csug/objects.stex

% Copyright 2005-2016 Cisco Systems, Inc.
% 
% Licensed under the Apache License, Version 2.0 (the "License");
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\chapter{Operations on Objects\label{CHPTOBJECTS}}

This chapter describes operations specific to {\ChezScheme} on
nonnumeric objects, including standard objects such as pairs and
numbers and {\ChezScheme} extensions such as boxes and records.
Chapter~\ref{CHPTNUMERIC} describes operations on numbers.
See Chapter~\ref{TSPL:CHPTOBJECTS} of {\TSPLFOUR} or the Revised$^6$ Report
on Scheme for a description of standard operations on objects.

\section{Missing R6RS Type Predicates\label{SECTMISSINGR6RSTYPEPREDS}}

%----------------------------------------------------------------------------
\noskipentryheader
\formdef{enum-set?}{\categoryprocedure}{(enum-set? \var{obj})}
\returns \scheme{#t} if \var{obj} is an enum set, \scheme{#f} otherwise
\listlibraries
\endnoskipentryheader

This predicate is not defined by the Revised$^6$ Report, but should be.

%----------------------------------------------------------------------------
\entryheader
\formdef{record-constructor-descriptor?}{\categoryprocedure}{(record-constructor-descriptor? \var{obj})}
\returns \scheme{#t} if \var{obj} is a record constructor descriptor, \scheme{#f} otherwise
\listlibraries
\endentryheader

This predicate is not defined by the Revised$^6$ Report, but should be.


\section{Pairs and Lists}

%----------------------------------------------------------------------------
\noskipentryheader
\formdef{atom?}{\categoryprocedure}{(atom? \var{obj})}
\returns \scheme{#t} if \var{obj} is not a pair, \scheme{#f} otherwise
\listlibraries
\endnoskipentryheader

\noindent
\scheme{atom?} is equivalent to \scheme{(lambda (x) (not (pair? x)))}.

\schemedisplay
(atom? '(a b c)) ;=> #f
(atom? '(3 . 4)) ;=> #f
(atom? '()) ;=> #t
(atom? 3) ;=> #t
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{list-head}{\categoryprocedure}{(list-head \var{list} \var{n})}
\returns a list of the first \var{n} elements of \var{list}
\listlibraries
\endentryheader

\noindent
\var{n} must be an exact nonnegative integer less than or equal to
the length of \var{list}.

\scheme{list-head} and the standard Scheme procedure \scheme{list-tail} 
may be used together to split a list into two separate lists.
While \scheme{list-tail} performs no allocation but instead returns a
sublist of the original list, \scheme{list-head} always returns a copy
of the first portion of the list.

\scheme{list-head} may be defined as follows.

\schemedisplay
(define list-head
  (lambda (ls n)
    (if (= n 0)
        '()
        (cons (car ls) (list-head (cdr ls) (- n 1))))))

(list-head '(a b c) 0) ;=> ()
(list-head '(a b c) 2) ;=> (a b)
(list-head '(a b c) 3) ;=> (a b c)
(list-head '(a b c . d) 2) ;=> (a b)
(list-head '(a b c . d) 3) ;=> (a b c)
(list-head '#1=(a . #1#) 5) ;=> (a a a a a)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{last-pair}{\categoryprocedure}{(last-pair \var{list})}
\returns the last pair of a \var{list}
\listlibraries
\endentryheader

\noindent
\var{list} must not be empty.
\scheme{last-pair} returns the last pair (not the last element) of \var{list}.
\var{list} may be an improper list, in which case the last pair is the
pair containing the last element and the terminating object.

\schemedisplay
(last-pair '(a b c d)) ;=> (d)
(last-pair '(a b c . d)) ;=> (c . d)
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{list-copy}{\categoryprocedure}{(list-copy \var{list})}
\returns a copy of \var{list}
\listlibraries
\endentryheader

\noindent
\scheme{list-copy} returns a list \scheme{equal?} to \var{list}, using new pairs
to reform the top-level list structure.

\schemedisplay
(list-copy '(a b c)) ;=> (a b c)

(let ([ls '(a b c)])
  (equal? ls (list-copy ls))) ;=> #t

(let ([ls '(a b c)])
  (let ([ls-copy (list-copy ls)])
    (or (eq? ls-copy ls)
        (eq? (cdr ls-copy) (cdr ls))
        (eq? (cddr ls-copy) (cddr ls))))) ;=> #f
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{list*}{\categoryprocedure}{(list* \var{obj} \dots \var{final-obj})}
\returns a list of \scheme{\var{obj} \dots} terminated by \var{final-obj}
\listlibraries
\endentryheader

\noindent
\scheme{list*} is identical to the Revised$^6$ Report \scheme{cons*}.


%----------------------------------------------------------------------------
\entryheader
\formdef{make-list}{\categoryprocedure}{(make-list \var{n})}
\formdef{make-list}{\categoryprocedure}{(make-list \var{n} \var{obj})}
\returns a list of \var{n} \var{objs}
\listlibraries
\endentryheader

\noindent
\var{n} must be a nonnegative integer.
If \var{obj} is omitted, the elements of the list are unspecified.

\schemedisplay
(make-list 0 '()) ;=> ()
(make-list 3 0) ;=> (0 0 0)
(make-list 2 "hi") ;=> ("hi" "hi")
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{iota}{\categoryprocedure}{(iota \var{n})}
\returns a list of integers from 0 (inclusive) to \var{n} (exclusive)
\listlibraries
\endentryheader

\var{n} must be an exact nonnegative integer.

\schemedisplay
(iota 0) ;=> ()
(iota 5) ;=> (0 1 2 3 4)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{enumerate}{\categoryprocedure}{(enumerate \var{ls})}
\returns a list of integers from 0 (inclusive) to the length of \var{ls} (exclusive)
\listlibraries
\endentryheader

\schemedisplay
(enumerate '()) ;=> ()
(enumerate '(a b c)) ;=> (0 1 2)
(let ([ls '(a b c)])
  (map cons ls (enumerate ls))) ;=> ((a . 0) (b . 1) (c . 2))
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{remq!}{\categoryprocedure}{(remq! \var{obj} \var{list})}
\formdef{remv!}{\categoryprocedure}{(remv! \var{obj} \var{list})}
\formdef{remove!}{\categoryprocedure}{(remove! \var{obj} \var{list})}
\returns a list containing the elements of \var{list} with all occurrences of \var{obj} removed
\listlibraries
\endentryheader

\noindent
These procedures are similar to the Revised$^6$ Report
\scheme{remq}, \scheme{remv}, and \scheme{remove} procedures, except
\scheme{remq!}, \scheme{remv!} and \scheme{remove!} use pairs from the
input list to build the output list.
They perform less allocation but are not
necessarily faster than their nondestructive counterparts.
Their use can easily lead to confusing or incorrect results if used
indiscriminately.

\schemedisplay
(remq! 'a '(a b a c a d)) ;=> (b c d)

(remv! #\a '(#\a #\b #\c)) ;=> (#\b #\c)

(remove! '(c) '((a) (b) (c))) ;=> ((a) (b))
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{substq}{\categoryprocedure}{(substq \var{new} \var{old} \var{tree})}
\formdef{substv}{\categoryprocedure}{(substv \var{new} \var{old} \var{tree})}
\formdef{subst}{\categoryprocedure}{(subst \var{new} \var{old} \var{tree})}
\formdef{substq!}{\categoryprocedure}{(substq! \var{new} \var{old} \var{tree})}
\formdef{substv!}{\categoryprocedure}{(substv! \var{new} \var{old} \var{tree})}
\formdef{subst!}{\categoryprocedure}{(subst! \var{new} \var{old} \var{tree})}
\returns a tree with \var{new} substituted for occurrences of \var{old} in \var{tree}
\listlibraries
\endentryheader

\noindent
These procedures traverse \var{tree}, replacing all objects equivalent to
the object \var{old} with the object \var{new}.

The equivalence test for \scheme{substq} and \scheme{substq!} is \scheme{eq?},
for \scheme{substv} and \scheme{substv!} is \scheme{eqv?},
and for \scheme{subst} and \scheme{subst!} is \scheme{equal?}.

\scheme{substq!}, \scheme{substv!}, and \scheme{subst!} perform the
substitutions destructively.
They perform less allocation but are not
necessarily faster than their nondestructive counterparts.
Their use can easily lead to confusing or incorrect results if used
indiscriminately.


\schemedisplay
(substq 'a 'b '((b c) b a)) ;=> ((a c) a a)

(substv 2 1 '((1 . 2) (1 . 4) . 1)) ;=> ((2 . 2) (2 . 4) . 2)

(subst 'a
       '(a . b)
       '((a . b) (c a . b) . c)) ;=> (a (c . a) . c)

(let ([tr '((b c) b a)])
  (substq! 'a 'b tr)
  tr) ;=> ((a c) a a)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{reverse!}{\categoryprocedure}{(reverse! \var{list})}
\returns a list containing the elements of \var{list} in reverse order
\listlibraries
\endentryheader

\noindent
\scheme{reverse!} destructively reverses \var{list}
by reversing its links.
Using \scheme{reverse!} in place of \scheme{reverse} reduces allocation but is not
necessarily faster than \scheme{reverse}.
Its use can easily lead to confusing or incorrect results if used
indiscriminately.

\schemedisplay
(reverse! '()) ;=> ()
(reverse! '(a b c)) ;=> (c b a)

(let ([x '(a b c)])
    (reverse! x)
    x) ;=> (a)

(let ([x '(a b c)])
    (set! x (reverse! x))
    x) ;=> (c b a)
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{append!}{\categoryprocedure}{(append! \var{list} \dots)}
\returns the concatenation of the input lists
\listlibraries
\endentryheader

\noindent
Like \scheme{append},
\scheme{append!} returns a new list consisting of the elements of the first
list followed by the elements of the second list, the elements of the
third list, and so on.
Unlike \scheme{append},
\scheme{append!} reuses the pairs in all of the
arguments in forming the new list.
That is, the last cdr of each list argument but the last is changed to
point to the next list argument.
If any argument but the last is the empty list, it is essentially ignored.
The final argument (which need not be a list) is not altered.

\scheme{append!} performs less allocation than \scheme{append} but is not
necessarily faster.
Its use can easily lead to confusing or incorrect results if used
indiscriminately.

\schemedisplay
(append! '(a b) '(c d)) ;=> (a b c d)

(let ([x '(a b)])
    (append! x '(c d))
    x) ;=> (a b c d)
\endschemedisplay


\section{Characters}

{\ChezScheme} extends the syntax of characters in two ways.
First, a \scheme{#\} prefix followed by exactly three octal digits is read
as a character whose numeric code is the octal value of the three digits,
e.g., \scheme{#\044} is read as \scheme{#\$}.
Second, it recognizes several nonstandard named characters:
\scheme{#\rubout} (which is the same as \scheme{#\delete}),
\scheme{#\bel} (which is the same as \scheme{#\alarm}),
\scheme{#\vt} (which is the same as \scheme{#\vtab}),
\scheme{#\nel} (the Unicode NEL character), and
\scheme{#\ls} (the Unicode LS character).
The set of nonstandard character names may be changed via the procedure
\index{\scheme{char-name}}\scheme{char-name} (page \ref{desc:char-name}).

These extensions are disabled in an input stream after \scheme{#!r6rs} has
been seen by the reader, unless \scheme{#!chezscheme} has been seen more
recently.

%----------------------------------------------------------------------------
\entryheader
\formdef{char=?}{\categoryprocedure}{(char=? \var{char_1} \var{char_2} \dots)}
\formdef{char<?}{\categoryprocedure}{(char<? \var{char_1} \var{char_2} \dots)}
\formdef{char>?}{\categoryprocedure}{(char>? \var{char_1} \var{char_2} \dots)}
\formdef{char<=?}{\categoryprocedure}{(char<=? \var{char_1} \var{char_2} \dots)}
\formdef{char>=?}{\categoryprocedure}{(char>=? \var{char_1} \var{char_2} \dots)}
\formdef{char-ci=?}{\categoryprocedure}{(char-ci=? \var{char_1} \var{char_2} \dots)}
\formdef{char-ci<?}{\categoryprocedure}{(char-ci<? \var{char_1} \var{char_2} \dots)}
\formdef{char-ci>?}{\categoryprocedure}{(char-ci>? \var{char_1} \var{char_2} \dots)}
\formdef{char-ci<=?}{\categoryprocedure}{(char-ci<=? \var{char_1} \var{char_2} \dots)}
\formdef{char-ci>=?}{\categoryprocedure}{(char-ci>=? \var{char_1} \var{char_2} \dots)}
\returns \scheme{#t} if the relation holds, \scheme{#f} otherwise
\listlibraries
\endentryheader

\noindent
These predicates are identical to the Revised$^6$ Report counterparts,
except they are extended to accept one or more rather than two or more
arguments.
When passed one argument, each of these predicates returns \scheme{#t}.

\schemedisplay
(char>? #\a) ;=> #t
(char<? #\a) ;=> #t
(char-ci=? #\a) ;=> #t
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{char-}{\categoryprocedure}{(char- \var{char_1} \var{char_2})}
\returns the integer difference between \var{char_1} and \var{char_2}
\listlibraries
\endentryheader

\noindent
\scheme{char-} subtracts the integer value of \var{char_2} from the
integer value of \var{char_1} and returns the difference.
The following examples assume that the integer representation is the
ASCII code for the character.

