XHTML} Strict 1.0 and
@acronym{XHTML} 1.1 documents.

@deffn procedure read-xml port [pi-handlers]
Read an @acronym{XML} document from @var{port} and return the
corresponding @acronym{XML} document record.

@var{Pi-handlers}, if specified, must be an association list.  Each
element of @var{pi-handlers} must be a list of two elements: a symbol
and a procedure.  When the parser encounters processing instructions
with a name that appears in @var{pi-handlers}, the procedure is called
with one argument, which is the text of the processing instructions.
The procedure must return a list of @acronym{XML} structure records that
are legal for the context of the processing instructions.
@end deffn

@deffn procedure read-xml-file pathname [pi-handlers]
This convenience procedure simplifies reading @acronym{XML} from a file.
It is roughly equivalent to

@example
@group
(define (read-xml-file pathname #!optional pi-handlers)
  (call-with-input-file pathname
    (lambda (port)
      (read-xml port pi-handlers))))
@end group
@end example
@end deffn

@deffn procedure string->xml string [start [end [pi-handlers]]]
This convenience procedure simplifies reading @acronym{XML} from a
string.  The @var{string} argument may be a string or a wide string.
It is roughly equivalent to

@example
@group
(define (string->xml string #!optional start end pi-handlers)
  (read-xml (open-input-string string start end)
            pi-handlers))
@end group
@end example
@end deffn


@node XML Output, XML Names, XML Input, XML Support
@subsection XML Output

@cindex XML output
@cindex output, XML
The following procedures serialize @acronym{XML} document records into
character sequences.  All are virtually identical except for the way
that the character sequence is represented.

Each procedure will accept either an @code{xml-document} record or any
of the other @acronym{XML} record types.  This makes it possible to
write fragments of @acronym{XML} documents, although you should keep in
mind that such fragments aren't documents and won't generally be
accepted by any @acronym{XML} parser.

If the @var{xml} being written is an @code{xml-document} record, the
procedures @code{write-xml} and @code{write-xml-file} will look for a
contained @code{xml-declaration} record and its @code{encoding}
attribute.  If the @code{encoding} is a supported value, the output will
be encoded as specified; otherwise it will be encoded as
@acronym{UTF-8}.

When an @acronym{XHTML} document record is written, named
@acronym{XHTML} characters are translated into their corresponding
entities.  For example, the character @samp{#\U+00A0} is written as
@samp{&nbsp;}.  In order for an @acronym{XML} document record to be
recognized as @acronym{XHTML}, it must have a @acronym{DTD} record that
satisfies the predicate @code{html-dtd?}.

@deffn procedure write-xml xml port
Write @var{xml} to @var{port}.  Note that character encoding will only
be done if @var{port} supports it.
@end deffn

@deffn procedure write-xml-file xml pathname
Write @var{xml} to the file specified by @var{pathname}.  Roughly
equivalent to

@example
@group
(define (write-xml-file xml pathname)
  (call-with-output-file pathname
    (lambda (port)
      (write-xml xml port))))
@end group
@end example
@end deffn

@deffn procedure xml->wide-string xml
Convert @var{xml} to a wide string.  No character encoding is used,
since wide strings can represent all characters without encoding.
Roughly equivalent to

@example
@group
(define (xml->wide-string xml)
  (call-with-wide-output-string
   (lambda (port)
     (write-xml xml port))))
@end group
@end example
@end deffn

@deffn procedure xml->string xml
Convert @var{xml} to a character string encoded as @acronym{UTF-8}.
Roughly equivalent to

@example
@group
(define (xml->string xml)
  (wide-string->utf8-string (xml->wide-string xml)))
@end group
@end example
@end deffn


@node XML Names, XML Structure, XML Output, XML Support
@subsection XML Names

@cindex XML names
@cindex names, XML
MIT/GNU Scheme implements @acronym{XML} names in a slightly complex way.
Unfortunately, this complexity is a direct consequence of the definition
of @acronym{XML} names rather than a mis-feature of this implementation.

The reason that @acronym{XML} names are complex is that @acronym{XML}
namespace support, which was added after @acronym{XML} was standardized,
is not very well integrated with the core @acronym{XML} definition.  The
most obvious problem is that names can't have associated namespaces when
they appear in the @acronym{DTD} of a document, even if the body of the
document uses them.  Consequently, it must be possible to compare
non-associated names with associated names.

@cindex Uniform Resource Identifier
@cindex URI, of XML name
@cindex qname, of XML name
An @acronym{XML} name consists of two parts: the @dfn{qname}, which is a
symbol, possibly including a namespace prefix; and the
@dfn{Uniform Resource Identifier} (@acronym{URI}), which
identifies an optional namespace.

@deffn procedure make-xml-name qname uri
Creates and returns an @acronym{XML} name.  @var{Qname} must be a symbol
whose name satisfies @code{string-is-xml-name?}.  @var{Uri} must satisfy
either @code{absolute-uri?} or @code{null-xml-namespace-uri?}.  The
returned value is an @acronym{XML} name that satisfies @code{xml-name?}.

If @var{uri} is the @dfn{null} namespace (satisfies
@code{null-xml-namespace-uri?}), the returned value is a symbol
equivalent to @var{qname}.  This means that an ordinary symbol can be
used as an @acronym{XML} name when there is no namespace associated with
the name.

For convenience, @var{qname} may be a string, in which case it is
converted to a symbol using @code{make-xml-qname}.

For convenience, @var{uri} may be any object that @code{->uri} is able
to convert to a @acronym{URI} record, provided the resulting
@acronym{URI} meets the above restrictions.
@end deffn

@deffn procedure xml-name? object
Returns @code{#t} if @var{object} is an @acronym{XML} name, and
@code{#f} otherwise.
@end deffn

@deffn procedure xml-name-qname xml-name
Returns the @dfn{qname} of @var{xml-name} as a symbol.
@end deffn

@deffn procedure xml-name-uri xml-name
Returns the @dfn{URI} of @var{xml-name}.  The result always satisfies
@code{absolute-uri?} or @code{null-xml-namespace-uri?}.
@end deffn

@deffn procedure xml-name-string xml-name
Returns the @dfn{qname} of @var{xml-name} as a string.  Equivalent to

@example
(symbol-name (xml-name-qname @var{xml-name}))
@end example
@end deffn

@cindex prefix, of XML name
@cindex local part, of XML name
The next two procedures get the @dfn{prefix} and @dfn{local part} of an
@acronym{XML} name, respectively.  The prefix of an @acronym{XML} name
is the part of the qname to the left of the colon, while the local part
is the part of the qname to the right of the colon.  If there is no
colon in the qname, the local part is the entire qname, and the prefix
is the null symbol (i.e.@: @samp{||}).

@deffn procedure xml-name-prefix xml-name
Returns the @dfn{prefix} of @var{xml-name} as a symbol.
@end deffn

@deffn procedure xml-name-local xml-name
Returns the @dfn{local part} of @var{xml-name} as a symbol.
@end deffn

@cindex comparison, of XML names
@cindex equality, of XML names
The next procedure compares two @acronym{XML} names for equality.  The
rules for equality are slightly complex, in order to permit comparing
names in the @acronym{DTD} with names in the document body.  So, if both
of the names have non-null namespace @acronym{URI}s, then the names are
equal if and only if their local parts are equal and their
@acronym{URI}s are equal.  (The prefixes of the names are not considered
in this case.)  Otherwise, the names are equal if and only if their
qnames are equal.

@deffn procedure xml-name=? xml-name-1 xml-name-2
Returns @code{#t} if @var{xml-name-1} and @var{xml-name-2} are the same
name, and @code{#f} otherwise.
@end deffn

These next procedures define the data abstraction for qnames.  While
qnames are represented as symbols, only symbols whose names satisfy
@code{string-is-xml-name?} are qnames.

@deffn procedure make-xml-qname string
@var{String} must satisfy @code{string-is-xml-name?}.  Returns the qname
corresponding to @var{string} (the symbol whose name is @var{string}).
@end deffn

@deffn procedure xml-qname? object
Returns @code{#t} if @var{object} is a qname, otherwise returns
@code{#f}.
@end deffn

@deffn procedure xml-qname-string qname
Returns a newly allocated string that is a copy of @var{qname}'s string.
Roughly equivalent to @code{symbol->utf8-string}.
@end deffn

@deffn procedure xml-qname-prefix qname
Returns the prefix of @var{qname} as a symbol.
@end deffn

@deffn procedure xml-qname-local qname
Returns the local part of @var{qname} as a symbol.
@end deffn

The prefix of a qname or @acronym{XML} name may be absent if there is no
colon in the name.  The absent, or null, prefix is abstracted by the
next two procedures.  Note that the null prefix is a symbol, just like
non-null prefixes.