\schemedisplay
(char- #\f #\e) ;=> 1

(define digit-value
 ; returns the digit value of the base-r digit c, or #f if c
 ; is not a valid digit
  (lambda (c r)
    (let ([v (cond
               [(char<=? #\0 c #\9) (char- c #\0)]
               [(char<=? #\A c #\Z) (char- c #\7)]
               [(char<=? #\a c #\z) (char- c #\W)]
               [else 36])])
      (and (fx< v r) v))))
(digit-value #\8 10) ;=> 8
(digit-value #\z 10) ;=> #f
(digit-value #\z 36) ;=> 35
\endschemedisplay

\noindent
\scheme{char-} might be defined as follows.

\schemedisplay
(define char-
  (lambda (c1 c2)
    (- (char->integer c1) (char->integer c2))))
\endschemedisplay

\section{Strings}

{\ChezScheme} extends the standard string syntax with two character
escapes: \scheme{\'}, which produces the single quote character, and
\scheme{\\var{nnn}}, i.e., backslash followed by 3 octal digits,
which produces the character equivalent of the octal value of
the 3 digits.
These extensions are disabled in an input stream after \scheme{#!r6rs} has
been seen by the reader, unless \scheme{#!chezscheme} has been seen more
recently.

The length and indices of a string in {\ChezScheme} are always fixnums.


%----------------------------------------------------------------------------
\entryheader
\formdef{string=?}{\categoryprocedure}{(string=? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string<?}{\categoryprocedure}{(string<? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string>?}{\categoryprocedure}{(string>? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string<=?}{\categoryprocedure}{(string<=? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string>=?}{\categoryprocedure}{(string>=? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string-ci=?}{\categoryprocedure}{(string-ci=? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string-ci<?}{\categoryprocedure}{(string-ci<? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string-ci>?}{\categoryprocedure}{(string-ci>? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string-ci<=?}{\categoryprocedure}{(string-ci<=? \var{string_1} \var{string_2} \var{string_3} \dots)}
\formdef{string-ci>=?}{\categoryprocedure}{(string-ci>=? \var{string_1} \var{string_2} \var{string_3} \dots)}
\returns \scheme{#t} if the relation holds, \scheme{#f} otherwise
\listlibraries
\endentryheader

\noindent
These predicates are identical to the Revised$^6$ Report counterparts,
except they are extended to accept one or more rather than two or more
arguments.
When passed one argument, each of these predicates returns \scheme{#t}.

\schemedisplay
(string>? "a") ;=> #t
(string<? "a") ;=> #t
(string-ci=? "a") ;=> #t
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{string-copy!}{\categoryprocedure}{(string-copy! \var{src} \var{src-start} \var{dst} \var{dst-start} \var{n})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\var{src} and \var{dst} must be strings.
\var{src-start}, \var{dst-start}, and \var{n} must be exact nonnegative
integers.
The sum of \var{src-start} and \var{n} must not exceed the length of \var{src},
and the sum of \var{dst-start} and \var{n} must not exceed the length of \var{dst}.

\scheme{string-copy!} overwrites the \var{n} bytes of \var{dst}
starting at \var{dst-start} with the \var{n} bytes of \var{dst}
starting at \var{src-start}.
This works even if \var{dst} is the same string as \var{src} and the
source and destination locations overlap.
That is, the destination is filled with the characters that appeared at the
source before the operation began.

\schemedisplay
(define s1 "to boldly go")
(define s2 (make-string 10 #\-))

(string-copy! s1 3 s2 1 3)
s2 ;=> "-bol------"
 
(string-copy! s1 7 s2 4 2)
s2 ;=> "-bolly----"
 
(string-copy! s2 2 s2 5 4)
s2 ;=> "-bollolly-"
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{substring-fill!}{\categoryprocedure}{(substring-fill! \var{string} \var{start} \var{end} \var{char})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
The characters of \var{string} from \var{start} (inclusive) to \var{end}
(exclusive) are set to \var{char}.
\var{start} and \var{end} must be nonnegative integers; \var{start}
must be strictly less than the length of \var{string}, while \var{end} may
be less than or equal to the length of \var{string}.
If $end\le start$, the string is left unchanged.

\schemedisplay
(let ([str (string-copy "a tpyo typo")])
  (substring-fill! str 2 6 #\X)
  str) ;=> "a XXXX typo"
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{string-truncate!}{\categoryprocedure}{(string-truncate! \var{string} \var{n})}
\returns \var{string} or the empty string
\listlibraries
\endentryheader

\noindent
\var{n} must be an exact nonnegative fixnum not greater than the length of
\var{string}.
If \var{n} is zero, \scheme{string-truncate!} returns the empty string.
Otherwise, \var{string-truncate!} destructively truncates \var{string} to
its first \var{n} characters and returns \var{string}.

\schemedisplay
(define s (make-string 7 #\$))
(string-truncate! s 0) ;=> ""
s ;=> "$$$$$$$"
(string-truncate! s 3) ;=> "$$$"
s ;=> "$$$"
\endschemedisplay


\section{Vectors}

{\ChezScheme} extends the syntax of vectors to allow the length of the
vector to be specified between the \scheme{#} and open parenthesis, e.g.,
\scheme{#3(a b c)}.
If fewer elements are supplied in the syntax than the specified length,
each element after the last printed element is the same as the last
printed element.
This extension is disabled in an input stream after \scheme{#!r6rs} has
been seen by the reader, unless \scheme{#!chezscheme} has been seen more
recently.

The length and indices of a vector in {\ChezScheme} are always fixnums.

%----------------------------------------------------------------------------
\entryheader
\formdef{vector-copy}{\categoryprocedure}{(vector-copy \var{vector})}
\returns a copy of \var{vector}
\listlibraries
\endentryheader

\noindent
\scheme{vector-copy} creates a new vector of the same length and contents
as \var{vector}.
The elements themselves are not copied.

\schemedisplay
(vector-copy '#(a b c)) ;=> #(a b c)

(let ([v '#(a b c)])
  (eq? v (vector-copy v))) ;=> #f
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{vector-set-fixnum!}{\categoryprocedure}{(vector-set-fixnum! \var{vector} \var{n} \var{fixnum})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\scheme{vector-set-fixnum!} changes the \var{n}th element of \var{vector} to \var{fixnum}.
\var{n} must be an exact nonnegative integer strictly less than
the length of \var{vector}.

It is faster to store a fixnum than an arbitrary value,
since for arbitrary values, the system has to record potential assignments from older to
younger objects to support generational garbage collection.
Care must be taken to ensure that the argument is indeed a fixnum, however;
otherwise, the collector may not properly track the assignment.
The primitive performs a fixnum check on the argument except at
optimization level~3.

See also the description of fixnum-only vectors (fxvectors) below.

\schemedisplay
(let ([v (vector 1 2 3 4 5)])
  (vector-set-fixnum! v 2 73)
  v) ;=> #(1 2 73 4 5)
\endschemedisplay

\section{Fixnum-Only Vectors\label{SECTFXVECTORS}}

Fixnum-only vectors, or ``fxvectors,'' are like vectors but contain
only fixnums.
Fxvectors are written with the \scheme{#vfx} prefix in place of the
\scheme{#} prefix for vectors, e.g., \scheme{#vfx(1 2 3)} or
\scheme{#10vfx(2)}.
The fxvector syntax is disabled in an input stream after \scheme{#!r6rs}
has been seen by the reader, unless \scheme{#!chezscheme} has been seen
more recently.

The length and indices of an fxvector are always fixnums.

Updating an fxvector is generally less expensive than updating a vector,
since for vectors, the system records potential assignments from older to
younger objects to support generational garbage collection.
The storage management system also takes advantage of the fact that
fxvectors contain no pointers to place them in an area of memory that
does not have to be traced during collection.

See also \scheme{vector-set-fixnum!} above.

%----------------------------------------------------------------------------
\entryheader
\formdef{fxvector?}{\categoryprocedure}{(fxvector? \var{obj})}
\returns \scheme{#t} if \var{obj} is an fxvector, \scheme{#f} otherwise
\listlibraries
\endentryheader

\noskip\schemedisplay
(fxvector? #vfx()) ;=> #t
(fxvector? #vfx(1 2 3)) ;=> #t
(fxvector? (fxvector 1 2 3)) ;=> #t
(fxvector? '#(a b c)) ;=> #f
(fxvector? '(a b c)) ;=> #f
(fxvector? "abc") ;=> #f
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{fxvector}{\categoryprocedure}{(fxvector \var{fixnum} \dots)}
\returns an fxvector of the fixnums \scheme{\var{fixnum} \dots}
\listlibraries
\endentryheader

\noskip\schemedisplay
(fxvector) ;=> #vfx()
(fxvector 1 3 5) ;=> #vfx(1 3 5)
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{make-fxvector}{\categoryprocedure}{(make-fxvector \var{n})}
\formdef{make-fxvector}{\categoryprocedure}{(make-fxvector \var{n} \var{fixnum})}
\returns an fxvector of length \var{n}
\listlibraries
\endentryheader

\noindent
\var{n} must be a fixnum.
If \var{fixnum} is supplied, each element of the fxvector is initialized
to \var{fixnum}; otherwise, the elements are unspecified.

\schemedisplay
(make-fxvector 0) ;=> #vfx()
(make-fxvector 0 7) ;=> #vfx()
(make-fxvector 5 7) ;=> #vfx(7 7 7 7 7)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{fxvector-length}{\categoryprocedure}{(fxvector-length \var{fxvector})}
\returns the number of elements in \var{fxvector}
\listlibraries
\endentryheader

\schemedisplay
(fxvector-length #vfx()) ;=> 0
(fxvector-length #vfx(1 2 3)) ;=> 3
(fxvector-length #10vfx(1 2 3)) ;=> 10
(fxvector-length (fxvector 1 2 3 4)) ;=> 4
(fxvector-length (make-fxvector 300)) ;=> 300
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{fxvector-ref}{\categoryprocedure}{(fxvector-ref \var{fxvector} \var{n})}
\returns the \var{n}th element (zero-based) of \var{fxvector}
\listlibraries
\endentryheader

\noindent
\var{n} must be a nonnegative fixnum strictly less than
the length of \var{fxvector}.

\schemedisplay
(fxvector-ref #vfx(-1 2 4 7) 0) ;=> -1
(fxvector-ref #vfx(-1 2 4 7) 1) ;=> 2
(fxvector-ref #vfx(-1 2 4 7) 3) ;=> 7
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{fxvector-set!}{\categoryprocedure}{(fxvector-set! \var{fxvector} \var{n} \var{fixnum})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\var{n} must be a nonnegative fixnum strictly less than
the length of \var{fxvector}.
\scheme{fxvector-set!} changes the \var{n}th element of \var{fxvector} to \var{fixnum}.

\schemedisplay
(let ([v (fxvector 1 2 3 4 5)])
  (fxvector-set! v 2 (fx- (fxvector-ref v 2)))
  v) ;=> #vfx(1 2 -3 4 5)
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{fxvector-fill!}{\categoryprocedure}{(fxvector-fill! \var{fxvector} \var{fixnum})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\scheme{fxvector-fill!} replaces each element of \var{fxvector} with \var{fixnum}.

\schemedisplay
(let ([v (fxvector 1 2 3)])
  (fxvector-fill! v 0)
  v) ;=> #vfx(0 0 0)
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{fxvector->list}{\categoryprocedure}{(fxvector->list \var{fxvector})}
\returns a list of the elements of \var{fxvector}
\listlibraries
\endentryheader

\schemedisplay
(fxvector->list (fxvector)) ;=> ()
(fxvector->list #vfx(7 5 2)) ;=> (7 5 2)

(let ([v #vfx(1 2 3 4 5)])
  (apply fx* (fxvector->list v))) ;=> 120
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{list->fxvector}{\categoryprocedure}{(list->fxvector \var{list})}
\returns an fxvector of the elements of \var{list}
\listlibraries
\endentryheader

\noindent
\var{list} must consist entirely of fixnums.

\schemedisplay
(list->fxvector '()) ;=> #vfx()
(list->fxvector '(3 5 7)) ;=> #vfx(3 5 7)

(let ([v #vfx(1 2 3 4 5)])
  (let ([ls (fxvector->list v)])
    (list->fxvector (map fx* ls ls)))) ;=> #vfx(1 4 9 16 25)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{fxvector-copy}{\categoryprocedure}{(fxvector-copy \var{fxvector})}
\returns a copy of \var{fxvector}
\listlibraries
\endnoskipentryheader

\noindent
\scheme{fxvector-copy} creates a new fxvector with the same length and contents
as \var{fxvector}.

\schemedisplay
(fxvector-copy #vfx(3 4 5)) ;=> #vfx(3 4 5)

(let ([v #vfx(3 4 5)])
  (eq? v (fxvector-copy v))) ;=> #f
\endschemedisplay

\section{Bytevectors\label{SECTBYTEVECTORS}}

As with vectors, {\ChezScheme} extends the syntax of bytevectors to allow
the length of the vector to be specified between the \scheme{#} and open
parenthesis, e.g., \scheme{#3vu8(1 105 73)}.
If fewer elements are supplied in the syntax than the specified length,
each element after the last printed element is the same as the last
printed element.
This extension is disabled in an input stream after \scheme{#!r6rs} has
been seen by the reader, unless \scheme{#!chezscheme} has been seen more
recently.