@deffn procedure null-xml-name-prefix
Returns the null prefix.
@end deffn

@deffn procedure null-xml-name-prefix? object
Returns @code{#t} if @var{object} is the null prefix, otherwise returns
@code{#f}.
@end deffn

The namespace @acronym{URI} of an @acronym{XML} name may be null,
meaning that there is no namespace associated with the name.  This
namespace is represented by a relative @acronym{URI} record whose string
representation is the null string.

@deffn procedure null-xml-namespace-uri
Returns the null namespace @acronym{URI} record.
@end deffn

@deffn procedure null-xml-namespace-uri? object
Returns @code{#t} if @var{object} is the null namespace @acronym{URI}
record, otherwise returns @code{#f}.
@end deffn

The following values are two distinguished @acronym{URI} records.

@defvr variable xml-uri
@code{xml-uri} is the @acronym{URI} reserved for use by the
@acronym{XML} recommendation.  This @acronym{URI} must be used with the
@samp{xml} prefix.
@end defvr

@defvr variable xmlns-uri
@code{xmlns-uri} is the @acronym{URI} reserved for use by the
@acronym{XML} namespace recommendation.  This @acronym{URI} must be used
with the @samp{xmlns} prefix.
@end defvr


@deffn procedure make-xml-nmtoken string
@end deffn

@deffn procedure xml-nmtoken? object
@end deffn

@deffn procedure xml-nmtoken-string xml-nmtoken
@end deffn


@deffn procedure string-is-xml-name? string
@end deffn

@deffn procedure string-is-xml-nmtoken? string
@end deffn


@deffn procedure make-xml-name-hash-table [initial-size]
@end deffn

@deffn procedure xml-name-hash xml-name modulus
@end deffn


@node XML Structure,  , XML Names, XML Support
@subsection XML Structure

The output from the @acronym{XML} parser and the input to the
@acronym{XML} output procedure is a complex data structure composed of
a heirarchy of typed components.  Each component is a record whose
fields correspond to parts of the @acronym{XML} structure that the
record represents.  There are no special operations on these records;
each is a tuple with named subparts.  The root record type is
@code{xml-document}, which represents a complete @acronym{XML}
document.

Each record type @var{type} has the following associated bindings:

@table @code
@item <@var{type}>
is a variable bound to the record-type descriptor for @var{type}.  The
record-type descriptor may be used as a specializer in @acronym{SOS}
method definitions, which greatly simplifies code to dispatch on these
types.

@item @var{type}?
is a predicate for records of type @var{type}.  It accepts one
argument, which can be any object, and returns @code{#t} if the object
is a record of this type, or @code{#f} otherwise.

@item make-@var{type}
is a constructor for records of type @var{type}.  It accepts one
argument for each field of @var{type}, in the same order that they are
written in the type description, and returns a newly-allocated record
of that type.

@item @var{type}-@var{field}
is an accessor procedure for the field @var{field} in records of type
@var{type}.  It accepts one argument, which must be a record of that
type, and returns the contents of the corresponding field in the
record.

@item set-@var{type}-@var{field}!
is a modifier procedure for the field @var{field} in records of type
@var{type}.  It accepts two arguments: the first must be a record of
that type, and the second is a new value for the corresponding field.
The record's field is modified to have the new value.
@end table

@deftp {record type} xml-document declaration misc-1 dtd misc-2 root misc-3
@vindex <xml-document>
@findex xml-document?
@findex make-xml-document
@findex xml-document-declaration
@findex xml-document-misc-1
@findex xml-document-dtd
@findex xml-document-misc-2
@findex xml-document-root
@findex xml-document-misc-3
@findex set-xml-document-declaration!
@findex set-xml-document-misc-1!
@findex set-xml-document-dtd!
@findex set-xml-document-misc-2!
@findex set-xml-document-root!
@findex set-xml-document-misc-3!
The @code{xml-document} record is the top-level record representing a
complete @acronym{XML} document.  @var{Declaration} is either an
@code{xml-declaration} object or @code{#f}.  @var{Dtd} is either an
@code{xml-dtd} object or @code{#f}.  @var{Root} is an @code{xml-element}
object.  @var{Misc-1}, @var{misc-2}, and @var{misc-3} are lists of
miscellaneous items; a miscellaneous item is either an
@code{xml-comment} object, an @code{xml-processing-instructions} object,
or a string of whitespace.
@end deftp

@deftp {record type} xml-declaration version encoding standalone
@vindex <xml-declaration>
@findex xml-declaration?
@findex make-xml-declaration
@findex xml-declaration-version
@findex xml-declaration-encoding
@findex xml-declaration-standalone
@findex set-xml-declaration-version!
@findex set-xml-declaration-encoding!
@findex set-xml-declaration-standalone!
The @code{xml-declaration} record represents the @samp{<?xml @dots{}
?>} declaration that optionally appears at the beginning of an
@acronym{XML} document.  @var{Version} is a version string, typically
@code{"1.0"}.  @var{Encoding} is either an encoding string or
@code{#f}.  @var{Standalone} is either @code{"yes"}, @code{"no"}, or
@code{#f}.
@end deftp

@deftp {record type} xml-element name attributes contents
@vindex <xml-element>
@findex xml-element?
@findex make-xml-element
@findex xml-element-name
@findex xml-element-attributes
@findex xml-element-content
@findex set-xml-element-name!
@findex set-xml-element-attributes!
@findex set-xml-element-content!
The @code{xml-element} record represents general @acronym{XML} elements;
the bulk of a typical @acronym{XML} document consists of these elements.
@var{Name} is the element name (an @acronym{XML} name).
@var{Attributes} is a list of @acronym{XML} attribute objects.
@var{Contents} is a list of the contents of the element.  Each element
of this list is either a string, an @code{xml-element} record or an
@code{xml-processing-instructions} record.
@end deftp

@deftp {record type} xml-processing-instructions name text
@vindex <xml-processing-instructions>
@findex xml-processing-instructions?
@findex make-xml-processing-instructions
@findex xml-processing-instructions-name
@findex xml-processing-instructions-text
@findex set-xml-processing-instructions-name!
@findex set-xml-processing-instructions-text!
The @code{xml-processing-instructions} record represents processing
instructions, which have the form @samp{<?@var{name} @dots{} ?>}.
These instructions are intended to contain non-@acronym{XML} data that
will be processed by another interpreter; for example they might
contain @acronym{PHP} programs.  The @var{name} field is the processor
name (a symbol), and the @var{text} field is the body of the
instructions (a string).
@end deftp

@deftp {record type} xml-dtd root external internal
@vindex <xml-dtd>
@findex xml-dtd?
@findex make-xml-dtd
@findex xml-dtd-root
@findex xml-dtd-external
@findex xml-dtd-internal
@findex set-xml-dtd-root!
@findex set-xml-dtd-external!
@findex set-xml-dtd-internal!
The @code{xml-dtd} record represents a document type declaration.  The
@var{root} field is an @acronym{XML} name for the root element of the
document.  @var{External} is either an @code{xml-external-id} record
or @code{#f}.  @var{Internal} is a list of @acronym{DTD} element
records (e.g.@: @code{xml-!element}, @code{xml-!attlist}, etc.).
@end deftp

The remaining record types are valid only within a @acronym{DTD}.

@deftp {record type} xml-!element name content-type
@vindex <xml-!element>
@findex xml-!element?
@findex make-xml-!element
@findex xml-!element-name
@findex xml-!element-content-type
@findex set-xml-!element-name!
@findex set-xml-!element-content-type!
The @code{xml-!element} record represents an element-type
declaration.  @var{Name} is the @acronym{XML} name of the type being
declared (a symbol).  @var{Content-type} describes the type and can
have several different values, as follows:

@itemize @bullet
@item
The @acronym{XML} names @samp{EMPTY} and @samp{ANY} correspond to the
@acronym{XML} keywords of the same name.

@item
A list @samp{(MIX @var{type} @dots{})} corresponds to the
@samp{(#PCDATA | @var{type} | @dots{})} syntax.
@end itemize
@end deftp

@deftp {record type} xml-!attlist name definitions
@vindex <xml-!attlist>
@findex xml-!attlist?
@findex make-xml-!attlist
@findex xml-!attlist-name
@findex xml-!attlist-definitions
@findex set-xml-!attlist-name!
@findex set-xml-!attlist-definitions!
The @code{xml-!attlist} record represents an attribute-list
declaration.  @var{Name} is the @acronym{XML} name of the type for
which attributes are being declared (a symbol).  @var{Definitions} is
a list of attribute definitions, each of which is a list of three
elements @code{(@var{name} @var{type} @var{default})}.  @var{Name} is
an @acronym{XML} name for the name of the attribute (a symbol).
@var{Type} describes the attribute type, and can have one of the
following values:

@itemize @bullet
@item
The @acronym{XML} names @samp{CDATA}, @samp{IDREFS}, @samp{IDREF},
@samp{ID}, @samp{ENTITY}, @samp{ENTITIES}, @samp{NMTOKENS}, and
@samp{NMTOKEN} correspond to the @acronym{XML} keywords of the same
names.