{\ChezScheme} also extends the set of bytevector primitives, including
primitives for loading and storing 3, 5, 6, and 7-byte quantities.

The length and indices of a bytevector in {\ChezScheme} are always fixnums.

%----------------------------------------------------------------------------
\entryheader
\formdef{bytevector}{\categoryprocedure}{(bytevector \var{fill} \dots)}
\returns a new bytevector containing \scheme{\var{fill} \dots}
\listlibraries
\endentryheader

Each \var{fill} value must be an exact integer representing a signed or
unsigned 8-bit value, i.e.,
a value in the range -128 to 255 inclusive.
A negative fill value is treated as its two's complement equivalent.

\schemedisplay
(bytevector) ;=> #vu8()
(bytevector 1 3 5) ;=> #vu8(1 3 5)
(bytevector -1 -3 -5) ;=> #vu8(255 253 251)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{bytevector->s8-list}{\categoryprocedure}{(bytevector->s8-list \var{bytevector})}
\returns a new list of the 8-bit signed elements of \var{bytevector}
\listlibraries
\endentryheader

The values in the returned list are exact eight-bit signed integers,
i.e., values in the range -128 to 127 inclusive.
\scheme{bytevector->s8-list} is similar to the Revised$^6$ Report 
\scheme{bytevector->u8-list} except the values in the returned list
are signed rather than unsigned.

\schemedisplay
(bytevector->s8-list (make-bytevector 0)) ;=> ()
(bytevector->s8-list #vu8(1 127 128 255)) ;=> (1 127 -128 -1)

(let ([v #vu8(1 2 3 255)])
  (apply * (bytevector->s8-list v))) ;=> -6
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{s8-list->bytevector}{\categoryprocedure}{(s8-list->bytevector \var{list})}
\returns a new bytevector of the elements of \var{list}
\listlibraries
\endentryheader

\var{list} must consist entirely of exact eight-bit signed integers, i.e.,
values in the range -128 to 127 inclusive.
\scheme{s8-list->bytevector} is similar to the Revised$^6$ Report 
procedure
\scheme{u8-list->bytevector}, except the elements of the input list
are signed rather than unsigned.

\schemedisplay
(s8-list->bytevector '()) ;=> #vu8()
(s8-list->bytevector '(1 127 -128 -1)) ;=> #vu8(1 127 128 255)

(let ([v #vu8(1 2 3 4 5)])
  (let ([ls (bytevector->s8-list v)])
    (s8-list->bytevector (map - ls)))) ;=> #vu8(255 254 253 252 251)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{bytevector-truncate!}{\categoryprocedure}{(bytevector-truncate! \var{bytevector} \var{n})}
\returns \var{bytevector} or the empty bytevector
\listlibraries
\endentryheader

\noindent
\var{n} must be an exact nonnegative fixnum not greater than the length of
\var{bytevector}.
If \var{n} is zero, \scheme{bytevector-truncate!} returns the empty bytevector.
Otherwise, \var{bytevector-truncate!} destructively truncates \var{bytevector} to
its first \var{n} bytes and returns \var{bytevector}.

\schemedisplay
(define bv (make-bytevector 7 19))
(bytevector-truncate! bv 0) ;=> #vu8()
bv ;=> #vu8(19 19 19 19 19 19 19)
(bytevector-truncate! bv 3) ;=> #vu8(19 19 19)
bv ;=> #vu8(19 19 19)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{bytevector-u24-ref}{\categoryprocedure}{(bytevector-u24-ref \var{bytevector} \var{n} \var{eness})}
\returns the 24-bit unsigned integer at index \var{n} (zero-based) of \var{bytevector} 
\formdef{bytevector-s24-ref}{\categoryprocedure}{(bytevector-s24-ref \var{bytevector} \var{n} \var{eness})}
\returns the 24-bit signed integer at index \var{n} (zero-based) of \var{bytevector} 
\formdef{bytevector-u40-ref}{\categoryprocedure}{(bytevector-u40-ref \var{bytevector} \var{n} \var{eness})}
\returns the 40-bit unsigned integer at index \var{n} (zero-based) of \var{bytevector} 
\formdef{bytevector-s40-ref}{\categoryprocedure}{(bytevector-s40-ref \var{bytevector} \var{n} \var{eness})}
\returns the 40-bit signed integer at index \var{n} (zero-based) of \var{bytevector} 
\formdef{bytevector-u48-ref}{\categoryprocedure}{(bytevector-u48-ref \var{bytevector} \var{n} \var{eness})}
\returns the 48-bit unsigned integer at index \var{n} (zero-based) of \var{bytevector} 
\formdef{bytevector-s48-ref}{\categoryprocedure}{(bytevector-s48-ref \var{bytevector} \var{n} \var{eness})}
\returns the 48-bit signed integer at index \var{n} (zero-based) of \var{bytevector} 
\formdef{bytevector-u56-ref}{\categoryprocedure}{(bytevector-u56-ref \var{bytevector} \var{n} \var{eness})}
\returns the 56-bit unsigned integer at index \var{n} (zero-based) of \var{bytevector} 
\formdef{bytevector-s56-ref}{\categoryprocedure}{(bytevector-s56-ref \var{bytevector} \var{n} \var{eness})}
\returns the 56-bit signed integer at index \var{n} (zero-based) of \var{bytevector} 
\listlibraries
\endentryheader

\noindent
\var{n} must be an exact nonnegative integer and
indexes the starting byte of the value.
The sum of \var{n} and the number of bytes occupied by the value
(3 for 24-bit values, 5 for 40-bit values, 6 for 48-bit values,
and 7 for 56-bit values) must not exceed the length of \var{bytevector}.
\var{eness} must be a valid endianness symbol naming the endianness.

The return value is an exact integer in the appropriate range for
the number of bytes occupied by the value.
Signed values are the equivalent of the stored value treated as a two's
complement value.

%----------------------------------------------------------------------------
\entryheader
\formdef{bytevector-u24-set!}{\categoryprocedure}{(bytevector-u24-set! \var{bytevector} \var{n} \var{u24} \var{eness})}
\formdef{bytevector-s24-set!}{\categoryprocedure}{(bytevector-s24-set! \var{bytevector} \var{n} \var{s24} \var{eness})}
\formdef{bytevector-u40-set!}{\categoryprocedure}{(bytevector-u40-set! \var{bytevector} \var{n} \var{u40} \var{eness})}
\formdef{bytevector-s40-set!}{\categoryprocedure}{(bytevector-s40-set! \var{bytevector} \var{n} \var{s40} \var{eness})}
\formdef{bytevector-u48-set!}{\categoryprocedure}{(bytevector-u48-set! \var{bytevector} \var{n} \var{u48} \var{eness})}
\formdef{bytevector-s48-set!}{\categoryprocedure}{(bytevector-s48-set! \var{bytevector} \var{n} \var{s48} \var{eness})}
\formdef{bytevector-u56-set!}{\categoryprocedure}{(bytevector-u56-set! \var{bytevector} \var{n} \var{u56} \var{eness})}
\formdef{bytevector-s56-set!}{\categoryprocedure}{(bytevector-s56-set! \var{bytevector} \var{n} \var{s56} \var{eness})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\var{n} must be an exact nonnegative integer and
indexes the starting byte of the value.
The sum of \var{n} and the number of bytes occupied by the value must
not exceed the length of \var{bytevector}.
\var{u24} must be a 24-bit unsigned value, i.e., a value in the range
0 to $2^{24}-1$ inclusive;
\var{s24} must be a 24-bit signed value, i.e., a value in the range
$-2^{23}$ to $2^{23}-1$ inclusive;
\var{u40} must be a 40-bit unsigned value, i.e., a value in the range
0 to $2^{40}-1$ inclusive;
\var{s40} must be a 40-bit signed value, i.e., a value in the range
$-2^{39}$ to $2^{39}-1$ inclusive;
\var{u48} must be a 48-bit unsigned value, i.e., a value in the range
0 to $2^{48}-1$ inclusive;
\var{s48} must be a 48-bit signed value, i.e., a value in the range
$-2^{47}$ to $2^{47}-1$ inclusive;
\var{u56} must be a 56-bit unsigned value, i.e., a value in the range
0 to $2^{56}-1$ inclusive; and
\var{s56} must be a 56-bit signed value, i.e., a value in the range
$-2^{55}$ to $2^{55}-1$ inclusive.
\var{eness} must be a valid endianness symbol naming the endianness.

These procedures store the given value in the 3, 5, 6, or 7 bytes starting
at index \var{n} (zero-based) of \var{bytevector}.
Negative values are stored as their two's complement equivalent.


\section{Boxes\label{SECTBOXES}}

\index{boxes}Boxes are single-cell objects that are primarily useful for providing
an ``extra level of indirection.''
This extra level of indirection is typically used to allow more than one body
of code or data structure to share a \index{reference}reference, or \index{pointer}pointer, to an object.
For example, boxes may be used to implement \index{call-by-reference}\emph{call-by-reference} semantics
in an interpreter for a language employing this parameter passing discipline.

\index{\scheme{#&} (box prefix)}Boxes are written with
the prefix \scheme{#&} (pronounced ``hash-ampersand'').
For example, \scheme{#&(a b c)} is a box holding the list \scheme{(a b c)}.
The box syntax is disabled in an input stream after \scheme{#!r6rs} has
been seen by the reader, unless \scheme{#!chezscheme} has been seen more
recently.

%----------------------------------------------------------------------------
\entryheader
\formdef{box?}{\categoryprocedure}{(box? \var{obj})}
\returns \scheme{#t} if \var{obj} is a box, \scheme{#f} otherwise
\listlibraries
\endentryheader

\noskip\schemedisplay
(box? '#&a) ;=> #t
(box? 'a) ;=> #f
(box? (box 3)) ;=> #t
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{box}{\categoryprocedure}{(box \var{obj})}
\returns a new box containing \var{obj}
\listlibraries
\endentryheader

\noskip\schemedisplay
(box 'a) ;=> #&a
(box (box '(a b c))) ;=> #&#&(a b c)
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{unbox}{\categoryprocedure}{(unbox \var{box})}
\returns contents of \var{box}
\listlibraries
\endentryheader

\noskip\schemedisplay
(unbox #&a) ;=> a
(unbox #&#&(a b c)) ;=> #&(a b c)

(let ([b (box "hi")])
  (unbox b)) ;=> "hi"
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{set-box!}{\categoryprocedure}{(set-box! \var{box} \var{obj})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\scheme{set-box!} sets the contents of \var{box} to \var{obj}.

\schemedisplay
(let ([b (box 'x)])
    (set-box! b 'y)
    b) ;=> #&y

(let ([incr!
       (lambda (x)
          (set-box! x (+ (unbox x) 1)))])
  (let ([b (box 3)])
    (incr! b)
    (unbox b))) ;=> 4
\endschemedisplay


\section{Symbols\label{SECTMISCSYMBOLS}}

{\ChezScheme} extends the standard symbol syntax in several ways:

\begin{itemize}
\item
Symbol names may begin with \scheme{@}, but \scheme{,@abc} is parsed
as \scheme{(unquote-splicing abc)}; to produce \scheme{(unquote @abc)}
one can type \scheme{, @abc}, \scheme{\x40;abc}, or \scheme{,|@abc|}.

\item 
The single-character sequences \scheme{\schlbrace} and \scheme{\schrbrace}
are read as symbols.

\item
A symbol's name may begin with any character that might normally start a
number, including a digit, \scheme{.}, \scheme{+}, \scheme{-}, as long as
the delimited sequence of characters starting with that character cannot
be parsed as a number.

\item
A symbol whose name contains arbitrary characters may be written by
escaping them them with \scheme{\} or with \scheme{|}.
\scheme{\} is used to escape a single character (except 'x', since
\scheme{\x} marks the start of a hex scalar value),
whereas \scheme{|} is used
to escape the group of characters that follow it up through the
matching \scheme{|}.
\end{itemize}

The printer always prints symbols using the standard R6RS syntax, so that,
e.g., \scheme{@abc} prints as \scheme{\x40;abc} and \scheme{1-} prints as
\scheme{\x31;-}.  '

Gensyms are printed
\index{\scheme{#\schlbrace} (gensym prefix)}\scheme{#\schlbrace} and
\scheme{\schrbrace} brackets that enclose both the ``pretty'' and ``unique''
names,
e.g., \scheme{#\schlbrace\raw{{}}g1426 e5g1c94g642dssw-a\schrbrace}.
They may also be printed using the pretty name only with the prefix
\index{\scheme{#:} (gensym prefix)}\scheme{#:}, e.g.,
\scheme{#:g1426}.

These extensions are disabled in an input stream after \scheme{#!r6rs} has
been seen by the reader, unless \scheme{#!chezscheme} has been seen more
recently.


%----------------------------------------------------------------------------
\entryheader\label{desc:gensym}
\formdef{gensym}{\categoryprocedure}{(gensym)}
\formdef{gensym}{\categoryprocedure}{(gensym \var{pretty-name})}
\formdef{gensym}{\categoryprocedure}{(gensym \var{pretty-name} \var{unique-name})}
\returns a unique generated symbol
\listlibraries
\endentryheader

\noindent
\index{gensyms}\index{generated symbols}Each
call to \scheme{gensym} returns a unique generated symbol, or \emph{gensym}.
Each generated symbol has two names: a ``pretty'' name and a
``unique'' name.

In the first form above, the pretty name is formed (lazily---see
below) by combining an
internal prefix with the value of an internal counter.
After each name is formed, the internal counter is incremented.
The parameters \scheme{gensym-prefix} and
\scheme{gensym-count}, described below, may be used to access and set
the internal prefix and counter.
By default, the prefix is the single-character string \scheme{"g"}.
In the second and third forms, the pretty name of the new gensym
is \var{pretty-name}, which must be a string.
The pretty name of a gensym is returned by the procedure
\scheme{symbol->string}.

In both the first and second forms, the unique name is an
automatically generated globally unique name.
Globally unique names are constructed (lazily---see below)
from some combination of a unique machine identifier (such as the
network address), the current process identifier (PID), and the
time at which the Scheme session began, along with an internal
counter.
In the third form of gensym, the unique name of the new gensym is
\var{unique-name}, which must be a string.
The unique name of a gensym may be obtained via the procedure
\scheme{gensym->unique-string}.

The unique name allows gensyms to be written in such a way that they
can be read back and reliably commonized on input.
\index{\scheme{#\schlbrace} (gensym prefix)}The syntax for gensyms
includes both the pretty name and the unique name, as shown in the
example below:

\schemedisplay
(gensym) ;=> #{g0 bcsfg5eq4e9b3h9o-a}
\endschemedisplay

\noindent
When the parameter \index{\scheme{print-gensym}}\scheme{print-gensym} is set to \scheme{pretty},
the printer prints the pretty name only, with a
\index{\scheme{#:} (gensym prefix)}\scheme{#:} syntax, so

\schemedisplay
(parameterize ([print-gensym 'pretty])
  (write (gensym)))
\endschemedisplay

prints \scheme{#:g0}.

When the reader sees the \scheme{#:} syntax, it produces a gensym with
the given pretty name, but the original unique name is lost.

When the parameter is set to \scheme{#f}, the printer prints just the
pretty name, so

\schemedisplay
(parameterize ([print-gensym #f])
  (write (gensym)))
\endschemedisplay

\noindent
prints \scheme{g0}.
This is useful only when gensyms do not need to be read back in
as gensyms.

In order to reduce construction and (when threaded) synchronization
overhead when gensyms are frequently created but rarely printed or
stored in an object file, generated pretty and unique names are created
lazily, i.e., not until first requested, either by the printer, fasl
writer, or explicitly by one of the procedures \scheme{symbol->string}
or \scheme{gensym->unique-string}.
In addition, a gensym is not placed into the system's internal symbol
table (the oblist; see page~\pageref{desc:oblist}) until the unique name
is requested.
This allows a gensym to be reclaimed by the storage manager
if no references to the gensym exist and no unique name exists by which to
access it, even if it has a top-level binding or a nonempty property
list.

\schemedisplay
(define x (gensym))
x                         ;=> #{g2 bcsfg5eq4e9b3h9o-c}
(symbol->string x)        ;=> "g2"
(gensym->unique-string x) ;=> "bcsfg5eq4e9b3h9o-c"
\endschemedisplay

Gensyms subsume the notion of \index{uninterned symbols}\emph{uninterned
symbols} supported by earlier versions of {\ChezScheme}.
Similarly, the predicate
\index{uninterned-symbol?}\scheme{uninterned-symbol?} has been replaced
by \scheme{gensym?}.

%----------------------------------------------------------------------------
\entryheader
\formdef{gensym-prefix}{\categorythreadparameter}{gensym-prefix}
\formdef{gensym-count}{\categorythreadparameter}{gensym-count}
\listlibraries
\endentryheader

\noindent
\index{\scheme{gensym}}The parameters \scheme{gensym-prefix} and
\scheme{gensym-count} are used to access and set the internal prefix
and counter from which the pretty name of a gensym
is generated when \scheme{gensym} is not given an explicit string
argument.
\scheme{gensym-prefix} defaults to the string \scheme{"g"} and may be
set to any object.
\scheme{gensym-count} starts at 0 and may be set to any nonnegative
integer.

As described above, {\ChezScheme} delays the creation
of the pretty name until the name is first requested by the printer or by
an explicit call to \scheme{symbol->string}.
These parameters are not consulted until that time; setting them when
\scheme{gensym} is called thus has no effect on the generated name.

\schemedisplay
(let ([x (parameterize ([gensym-prefix "genny"]
                        [gensym-count 17]
                        [print-gensym 'pretty])
           (gensym))])
  (format "~s" x))                       ;=> "#{g4 bcsfg5eq4e9b3h9o-e}"
(let ([x (gensym)])
  (parameterize ([gensym-prefix "genny"]
                 [gensym-count 17]
                 [print-gensym #f])
    (format "~s" (gensym))))             ;=> "genny17"
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{gensym->unique-string}{\categoryprocedure}{(gensym->unique-string \var{gensym})}
\returns the unique name of \var{gensym}
\listlibraries
\endentryheader

\noskip\schemedisplay
(gensym->unique-string (gensym)) ;=> "bd3kufa7ypjcuvut-g"
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{gensym?}{\categoryprocedure}{(gensym? \var{obj})}
\returns \scheme{#t} if \var{obj} is gensym, \scheme{#f} otherwise
\listlibraries
\endentryheader

\noskip\schemedisplay
(gensym? (string->symbol "z")) ;=> #f
(gensym? (gensym "z")) ;=> #t
(gensym? 'a) ;=> #f
(gensym? 3) ;=> #f
(gensym? (gensym)) ;=> #t
(gensym? '#{g2 bcsfg5eq4e9b3h9o-c}) ;=> #t
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader\label{property-lists}
\formdef{putprop}{\categoryprocedure}{(putprop \var{symbol} \var{key} \var{value})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
{\ChezScheme} associates a \index{property lists}\emph{property list} with
each symbol, allowing multiple \var{key-value} pairs to be stored
directly with the symbol.
New key-value pairs may be placed in the property list or retrieved in
a manner analogous to the use of association lists, using the procedures
\scheme{putprop} and \scheme{getprop}.
Property lists are often used to store information related to the symbol
itself.
For example, a natural language program might use symbols to represent
words, using their property lists to store information about use and
meaning.

\scheme{putprop} associates \var{value} with \var{key} on the
property list of \var{symbol}.
\var{key} and \var{value} may be any types of object, although \var{key} is
typically a symbol.

\scheme{putprop} may be used to establish a new property or to change
an existing property.

See the examples under \scheme{getprop} below.


%----------------------------------------------------------------------------
\entryheader
\formdef{getprop}{\categoryprocedure}{(getprop \var{symbol} \var{key})}
\formdef{getprop}{\categoryprocedure}{(getprop \var{symbol} \var{key} \var{default})}
\returns the value associated with \var{key} on the property list of \var{symbol}
\listlibraries
\endentryheader

\noindent
\index{property lists}\scheme{getprop} searches the property list of
\var{symbol} for a key identical to \var{key} (in the sense of
\scheme{eq?}), and returns the value associated with this key, if any.
If no value is associated with \var{key} on the property list of
\var{symbol}, \scheme{getprop} returns \var{default}, or \scheme{#f} if
the \var{default} argument is not supplied.


\schemedisplay
(putprop 'fred 'species 'snurd)
(putprop 'fred 'age 4)  
(putprop 'fred 'colors '(black white))

(getprop 'fred 'species) ;=> snurd
(getprop 'fred 'colors) ;=> (black white)
(getprop 'fred 'nonkey) ;=> #f
(getprop 'fred 'nonkey 'unknown) ;=> unknown
 
(putprop 'fred 'species #f)
(getprop 'fred 'species 'unknown) ;=> #f
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{remprop}{\categoryprocedure}{(remprop \var{symbol} \var{key})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\scheme{remprop} removes the property with key \var{key} from the property
list of \var{symbol}, if such a property exists\index{Fred}.

\schemedisplay
(putprop 'fred 'species 'snurd)
(getprop 'fred 'species) ;=> snurd

(remprop 'fred 'species)
(getprop 'fred 'species 'unknown) ;=> unknown
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{property-list}{\categoryprocedure}{(property-list \var{symbol})}
\returns a copy of the internal property list for \var{symbol}
\listlibraries
\endentryheader

\noindent
A property list is a list of alternating keys and values,
i.e., \scheme{(\var{key} \var{value} \dots)}.

\schemedisplay
(putprop 'fred 'species 'snurd)
(putprop 'fred 'colors '(black white))
(property-list 'fred) ;=> (colors (black white) species snurd)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader\label{desc:oblist}
\formdef{oblist}{\categoryprocedure}{(oblist)}
\returns a list of interned symbols
\listlibraries
\endentryheader

\noindent
The system maintains an internal symbol table used
to insure that any two occurrences of the same
symbol name resolve to the same symbol object.
The \scheme{oblist} procedure returns a list of the symbols currently in
this symbol table.

The list of interned symbols grows when a new symbol
is introduced into the system or when the unique name of a
gensym (see page~\pageref{desc:gensym}) is requested.
It shrinks when the garbage collector determines that it is
safe to discard a symbol.
It is safe to discard a symbol only if the symbol is not accessible except
through the oblist,
has no top-level binding, and has no properties on its property
list.

\schemedisplay
(if (memq 'tiger (oblist)) 'yes 'no) ;=> yes
(equal? (oblist) (oblist)) ;=> #t
(= (length (oblist)) (length (oblist))) ;=> #t \var{or} #f
\endschemedisplay

\noindent
The first example above follows from the property that all interned
symbols are in the oblist from the time they are read, which happens
prior to evaluation.
The second example follows from the fact that no symbols can be
removed from the oblist while references to those symbols exist, in
this case, within the list returned by the first call to
\scheme{oblist} (whichever call is performed first).
The expression in the third example can return \scheme{#f} only if a garbage
collection occurs sometime between the two calls to \scheme{oblist}, and only
if one or more symbols are removed from the oblist by that collection.

\section{Void\label{SECTMISCVOID}}

Many Scheme operations return an unspecified result.
{\ChezScheme} typically returns a special \emph{void} object when the
value returned by an operation is unspecified.
The {\ChezScheme} void object is not meant to be used as a datum, and
consequently does not have a reader syntax.
As for other objects without a reader syntax, such as procedures and
ports, {\ChezScheme} output procedures print the void object using a
nonreadable representation, i.e., \scheme{#<void>}.
Since the void object should be returned only by operations that do not
have ``interesting'' values, the default waiter printer (see
\scheme{waiter-write}) suppresses the printing of the void object.
\scheme{set!}, \scheme{set-car!}, \scheme{load}, and \scheme{write} are examples of {\ChezScheme}
operations that return the void object.

%----------------------------------------------------------------------------
\entryheader
\formdef{void}{\categoryprocedure}{(void)}
\returns the void object
\listlibraries
\endentryheader

\noindent
\scheme{void} is a procedure of no arguments that returns the void object.
It can be used to force expressions that are used for effect or whose
values are otherwise unspecified to evaluate to a consistent, trivial
value.
Since most {\ChezScheme} operations that are used for effect
return the void object, however, it is rarely necessary to explicitly
invoke the \scheme{void} procedure.

Since the void object is used explicitly as an ``unspecified'' value,
it is a bad idea to use it for any other purpose or to count on any
given expression evaluating to the void object.

The default waiter printer suppresses the void object; that is, nothing
is printed for expressions that evaluate to the void object.

\schemedisplay
(eq? (void) #f) ;=> #f
(eq? (void) #t) ;=> #f
(eq? (void) '()) ;=> #f
\endschemedisplay

\section{Sorting\label{SECTMISCSORTING}}

%----------------------------------------------------------------------------
\noskipentryheader
\formdef{sort}{\categoryprocedure}{(sort \var{predicate} \var{list})}
\formdef{sort!}{\categoryprocedure}{(sort! \var{predicate} \var{list})}
\returns a list containing the elements of \var{list} sorted according to \var{predicate}
\listlibraries
\endnoskipentryheader

\noindent
\scheme{sort} is identical to the Revised$^6$ Report \scheme{list-sort},
and \scheme{sort!} is a destructive version of \scheme{sort}, i.e., it
reuses pairs from the input list to form the output list.

\schemedisplay
(sort < '(3 4 2 1 2 5)) ;=> (1 2 2 3 4 5)
(sort! < '(3 4 2 1 2 5)) ;=> (1 2 2 3 4 5)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{merge}{\categoryprocedure}{(merge \var{predicate} \var{list_1} \var{list_2})}
\formdef{merge!}{\categoryprocedure}{(merge! \var{predicate} \var{list_1} \var{list_2})}
\returns \var{list_1} merged with \var{list_2} in the order specified by \var{predicate}
\listlibraries
\endentryheader

\noindent
\var{predicate} should be a procedure that expects two arguments and
returns \scheme{#t} if its first argument must precede its second in
the merged list.
It should not have any side effects.
That is, if \var{predicate} is applied to two objects \var{x} and
\var{y}, where \var{x} is taken from the second list and \var{y}
is taken from the first list,
it should return true only if \var{x} should appear before \var{y}
in the output list.
If this constraint is met,
\scheme{merge} and \scheme{merge!} are stable, in that items from \var{list_1} are
placed in front of equivalent items from \var{list_2} in the output list.
Duplicate elements are included in the merged list.