@item
A list @samp{(NOTATION @var{name1} @var{name2} @dots{})} corresponds
to the @samp{NOTATION (@var{name1} | @var{name2} @dots{})} syntax.

@item
A list @samp{(ENUMERATED @var{name1} @var{name2} @dots{})} corresponds
to the @samp{(@var{name1} | @var{name2} @dots{})} syntax.
@end itemize

@var{Default} describes the default value for the attribute, and can
have one of the following values:

@itemize @bullet
@item
The @acronym{XML} names @samp{#REQUIRED} and @samp{#IMPLIED}
correspond to the @acronym{XML} keywords of the same names.

@item
A list @samp{(#FIXED @var{value})} corresponds to the @samp{#FIXED
"@var{value}"} syntax.  @var{Value} is represented as a string.

@item
A list @samp{(DEFAULT @var{value})} corresponds to the
@samp{"@var{value}"} syntax.  @var{Value} is represented as a string.
@end itemize
@end deftp

@deftp {record type} xml-!entity name value
@vindex <xml-!entity>
@findex xml-!entity?
@findex make-xml-!entity
@findex xml-!entity-name
@findex xml-!entity-value
@findex set-xml-!entity-name!
@findex set-xml-!entity-value!
The @code{xml-!entity} record represents a general entity declaration.
@var{Name} is an @acronym{XML} name for the entity.  @var{Value} is the
entity's value, either a string or an @code{xml-external-id} record.
@end deftp

@deftp {record type} xml-parameter-!entity name value
@vindex <xml-parameter-!entity>
@findex xml-parameter-!entity?
@findex make-xml-parameter-!entity
@findex xml-parameter-!entity-name
@findex xml-parameter-!entity-value
@findex set-xml-parameter-!entity-name!
@findex set-xml-parameter-!entity-value!
The @code{xml-parameter-!entity} record represents a parameter entity
declaration.  @var{Name} is an @acronym{XML} name for the entity.
@var{Value} is the entity's value, either a string or an
@code{xml-external-id} record.
@end deftp

@deftp {record type} xml-unparsed-!entity name id notation
@vindex <xml-unparsed-!entity>
@findex xml-unparsed-!entity?
@findex make-xml-unparsed-!entity
@findex xml-unparsed-!entity-name
@findex xml-unparsed-!entity-id
@findex xml-unparsed-!entity-notation
@findex set-xml-unparsed-!entity-name!
@findex set-xml-unparsed-!entity-id!
@findex set-xml-unparsed-!entity-notation!
The @code{xml-unparsed-!entity} record represents an unparsed entity
declaration.  @code{Name} is an @acronym{XML} name for the entity.
@var{Id} is an @code{xml-external-id} record.  @var{Notation} is an
@acronym{XML} name for the notation.
@end deftp

@deftp {record type} xml-!notation name id
@vindex <xml-!notation>
@findex xml-!notation?
@findex make-xml-!notation
@findex xml-!notation-name
@findex xml-!notation-id
@findex set-xml-!notation-name!
@findex set-xml-!notation-id!
The @code{xml-!notation} record represents a notation declaration.
@code{Name} is an @acronym{XML} name for the notation.  @var{Id} is an
@code{xml-external-id} record.
@end deftp

@deftp {record type} xml-external-id id uri
@vindex <xml-external-id>
@findex xml-external-id?
@findex make-xml-external-id
@findex xml-external-id-id
@findex xml-external-id-uri
@findex set-xml-external-id-id!
@findex set-xml-external-id-uri!
The @code{xml-external-id} record is a reference to an external
@acronym{DTD}.  This reference consists of two parts: @var{id} is a
public @acronym{ID} literal, corresponding to the @samp{PUBLIC}
keyword, while @var{uri} is a system literal, corresponding to the
@samp{SYSTEM} keyword.  Either or both may be present, depending on
the context.  @var{Id} is represented as a string, while @var{uri} is
represented as a @acronym{URI} record.
@end deftp