\scheme{merge!} combines the lists destructively, using pairs from the input
lists to form the output list.

\schemedisplay
(merge char<?
       '(#\a #\c)
       '(#\b #\c #\d)) ;=> (#\a #\b #\c #\c #\d)
(merge <
       '(1/2 2/3 3/4)
       '(0.5 0.6 0.7)) ;=> (1/2 0.5 0.6 2/3 0.7 3/4)
\endschemedisplay


\section{Hashtables\label{SECTMISCHASHTABLES}}

{\ChezScheme} provides several extensions to the hashtable mechanism,
including a mechanism for directly accessing a key, value pair in a
hashtable, support for weak eq and eqv hashtables, and a set of procedures
specialized to eq and symbol hashtables.

%----------------------------------------------------------------------------
\entryheader
\formdef{hashtable-cell}{\categoryprocedure}{(hashtable-cell \var{hashtable} \var{key} \var{default})}
\returns a pair (see below)
\listlibraries
\endentryheader

\var{hashtable} must be a hashtable.
\var{key} and \var{default} may be any Scheme values.

If no value is associated with \var{key} in \var{hashtable},
\scheme{hashtable-cell} modifies \var{hashtable} to associate \var{key} with
\var{default}.
It returns a pair whose car is \var{key} and whose cdr is
the associated value.
Changing the cdr of this pair effectively updates the table to
associate \var{key} with a new value.
The \var{key} in the car field should not be changed.
The advantage of this procedure over the Revised$^6$ Report procedures
for manipulating hashtable entries is that the value associated with
a key may be read or written many times with only a single hashtable
lookup.

\schemedisplay
(define ht (make-eq-hashtable))
(define v (vector 'a 'b 'c))
(define cell (hashtable-cell ht v 3))
cell ;=> (#(a b c) . 3)
(hashtable-ref ht v 0) ;=> 3
(set-cdr! cell 4)
(hashtable-ref ht v 0) ;=> 4
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{hashtable-values}{\categoryprocedure}{(hashtable-values \var{hashtable})}
\returns a vector containing the values in \var{hashtable}
\listlibraries
\endentryheader

\noindent
Each value is the value of one of the keys in \var{hashtable}.
Duplicate values are not removed.
The values may appear in any order in the returned vector.

\schemedisplay
(define ht (make-eq-hashtable))
(define p1 (cons 'a 'b))
(define p2 (cons 'a 'b))
(hashtable-set! ht p1 "one")
(hashtable-set! ht p2 "two")
(hashtable-set! ht 'q "two")
(hashtable-values ht) ;=> #("one" "two" "two")
\endschemedisplay

This procedure is equivalent to:

\schemedisplay
(lambda (ht)
  (let-values ([(keys values) (hashtable-entries ht)])
    values))
\endschemedisplay

but more efficient since the separate vector of keys need
not be created.

%----------------------------------------------------------------------------
\entryheader
\formdef{make-weak-eq-hashtable}{\categoryprocedure}{(make-weak-eq-hashtable)}
\formdef{make-weak-eq-hashtable}{\categoryprocedure}{(make-weak-eq-hashtable \var{size})}
\formdef{make-weak-eqv-hashtable}{\categoryprocedure}{(make-weak-eqv-hashtable)}
\formdef{make-weak-eqv-hashtable}{\categoryprocedure}{(make-weak-eqv-hashtable \var{size})}
\returns a new weak eq hashtable
\listlibraries
\endentryheader

These procedures are like the Revised$^6$ Report procedures \scheme{make-eq-hashtable}
and \scheme{make-eqv-hashtable}
except the keys of the hashtable are held weakly, i.e., they are not
protected from the garbage collector.
Keys reclaimed by the garbage collector are removed from the table,
and their associated values are dropped the next time the table
is modified, if not sooner.

A copy of a weak eq or eqv hashtable created by \scheme{hashtable-copy} is
also weak.
If the copy is immutable, inaccessible keys may still be dropped from the
hashtable, even though the contents of the table is otherwise unchanging.
The effect of this can be observed via \scheme{hashtable-keys} and
\scheme{hashtable-entries}.

\schemedisplay
(define ht1 (make-weak-eq-hashtable))
(define ht2 (make-weak-eq-hashtable 32))
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{hashtable-weak?}{\categoryprocedure}{(hashtable-weak? \var{obj})}
\returns \scheme{#t} if \var{obj} is a weak eq or eqv hashtable, \scheme{#f} otherwise
\listlibraries
\endentryheader

\schemedisplay
(define ht1 (make-weak-eq-hashtable))
(define ht2 (hashtable-copy ht1))
(hashtable-weak? ht2) ;=> #t
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{eq-hashtable?}{\categoryprocedure}{(eq-hashtable? \var{obj})}
\returns \scheme{#t} if \var{obj} is an eq hashtable, \scheme{#f} otherwise
\listlibraries
\endentryheader

\noskip\schemedisplay
(eq-hashtable? (make-eq-hashtable)) ;=> #t
(eq-hashtable? '(not a hash table)) ;=> #f
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{eq-hashtable-weak?}{\categoryprocedure}{(eq-hashtable-weak? \var{hashtable})}
\returns \scheme{#t} if \var{hashtable} is weak, \scheme{#f} otherwise
\listlibraries
\endentryheader

\var{hashtable} must be an eq hashtable.

\schemedisplay
(eq-hashtable-weak? (make-eq-hashtable)) ;=> #f
(eq-hashtable-weak? (make-weak-eq-hashtable)) ;=> #t
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{eq-hashtable-set!}{\categoryprocedure}{(eq-hashtable-set! \var{hashtable} \var{key} \var{value})}
\returns unspecified
\listlibraries
\endentryheader

\var{hashtable} must be a mutable eq hashtable.
\var{key} and \var{value} may be any Scheme values.

\scheme{eq-hashtable-set!} associates the value
\var{value} with the key \var{key} in \var{hashtable}.

\schemedisplay
(define ht (make-eq-hashtable))
(eq-hashtable-set! ht 'a 73)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{eq-hashtable-ref}{\categoryprocedure}{(eq-hashtable-ref \var{hashtable} \var{key} \var{default})}
\returns see below
\listlibraries
\endentryheader

\var{hashtable} must be an eq hashtable.
\var{key} and \var{default} may be any Scheme values.

\scheme{eq-hashtable-ref} returns the value
associated with \var{key} in \var{hashtable}.
If no value is associated with \var{key} in \var{hashtable},
\scheme{eq-hashtable-ref} returns \var{default}.

% Key comparisons are performed with \var{eq?}.

\schemedisplay
(define ht (make-eq-hashtable))
(define p1 (cons 'a 'b))
(define p2 (cons 'a 'b))
(eq-hashtable-set! ht p1 73)
(eq-hashtable-ref ht p1 55) ;=> 73
(eq-hashtable-ref ht p2 55) ;=> 55
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{eq-hashtable-contains?}{\categoryprocedure}{(eq-hashtable-contains? \var{hashtable} \var{key})}
\returns \scheme{#t} if an association for \var{key} exists in \var{hashtable}, \scheme{#f} otherwise
\listlibraries
\endentryheader

\var{hashtable} must be an eq hashtable.
\var{key} may be any Scheme value.

\schemedisplay
(define ht (make-eq-hashtable))
(define p1 (cons 'a 'b))
(define p2 (cons 'a 'b))
(eq-hashtable-set! ht p1 73)
(eq-hashtable-contains? ht p1) ;=> #t
(eq-hashtable-contains? ht p2) ;=> #f
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{eq-hashtable-update!}{\categoryprocedure}{(eq-hashtable-update! \var{hashtable} \var{key} \var{procedure} \var{default})}
\returns unspecified
\listlibraries
\endentryheader

\var{hashtable} must be a mutable eq hashtable.
\var{key} and \var{default} may be any Scheme values.
\var{procedure} should accept one argument, should return one value, and
should not modify \var{hashtable}.

\scheme{eq-hashtable-update!} applies \var{procedure} to the value associated with
\var{key} in \var{hashtable}, or to \var{default} if no value is associated with
\var{key} in \var{hashtable}.
If \var{procedure} returns, \scheme{eq-hashtable-update!} associates \var{key}
with the value returned by \var{procedure}, replacing the old association,
if any.

A version of \scheme{eq-hashtable-update!} that does not verify that it receives
arguments of the proper type might be defined as follows.

\schemedisplay
(define eq-hashtable-update!
  (lambda (ht key proc value)
    (eq-hashtable-set! ht key
      (proc (eq-hashtable-ref ht key value)))))
\endschemedisplay

An implementation may, however, be able to implement
\scheme{eq-hashtable-update!} more efficiently by avoiding multiple
hash computations and hashtable lookups.

\schemedisplay
(define ht (make-eq-hashtable))
(eq-hashtable-update! ht 'a
  (lambda (x) (* x 2))
  55)
(eq-hashtable-ref ht 'a 0) ;=> 110
(eq-hashtable-update! ht 'a
  (lambda (x) (* x 2))
  0)
(eq-hashtable-ref ht 'a 0) ;=> 220
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{eq-hashtable-cell}{\categoryprocedure}{(eq-hashtable-cell \var{hashtable} \var{key} \var{default})}
\returns a pair (see below)
\listlibraries
\endentryheader

\var{hashtable} must be an eq hashtable.
\var{key} and \var{default} may be any Scheme values.

If no value is associated with \var{key} in \var{hashtable},
\scheme{eq-hashtable-cell} modifies \var{hashtable} to associate \var{key} with
\var{default}.
It returns a pair whose car is \var{key} and whose cdr is
the associated value.
Changing the cdr of this pair effectively updates the table to
associate \var{key} with a new value.
The \var{key} should not be changed.

\schemedisplay
(define ht (make-eq-hashtable))
(define v (vector 'a 'b 'c))
(define cell (eq-hashtable-cell ht v 3))
cell ;=> (#(a b c) . 3)
(eq-hashtable-ref ht v 0) ;=> 3
(set-cdr! cell 4)
(eq-hashtable-ref ht v 0) ;=> 4
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{eq-hashtable-delete!}{\categoryprocedure}{(eq-hashtable-delete! \var{hashtable} \var{key})}
\returns unspecified
\listlibraries
\endentryheader

\var{hashtable} must be a mutable eq hashtable.
\var{key} may be any Scheme value.

\scheme{eq-hashtable-delete!} drops any association
for \var{key} from \var{hashtable}.

\schemedisplay
(define ht (make-eq-hashtable))
(define p1 (cons 'a 'b))
(define p2 (cons 'a 'b))
(eq-hashtable-set! ht p1 73)
(eq-hashtable-contains? ht p1) ;=> #t
(eq-hashtable-delete! ht p1)
(eq-hashtable-contains? ht p1) ;=> #f
(eq-hashtable-contains? ht p2) ;=> #f
(eq-hashtable-delete! ht p2)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{symbol-hashtable?}{\categoryprocedure}{(symbol-hashtable? \var{obj})}
\returns \scheme{#t} if \var{obj} is an eq hashtable, \scheme{#f} otherwise
\listlibraries
\endentryheader

\noskip\schemedisplay
(symbol-hashtable? (make-hashtable symbol-hash eq?)) ;=> #t
(symbol-hashtable? (make-eq-hashtable)) ;=> #f
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{symbol-hashtable-set!}{\categoryprocedure}{(symbol-hashtable-set! \var{hashtable} \var{key} \var{value})}
\returns unspecified
\listlibraries
\endentryheader

\var{hashtable} must be a mutable symbol hashtable.
(A symbol hashtable is a hashtable created with hash function \scheme{symbol-hash}
and equivalence function \scheme{eq?}, \scheme{eqv?}, \scheme{equal?}, or \scheme{symbol=?}.)
\var{key} must be a symbol, and \var{value} may be any Scheme value.

\scheme{symbol-hashtable-set!} associates the value
\var{value} with the key \var{key} in \var{hashtable}.

\schemedisplay
(define ht (make-hashtable symbol-hash eq?))
(symbol-hashtable-ref ht 'a #f) ;=> #f
(symbol-hashtable-set! ht 'a 73)
(symbol-hashtable-ref ht 'a #f) ;=> 73
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{symbol-hashtable-ref}{\categoryprocedure}{(symbol-hashtable-ref \var{hashtable} \var{key} \var{default})}
\returns see below
\listlibraries
\endentryheader

\var{hashtable} must be a symbol hashtable.
(A symbol hashtable is a hashtable created with hash function \scheme{symbol-hash}
and equivalence function \scheme{eq?}, \scheme{eqv?}, \scheme{equal?}, or \scheme{symbol=?}.)
\var{key} must be a symbol, and \var{default} may be any Scheme value.

\scheme{symbol-hashtable-ref} returns the value
associated with \var{key} in \var{hashtable}.
If no value is associated with \var{key} in \var{hashtable},
\scheme{symbol-hashtable-ref} returns \var{default}.

% Key comparisons are performed with \var{eq?}.

\schemedisplay
(define ht (make-hashtable symbol-hash eq?))
(define k1 'abcd)
(define k2 'not-abcd)
(symbol-hashtable-set! ht k1 "hi")
(symbol-hashtable-ref ht k1 "bye") ;=> "hi"
(symbol-hashtable-ref ht k2 "bye") ;=> "bye"
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{symbol-hashtable-contains?}{\categoryprocedure}{(symbol-hashtable-contains? \var{hashtable} \var{key})}
\returns \scheme{#t} if an association for \var{key} exists in \var{hashtable}, \scheme{#f} otherwise
\listlibraries
\endentryheader

\var{hashtable} must be a symbol hashtable.
(A symbol hashtable is a hashtable created with hash function \scheme{symbol-hash}
and equivalence function \scheme{eq?}, \scheme{eqv?}, \scheme{equal?}, or \scheme{symbol=?}.)
\var{key} must be a symbol.

\schemedisplay
(define ht (make-hashtable symbol-hash eq?))
(define k1 'abcd)
(define k2 'not-abcd)
(symbol-hashtable-set! ht k1 "hi")
(symbol-hashtable-contains? ht k1) ;=> #t
(symbol-hashtable-contains? ht k2 ) ;=> #f
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{symbol-hashtable-update!}{\categoryprocedure}{(symbol-hashtable-update! \var{hashtable} \var{key} \var{procedure} \var{default})}
\returns unspecified
\listlibraries
\endentryheader

\var{hashtable} must be a mutable symbol hashtable.
(A symbol hashtable is a hashtable created with hash function \scheme{symbol-hash}
and equivalence function \scheme{eq?}, \scheme{eqv?}, \scheme{equal?}, or \scheme{symbol=?}.)
\var{key} must be a symbol, and \var{default} may be any Scheme value.
\var{procedure} should accept one argument, should return one value, and
should not modify \var{hashtable}.

\scheme{symbol-hashtable-update!} applies \var{procedure} to the value associated with
\var{key} in \var{hashtable}, or to \var{default} if no value is associated with
\var{key} in \var{hashtable}.
If \var{procedure} returns, \scheme{symbol-hashtable-update!} associates \var{key}
with the value returned by \var{procedure}, replacing the old association,
if any.

A version of \scheme{symbol-hashtable-update!} that does not verify that it receives
arguments of the proper type might be defined as follows.

\schemedisplay
(define symbol-hashtable-update!
  (lambda (ht key proc value)
    (symbol-hashtable-set! ht key
      (proc (symbol-hashtable-ref ht key value)))))
\endschemedisplay

An implementation may, however, be able to implement
\scheme{symbol-hashtable-update!} more efficiently by avoiding multiple
hash computations and hashtable lookups.

\schemedisplay
(define ht (make-hashtable symbol-hash eq?))
(symbol-hashtable-update! ht 'a
  (lambda (x) (* x 2))
  55)
(symbol-hashtable-ref ht 'a 0) ;=> 110
(symbol-hashtable-update! ht 'a
  (lambda (x) (* x 2))
  0)
(symbol-hashtable-ref ht 'a 0) ;=> 220
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{symbol-hashtable-cell}{\categoryprocedure}{(symbol-hashtable-cell \var{hashtable} \var{key} \var{default})}
\returns a pair (see below)
\listlibraries
\endentryheader

\var{hashtable} must be a mutable symbol hashtable.
(A symbol hashtable is a hashtable created with hash function \scheme{symbol-hash}
and equivalence function \scheme{eq?}, \scheme{eqv?}, \scheme{equal?}, or \scheme{symbol=?}.)
\var{key} must be a symbol, and \var{default} may be any Scheme value.

If no value is associated with \var{key} in \var{hashtable},
\scheme{symbol-hashtable-cell} modifies \var{hashtable} to associate \var{key} with
\var{default}.
It returns a pair whose car is \var{key} and whose cdr is
the associated value.
Changing the cdr of this pair effectively updates the table to
associate \var{key} with a new value.
The \var{key} should not be changed.

\schemedisplay
(define ht (make-hashtable symbol-hash eq?))
(define k 'a-key)
(define cell (symbol-hashtable-cell ht k 3))
cell ;=> (a-key . 3)
(symbol-hashtable-ref ht k 0) ;=> 3
(set-cdr! cell 4)
(symbol-hashtable-ref ht k 0) ;=> 4
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{symbol-hashtable-delete!}{\categoryprocedure}{(symbol-hashtable-delete! \var{hashtable} \var{key})}
\returns unspecified
\listlibraries
\endentryheader

\var{hashtable} must be a mutable symbol hashtable.
(A symbol hashtable is a hashtable created with hash function \scheme{symbol-hash}
and equivalence function \scheme{eq?}, \scheme{eqv?}, \scheme{equal?}, or \scheme{symbol=?}.)
\var{key} must be a symbol.

\scheme{symbol-hashtable-delete!} drops any association
for \var{key} from \var{hashtable}.

\schemedisplay
(define ht (make-hashtable symbol-hash eq?))
(define k1 (gensym))
(define k2 (gensym))
(symbol-hashtable-set! ht k1 73)
(symbol-hashtable-contains? ht k1) ;=> #t
(symbol-hashtable-delete! ht k1)
(symbol-hashtable-contains? ht k1) ;=> #f
(symbol-hashtable-contains? ht k2) ;=> #f
(symbol-hashtable-delete! ht k2)
\endschemedisplay


\section{Record Types\label{SECTR6RSRECORDS}}

\index{\scheme{define-record-type}}\index{\scheme{require-nongenerative-clause}}%
Chez Scheme extends the Revised$^6$ Report's \scheme{define-record-type}
syntax in one way, which is that it allows a generative record type
to be declared explicitly as such (in a double-negative sort of way)
by including a \scheme{nongenerative} clause with \scheme{#f} as the
uid, i.e.:

\schemedisplay
(nongenerative #f)
\endschemedisplay

This can be used in conjunction with the parameter
\scheme{require-nongenerative-clause} to catch the accidental use of
generative record types while avoiding spurious errors for record types
that must be generative.
Generative record types are rarely needed and are generally less
efficient since a run-time representation of the type is created each
time the \scheme{define-record-clause} is evaluated, rather than once
at compile (expansion) time.

%----------------------------------------------------------------------------
\entryheader
\formdef{require-nongenerative-clause}{\categorythreadparameter}{require-nongenerative-clause}
\listlibraries
\endentryheader

This parameter holds a boolean value that determines whether
\index{\scheme{define-record-type}}\scheme{define-record-type}
requires a nongenerative clause.
The default value is \scheme{#f}.
The lead-in above describes why one might want to set this to \scheme{#t}.


\section{Legacy Record Types\label{SECTCSV7RECORDS}}

\index{records}\index{\scheme{define-record}}\index{\scheme{make-record-type}}%
In addition to the Revised$^6$ Report record-type creation and definition
mechanisms, which are described in Chapter~\ref{TSPL:CHPTRECORDS} of {\TSPLFOUR},
{\ChezScheme} continues to support pre-R6RS mechanisms for creating new
data types, or \emph{record types}, with fixed sets of named fields.
Many of the procedures described in this section are available only when
imported from the \scheme{(chezscheme csv7)} library.

Code intended to be portable should use the R6RS mechanism instead.

Records may be defined via the \scheme{define-record} syntactic form or
via the \scheme{make-record-type} procedure.
The underlying representation of records and record-type descriptors is the
same for the Revised$^6$ Report mechanism and the alternative mechanism.
Record types created by one can be used as parent record types for the
other via the procedural mechanisms, though not via the syntactic mechanisms.

% undocumented:
%   record-type-interfaces

The syntactic (\scheme{define-record})
interface is the most commonly used interface.
Each \scheme{define-record} form defines a constructor
procedure for records of the new type, a type predicate that returns
true only for records of the new type, an access procedure for each field,
and an assignment procedure for each mutable field.
For example,

\schemedisplay
(define-record point (x y))
\endschemedisplay

creates a new \scheme{point} record type with two fields, \scheme{x}
and \scheme{y}, and defines the following procedures:

\begin{tabular}{ll}
\scheme{(make-point \var{x} \var{y})} & constructor\\
\scheme{(point? \var{obj})} & predicate\\
\scheme{(point-x \var{p})} & accessor for field \scheme{x}\\
\scheme{(point-y \var{p})} & accessor for field \scheme{y}\\
\scheme{(set-point-x! \var{p} \var{obj})} & mutator for field \scheme{x}\\
\scheme{(set-point-y! \var{p} \var{obj})} & mutator for field \scheme{y}
\end{tabular}

The names of these procedures follow a regular naming convention by
default, but the programmer can override the defaults if desired.
\scheme{define-record} allows the programmer to control which fields
are arguments to the generated constructor procedure and which 
are explicitly initialized by the constructor procedure.
Fields are mutable by default, but may be declared immutable.
Fields can generally contain any Scheme value, but the internal
representation of each field may be specified, which places implicit
constraints on the type of value that may be stored there.
These customization options are covered in the formal description
of \scheme{define-record} later in this section.

The procedural (\scheme{make-record-type}) interface may be used to
implement interpreters that must handle \scheme{define-record} forms.
Each call to \scheme{make-record-type} returns a \emph{record-type
descriptor} representing the record type.
Using this record-type descriptor, programs may generate constructors,
type predicates, field accessors, and field mutators dynamically.
The following code demonstrates how the procedural interface might
be used to create a similar \scheme{point} record type and associated
definitions.

\schemedisplay
(define point (make-record-type "point" '(x y)))
(define make-point (record-constructor point))
(define point? (record-predicate point))
(define point-x (record-field-accessor point 'x))
(define point-y (record-field-accessor point 'y))
(define set-point-x! (record-field-mutator point 'x))
(define set-point-y! (record-field-mutator point 'y))
\endschemedisplay

The procedural interface is more flexible than the syntactic interface,
but this flexibility can lead to less readable programs and
compromises the compiler's ability to generate efficient code.
Programmers should use the syntactic interface whenever it suffices.

A record-type descriptor may also be extracted from an instance
of a record type, whether the record type was produced by
\scheme{define-record} or \scheme{make-record-type}, and the extracted
descriptor may also be used to produce constructors, predicates,
accessors, and mutators, with a few limitations noted in the description
of \scheme{record-type-descriptor} below.
This is a powerful feature that permits the coding of portable printers
and object inspectors.
For example, the printer employs this feature in its default record
printer, and the inspector uses it to allow inspection and mutation of
system- and user-defined records during debugging.

\index{record inheritance}\index{inheritance in records}A parent record
may be specified in the \scheme{define-record} syntax or as an optional
argument to \scheme{make-record-type}.
A new record inherits the parent record's fields, and each instance
of the new record type is considered to be an instance of the parent
type as well, so that accessors and mutators for the parent type may
be used on instances of the new type.

\index{record generativity}\index{generativity of record definitions}Record
type definitions may be classified as either generative or nongenerative.
A new type results for each \emph{generative} record definition,
while only one type results for all occurrences of a given
\emph{nongenerative} record definition.
This distinction is important semantically since record accessors
and setters are applicable only to objects with the same type.

Syntactic (\scheme{define-record}) record definitions are
\index{expand-time generativity}\emph{expand-time generative} by default, which means that a new
record is created when the code is expanded.
Expansion happens once for each form prior to compilation or
interpretation, as when it is entered interactively, loaded from source,
or compiled by \scheme{compile-file}.
As a result, multiple evaluations of a single \scheme{define-record}
form, e.g., in the body of a procedure called multiple times, always
produce the same record type.

\index{nongenerative record definitions}Separate \scheme{define-record} forms
usually produce different types, even if the forms are textually
identical.
The only exception occurs when the name of a record is specified as
a generated symbol, or \emph{gensym} (page~\pageref{desc:gensym}).
Multiple copies of a record definition whose name is given by a gensym
always produce the same record type; i.e., such definitions are
nongenerative.
Each copy of the record definition must contain the same fields and field
modifiers in the same order; an exception is raised with condition-type
\scheme{&assertion} when two differing
record types with the same generated name are loaded into the same
Scheme process.

Procedural (\scheme{make-record-type}) record definitions are
\index{run-time generativity}\emph{run-time generative} by default.
That is, each call to \scheme{make-record-type} usually produces a new
record type.
As with the syntactic interface,
the only exception occurs when the name of the record is specified
as a gensym, in which case the record type is
fully nongenerative.

By default, a record is printed with the syntax

\schemedisplay
#[\var{type-name} \var{field} \dots]
\endschemedisplay

\noindent
where \scheme{\var{field} \dots} are the printed representations of
the contents of the fields of the record, and
\var{type-name} is a generated symbol, or \emph{gensym}
(page~\pageref{desc:gensym}), that uniquely identifies the record type.
For nongenerative records, \var{type-name} is the gensym
provided by the program.
Otherwise, it is a gensym whose ``pretty'' name
(page~\pageref{desc:gensym}) is the name given to the record by
\scheme{define-record} or \scheme{make-record-type}.

The default printing of records of a given type may be overridden
with \var{record-writer}.

The default syntax may be used as input to the reader as well, as long
as the corresponding record type has already been defined in the Scheme
session in which the read occurs.
The parameter \scheme{record-reader} may be used to specify a
different name to be recognized by the reader in place of the
generated name.
Specifying a different name in this manner also changes the name used
when the record is printed.
This reader extension is disabled in an input stream after \scheme{#!r6rs}
has been seen by the reader, unless \scheme{#!chezscheme} has been seen
more recently.

The mark (\scheme{#\var{n}=}) and reference (\scheme{#\var{n}#})
syntaxes may be used within the record syntax, with the result
of creating shared or cyclic structure as desired.
All cycles must be resolvable, however, without mutation of an
immutable record field.
That is, any cycle must contain at least one pointer through a
mutable field, whether it is a mutable record field or a mutable
field of a built-in object type such as a pair or vector.

When the parameter \scheme{print-record} is set to \scheme{#f}, records
are printed using the simpler syntax

\schemedisplay
#<record of type \var{name}>
\endschemedisplay

\noindent
where \var{name} is the ``pretty'' name of the record (not the full
gensym) or the reader name first assigned to the record
type.

%%% need more define-record examples 
%%%   - show use of field types other than ptr
%%%   - show non-top-level use 

%%% make-record-type and company
%%%   - (scan primvars for record-related primitives)
%%%   - adapt some of the define-record examples
%%%   - show definition of default print method

%----------------------------------------------------------------------------
\entryheader
\formdef{define-record}{\categorysyntax}{(define-record \var{name} (\var{fld_1} \dots) ((\var{fld_2} \var{init}) \dots) (\var{opt} \dots))}
\formdef{define-record}{\categorysyntax}{(define-record \var{name} \var{parent} (\var{fld_1} \dots) ((\var{fld_2} \var{init}) \dots) (\var{opt} \dots))}
\returns unspecified
\listlibraries
\endentryheader

\noindent
A \scheme{define-record} form is a definition and may appear anywhere
and only where other definitions may appear.

\scheme{define-record} creates a new record type containing a specified
set of named fields and defines a set of procedures for creating and
manipulating instances of the record type.

\var{name} must be an identifier.
If \var{name} is a generated symbol (gensym), the record definition is
\emph{nongenerative}, otherwise it is \emph{expand-time generative}.
(See the discussion of generativity earlier in this section.)

Each \var{fld} must be an identifier \var{field-name}, or it must take
the form

\schemedisplay
(\var{class} \var{type} \var{field-name})
\endschemedisplay

\noindent
where \var{class} and \var{type} are optional and
\var{field-name} is an identifier.
\var{class}, if present, must be the keyword \scheme{immutable}
or the keyword \scheme{mutable}.
If the \scheme{immutable} class specifier is present, the field is
immutable; otherwise, the field is mutable.
\var{type}, if present, specifies how the field is represented,
as described below.
 
\begin{tabular}{ll}\label{record-field-types}
\scheme{ptr} &            any Scheme object\\
\scheme{scheme-object} &  same as \scheme{ptr}\\
\scheme{int} &            a C \scheme{int}\\
\scheme{unsigned} &       a C \scheme{unsigned int}\\
\scheme{short} &          a C \scheme{short}\\
\scheme{unsigned-short} & a C \scheme{unsigned short}\\
\scheme{long} &           a C \scheme{long}\\
\scheme{unsigned-long} &  a C \scheme{unsigned long}\\
\scheme{iptr} &           a signed integer the size of a \scheme{ptr}\\
\scheme{uptr} &           an unsigned integer the size of a \scheme{ptr}\\
\scheme{float} &          a C \scheme{float}\\
\scheme{double} &         a C \scheme{double}\\
\scheme{integer-8} &      an eight-bit signed integer\\
\scheme{unsigned-8} &     an eight-bit unsigned integer\\
\scheme{integer-16} &     a 16-bit signed integer\\
\scheme{unsigned-16} &    a 16-bit unsigned integer\\
\scheme{integer-32} &     a 32-bit signed integer\\
\scheme{unsigned-32} &    a 32-bit unsigned integer\\
\scheme{integer-64} &     a 64-bit signed integer\\
\scheme{unsigned-64} &    a 64-bit unsigned integer\\
\scheme{single-float} &   a 32-bit single floating point number\\
\scheme{double-float} &   a 64-bit double floating point number
\end{tabular}

\noindent
If a type is specified, the field can contain objects only of the
specified type.
If no type is specified, the field is of type \scheme{ptr},
meaning that it can contain any Scheme object.

The field identifiers name the fields of the record.
The values of the $n$ fields described by \scheme{\var{fld_1} \dots} are
specified by the $n$ arguments to the generated constructor procedure.
The values of the remaining fields, \scheme{\var{fld_2} \dots}, are
given by the corresponding expressions, \scheme{\var{init} \dots}.
Each \var{init} is evaluated within the scope of the set of field names
given by \scheme{\var{fld_1} \dots} and each field in
\scheme{\var{fld_2} \dots} that precedes it, as if within a
\scheme{let*} expression.
Each of these field names is bound to the value of the corresponding field
during initialization.

\index{record inheritance}\index{inheritance in records}If
\var{parent} is present, the record type named by \var{parent}
is the parent of the record.
The new record type inherits each of the parent record's fields,
and records of the new type are considered records of the
parent type.
If \var{parent} is not present, the parent record type is
a base record type with no fields.

The following procedures are defined by \scheme{define-record}:

\begin{itemize}
\item
a constructor procedure whose name is \scheme{make-\var{name}},

\item
a type predicate whose name is \scheme{\var{name}?},

\item
an access procedure whose name is \scheme{\var{name}-\var{fieldname}}
for each noninherited field, and

\item
an assignment procedure whose name is
\scheme{set-\var{name}-\var{fieldname}!}
for each noninherited mutable field.
\end{itemize}

\noindent
If no parent record type is specified,
the constructor behaves as if defined as

\schemedisplay
(define make-\var{name}
  (lambda (\var{id_1} \dots)
    (let* ([\var{id_2} \var{init}] \dots)
      \var{body})))
\endschemedisplay

\noindent
where \scheme{\var{id_1} \dots} are the names of the fields defined by
\scheme{\var{fld_1} \dots},
\scheme{\var{id_2} \dots} are the names of the fields defined by
\scheme{\var{fld_2} \dots},
and \var{body} builds the record from the values of the identifiers
\scheme{\var{id_1} \dots} and \scheme{\var{id_2} \dots}.

If a parent record type is specified, the parent arguments appear first,
and the parent fields are inserted into the record before the child
fields.

The options \scheme{\var{opt} \dots} control the selection of names
of the generated constructor, predicate, accessors, and mutators.

\schemedisplay
(constructor \var{id})
(predicate \var{id})
(prefix \var{string})
\endschemedisplay

\noindent
The option
\scheme{(constructor \var{id})} causes the generated constructor's name
to be \var{id} rather than \scheme{make-\var{name}}.
The option \scheme{(predicate \var{id})} likewise causes the generated
predicate's name to be \var{id} rather than \scheme{\var{name}?}.
The option \scheme{(prefix \var{string})} determines the prefix
to be used in the generated accessor and mutator names in place of
\scheme{\var{name}-}.

If no options are needed, the third subexpression,
\scheme{(\var{opt} \dots)}, may be omitted.
If no options and no fields other than those initialized by the arguments
to the
constructor procedure are needed, both the second and third subexpressions
may be omitted.
If options are specified, the second subexpression must be present,
even if it contains no field specifiers.

Here is a simple example with no inheritance and no options.

\schemedisplay
(define-record marble (color quality))
(define x (make-marble 'blue 'medium))
(marble? x) ;=> #t
(pair? x) ;=> #f
(vector? x) ;=> #f
(marble-color x) ;=> blue
(marble-quality x) ;=> medium
(set-marble-quality! x 'low)
(marble-quality x) ;=> low

(define-record marble ((immutable color) (mutable quality))
  (((mutable shape) (if (eq? quality 'high) 'round 'unknown))))
(marble-shape (make-marble 'blue 'high)) ;=> round
(marble-shape (make-marble 'blue 'low)) ;=> unknown
(define x (make-marble 'blue 'high))
(set-marble-quality! x 'low)
(marble-shape x) ;=> round
(set-marble-shape! x 'half-round)
(marble-shape x) ;=> half-round
\endschemedisplay

\pagebreak
The following example illustrates inheritance.

\schemedisplay
(define-record shape (x y))
(define-record point shape ())
(define-record circle shape (radius))

(define a (make-point 7 -3))
(shape? a) ;=> #t
(point? a) ;=> #t
(circle? a) ;=> #f

(shape-x a) ;=> 7
(set-shape-y! a (- (shape-y a) 1))
(shape-y a) ;=> -4

(define b (make-circle 7 -3 1))
(shape? b) ;=> #t
(point? b) ;=> #f
(circle? b) ;=> #t

(circle-radius b) ;=> 1
(circle-radius a) ;=> \var{exception: not of type circle}

(define c (make-shape 0 0))
(shape? c) ;=> #t
(point? c) ;=> #f
(circle? c) ;=> #f
\endschemedisplay

This example demonstrates the use of options:

\schemedisplay
(define-record pair (car cdr)
  ()
  ((constructor cons)
   (prefix "")))

(define x (cons 'a 'b))
(car x) ;=> a
(cdr x) ;=> b
(pair? x) ;=> #t

(pair? '(a b c)) ;=> #f
x ;=> #[#{pair bdhavk1bwafxyss1-a} a b]
\endschemedisplay

This example illustrates the use a specified reader name, immutable
fields, and the graph mark and reference syntax.

\schemedisplay
(define-record triple ((immutable x1) (mutable x2) (immutable x3)))
(record-reader 'triple (type-descriptor triple))

(let ([t '#[triple #1=(1 2) (3 4) #1#]])
  (eq? (triple-x1 t) (triple-x3 t))) ;=> #t
(let ([x '(#1=(1 2) . #[triple #1# b c])])
  (eq? (car x) (triple-x1 (cdr x)))) ;=> #t
(let ([t #[triple #1# (3 4) #1=(1 2)]])
  (eq? (triple-x1 t) (triple-x3 t))) ;=> #t
(let ([t '#1=#[triple a #1# c]])
  (eq? t (triple-x2 t))) ;=> #t
(let ([t '#1=(#[triple #1# b #1#])])
  (and (eq? t (triple-x1 (car t)))
       (eq? t (triple-x1 (car t))))) ;=> #t
\endschemedisplay

\noindent
Cycles established with the mark and reference syntax can be
resolved only if a mutable record field or mutable location
of some other object is involved the cycle, as in the last
two examples above.
An exception is raised with condition type \scheme{&lexical} if only
immutable fields are involved.

\schemedisplay
'#1=#[triple #1# (3 4) #1#] ;=> \var{exception}
\endschemedisplay

\index{nongenerative record definitions}The following example demonstrates
the use of nongenerative record definitions.

\schemedisplay
(module A (point-disp)
  (define-record #{point bdhavk1bwafxyss1-b} (x y))
  (define square (lambda (x) (* x x)))
  (define point-disp
    (lambda (p1 p2)
      (sqrt (+ (square (- (point-x p1) (point-x p2)))
               (square (- (point-y p1) (point-y p2))))))))

(module B (base-disp)
  (define-record #{point bdhavk1bwafxyss1-b} (x y))
  (import A)
  (define base-disp
    (lambda (p)
      (point-disp (make-point 0 0) p))))

(let ()
  (import B)
  (define-record #{point bdhavk1bwafxyss1-b} (x y))
  (base-disp (make-point 3 4))) ;=> 5
\endschemedisplay

\noindent
This works even if the different program components are loaded from
different source files or are compiled separately and loaded from
different object files.

%----------------------------------------------------------------------------
%----------------------------------------------------------------------------
\entryheader
\formdef{predicate}{\categorysyntax}{predicate}
\formdef{prefix}{\categorysyntax}{prefix}
\formdef{constructor}{\categorysyntax}{constructor}
% \formdef{mutable}{\categorysyntax}{mutable}
% \formdef{immutable}{\categorysyntax}{immutable}
\listlibraries
\endentryheader

\noindent
\index{mutable}\index{immutable}%
These identifiers are auxiliary keywords for \scheme{define-record}.
It is a syntax violation to reference these identifiers except in
contexts where they are recognized as auxiliary keywords.
\scheme{mutable} and \scheme{immutable} are also auxiliary keywords for
\scheme{define-record}, shared with the Revised$^6$ Report
\scheme{define-record-type}.


%----------------------------------------------------------------------------
%----------------------------------------------------------------------------
\entryheader
\formdef{type-descriptor}{\categorysyntax}{(type-descriptor \var{name})}
\returns the record-type descriptor associated with \var{name}
\listlibraries
\endentryheader

\noindent
\var{name} must name a record type defined by \scheme{define-record}
of \scheme{define-record-type}.

This form is equivalent to the Revised$^6$ Report
\scheme{record-type-descriptor} form.

The record-type descriptor is useful for overriding the default
read and write syntax using \scheme{record-reader} and
\scheme{record-writer} and may also be used with the procedural
interface routines described later in this section.

\schemedisplay
(define-record frob ())
(type-descriptor frob) ;=> #<record type frob>
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{record-reader}{\categoryprocedure}{(record-reader \var{name})}
\returns the record-type descriptor associated with \var{name}
\formdef{record-reader}{\categoryprocedure}{(record-reader \var{rtd})}
\returns the first name associated with \var{rtd}
\formdef{record-reader}{\categoryprocedure}{(record-reader \var{name} \var{rtd})}
\returns unspecified
\formdef{record-reader}{\categoryprocedure}{(record-reader \var{name} #f)}
\returns unspecified
\formdef{record-reader}{\categoryprocedure}{(record-reader \var{rtd} #f)}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\var{name} must be a symbol, and \var{rtd} must be a
record-type descriptor.

With one argument, \scheme{record-reader} is used to retrieve the record
type associated with a name or name associated with a record type.
If no association has been created, \scheme{record-reader} returns
\scheme{#f}

With arguments \var{name} and \var{rtd}, \scheme{record-reader} registers
\var{rtd} as the record-type descriptor to be used whenever the
\scheme{read} procedure encounters a record named by \var{name} and
printed in the default record syntax.

With arguments \var{name} and \scheme{#f}, \scheme{record-reader} removes
any association for \var{name} to a record-type descriptor.
Similarly, with arguments \var{rtd} and \scheme{#f}, \scheme{record-reader}
removes any association for \var{rtd} to a name.

\schemedisplay
(define-record marble (color quality))
(define m (make-marble 'blue 'perfect))
m ;=> #[#{marble bdhavk1bwafxyss1-c} blue perfect]

(record-reader (type-descriptor marble)) ;=> #f
(record-reader 'marble) ;=> #f

(record-reader 'marble (type-descriptor marble))
(marble-color '#[marble red miserable]) ;=> red

(record-reader (type-descriptor marble)) ;=> marble
(record-reader 'marble) ;=> #<record type marble>

(record-reader (type-descriptor marble) #f)
(record-reader (type-descriptor marble)) ;=> #f
(record-reader 'marble) ;=> #f

(record-reader 'marble (type-descriptor marble))
(record-reader 'marble #f)
(record-reader (type-descriptor marble)) ;=> #f
(record-reader 'marble) ;=> #f
\endschemedisplay

\noindent
The introduction of a record reader also changes the default
printing of records.
The printer always chooses the reader name first assigned
to the record, if any, in place of the unique record name, as this
continuation of the example above demonstrates.

\schemedisplay
(record-reader 'marble (type-descriptor marble))
(make-marble 'pink 'splendid) ;=> #[marble pink splendid]
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{record-writer}{\categoryprocedure}{(record-writer \var{rtd})}
\returns the record writer associated with \var{rtd}
\formdef{record-writer}{\categoryprocedure}{(record-writer \var{rtd} \var{procedure})}
\returns unspecified
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor, and \var{procedure} should
accept three arguments, as described below.

When passed only one argument, \scheme{record-writer} returns the
record writer associated with \var{rtd}, which is initially the
default record writer for all records.
The default print method prints all records in a uniform syntax that
includes the generated name for the record 
and the values of each of the fields, as described in the introduction
to this section.

When passed two arguments, \scheme{record-writer} establishes a
new association between \var{rtd} and \var{procedure} so that
\var{procedure} will be used by the printer in place of the default
printer for records of the given type.
The printer passes \var{procedure} three arguments:
the record \var{r}, a port \var{p}, and a procedure \var{wr} that
should be used to write out the values of arbitrary Scheme objects that
the print method chooses to include in the printed representation of the
record, e.g., values of the record's fields.

\schemedisplay
(define-record marble (color quality))
(define m (make-marble 'blue 'medium))

m ;=> #[#{marble bdhavk1bwafxyss1-d} blue medium]

(record-writer (type-descriptor marble)
  (lambda (r p wr)
    (display "#<" p)
    (wr (marble-quality r) p)
    (display " quality " p)
    (wr (marble-color r) p)
    (display " marble>" p)))

m ;=> #<medium quality blue marble>
\endschemedisplay

The record writer is used only when \scheme{print-record} is true
(the default).
When the parameter \scheme{print-record} is set to \scheme{#f}, records
are printed using a compressed syntax that identifies only the type
of record.

\schemedisplay
(parameterize ([print-record #f])
  (format "~s" m)) ;=> "#<record of type marble>"
\endschemedisplay

A print method may be called more than once during the printing of a
single record to support cycle detection and graph printing
(see \index{\scheme{print-graph}}\scheme{print-graph}),
so print
methods that perform side effects other than printing to the given
port are discouraged.
Whenever a print method is called more than once during the printing
of a single record, in all but one call, a generic ``bit sink'' port
is used to suppress output automatically so that only one copy of
the object appears on the actual port.
In order to avoid confusing the cycle detection and graph printing
algorithms, a print method should always produce the same printed
representation for each object.
Furthermore, a print method should normally use the supplied procedure
\var{wr} to print subobjects, though atomic values, such as strings
or numbers, may be printed by direct calls to \scheme{display} or
\scheme{write} or by other means.

\schemedisplay
(let ()
  (define-record ref () ((contents 'nothing)))
  (record-writer (type-descriptor ref)
    (lambda (r p wr)
      (display "<" p)
      (wr (ref-contents r) p)
      (display ">" p)))
  (let ([ref-lexive (make-ref)])
    (set-ref-contents! ref-lexive ref-lexive)
    ref-lexive)) ;=> #0=<#0#>
\endschemedisplay

Print methods need not be concerned with handling nonfalse values of
the parameters
\index{\scheme{print-length}}\scheme{print-level}.
The printer handles \scheme{print-level} automatically even when
user-defined print procedures are used.
Since records typically contain a small, fixed number of fields, it
is usually possible to ignore nonfalse values of
\index{\scheme{print-length}}\scheme{print-length} as well.

\schemedisplay
(print-level 3)
(let ()
  (define-record ref () ((contents 'nothing)))
  (record-writer (type-descriptor ref)
    (lambda (r p wr)
      (display "<" p)
      (wr (ref-contents r) p)
      (display ">" p)))
  (let ([ref-lexive (make-ref)])
    (set-ref-contents! ref-lexive ref-lexive)
    ref-lexive)) ;=> <<<<#[...]>>>>
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{print-record}{\categorythreadparameter}{print-record}
\listlibraries
\endentryheader

\noindent
This parameter controls the printing of records.
If set to true (the default) the record writer associated with a
record type is used to print records of that type.
If set to false, all records are printed with the syntax
\scheme{#<record of type \var{name}>}, where \var{name} is the
name of the record type as returned by \scheme{record-type-name}.

% not documented:
%   extended version with base-rtd, parent, name, fields, interfaces, and extras

%----------------------------------------------------------------------------
\entryheader
\formdef{make-record-type}{\categoryprocedure}{(make-record-type \var{type-name} \var{fields})}
\formdef{make-record-type}{\categoryprocedure}{(make-record-type \var{parent-rtd} \var{type-name} \var{fields})}
\returns a record-type descriptor for a new record type
\listlibraries
\endentryheader

\noindent
\scheme{make-record-type} creates a new data type and returns a
record-type descriptor, a value representing the new data type.
The new type is disjoint from all others.

If present, \var{parent-rtd} must be a record-type descriptor.

\var{type-name} must be a string or gensym.
If \var{type-name} is a string, a new record type is generated.
If \var{type-name} is a gensym, a new record type is generated only
if one with the same gensym has not already been defined.
If one has already been defined, the parent and fields must be identical
to those of the existing record type, and the
existing record type is used.
If the parent and fields are not identical, an exception is raised with
condition-type \scheme{&assertion}.

\var{fields} must be a list of field descriptors, each of which
describes one field of instances of the new record type.
A field descriptor is either a symbol or a list in the following form:

\schemedisplay
(\var{class} \var{type} \var{field-name})
\endschemedisplay

\noindent
where \var{class} and \var{type} are optional.
\var{field-name} must be a symbol.
\var{class}, if present, must be the symbol \scheme{immutable} or
the symbol \scheme{mutable}.
If the \scheme{immutable} class-specifier is present, the field is
immutable; otherwise, the field is mutable.
\var{type}, if present, specifies how the field is represented.
The types are the same as those given in the description
of \scheme{define-record} on page~\pageref{record-field-types}.

\noindent
If a type is specified, the field can contain objects only of the
specified type.
If no type is specified, the field is of type \scheme{ptr},
meaning that it can contain any Scheme object.

The behavior of a program that modifies the string \var{type-name}
or the list \var{fields} or any of its sublists is unspecified.

The record-type descriptor may be passed as an argument to any of the
Revised$^6$ Report procedures

\begin{itemize}
\item \scheme{record-constructor},
\item \scheme{record-predicate},
\item \scheme{record-accessor}, and
\item \scheme{record-mutator},
\end{itemize}

or to the {\ChezScheme} variants

\begin{itemize}
\item \scheme{record-constructor},
\item \scheme{record-field-accessor}, and
\item \scheme{record-field-mutator}
\end{itemize}

\noindent
to obtain procedures for creating and manipulating records of the
new type.

\schemedisplay
(define marble
  (make-record-type "marble"
    '(color quality)
    (lambda (r p wr)
      (display "#<" p)
      (wr (marble-quality r) p)
      (display " quality " p)
      (wr (marble-color r) p)
      (display " marble>" p))))
(define make-marble
  (record-constructor marble))
(define marble?
  (record-predicate marble))
(define marble-color
  (record-field-accessor marble 'color))
(define marble-quality
  (record-field-accessor marble 'quality))
(define set-marble-quality!
  (record-field-mutator marble 'quality))
(define x (make-marble 'blue 'high))
(marble? x) ;=> #t
(marble-quality x) ;=> high
(set-marble-quality! x 'low)
(marble-quality x) ;=> low
x ;=> #<low quality blue marble>
\endschemedisplay

The order in which the fields appear in \var{fields} is important.
While field names are generally distinct, it is permissible for one field
name to be the same as another in the list of fields or the same as
an inherited name.
In this case, \index{ordinals}\index{record field ordinals}field ordinals
must be used to select fields in calls to \scheme{record-field-accessor}
and \scheme{record-field-mutator}.
Ordinals range from zero through one less than the number of fields.
Parent fields come first, if any, followed by the fields in
\var{fields}, in the order given.

\schemedisplay
(define r1 (make-record-type "r1" '(t t)))
(define r2 (make-record-type r1 "r2" '(t)))
(define r3 (make-record-type r2 "r3" '(t t t)))

(define x ((record-constructor r3) 'a 'b 'c 'd 'e 'f))
((record-field-accessor r3 0) x) ;=> a
((record-field-accessor r3 2) x) ;=> c
((record-field-accessor r3 4) x) ;=> e
((record-field-accessor r3 't) x) ;=> \var{unspecified}
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{record-constructor}{\categoryprocedure}{(record-constructor \var{rcd})}
\formdef{record-constructor}{\categoryprocedure}{(record-constructor \var{rtd})}
\returns a constructor for records of the type represented by \var{rtd}
\listlibraries
\endentryheader

\noindent
Like the Revised$^6$ Report version of this procedure, this procedure
may be passed a record-constructor descriptor, \var{rcd}, which determines
the behavior of the constructor.
It may also be passed a record-type descriptor, \var{rtd}, in which
case the constructor accepts as many arguments as there are fields in the
record; these arguments are the initial values of the fields in the
order given when the record-type descriptor was created.


%----------------------------------------------------------------------------
\entryheader
\formdef{record-field-accessor}{\categoryprocedure}{(record-field-accessor \var{rtd} \var{field-id})}
\returns an accessor for the identified field
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor, \var{field-id} must be
a symbol or field ordinal, i.e., a nonnegative exact integer less than
the number of fields of the given record type.
The specified field must be accessible.

The generated accessor expects one argument, which must be a record of
the type represented by \var{rtd}.
It returns the contents of the specified field of the record.


%----------------------------------------------------------------------------
\entryheader
\formdef{record-field-accessible?}{\categoryprocedure}{(record-field-accessible? \var{rtd} \var{field-id})}
\returns \scheme{#t} if the specified field is accessible, otherwise \scheme{#f}
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor, \var{field-id} must be
a symbol or field ordinal, i.e., a nonnegative exact integer less than
the number of fields of the given record type.

The compiler is free to eliminate a record field if it can prove that
the field is not accessed.
In making this determination, the compiler is free to ignore the
possibility that an accessor might be created from a record-type
descriptor obtained by calling \scheme{record-type-descriptor} on an
instance of the record type.


%----------------------------------------------------------------------------
\entryheader
\formdef{record-field-mutator}{\categoryprocedure}{(record-field-mutator \var{rtd} \var{field-id})}
\returns a mutator for the identified field
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor, \var{field-id} must be
a symbol or field ordinal, i.e., a nonnegative exact integer less than
the number of fields of the given record type.
The specified field must be mutable.

\noindent
The mutator expects two arguments, \var{r} and \var{obj}.
\var{r} must be a record of the type represented by \var{rtd}.
\var{obj} must be a value that is compatible with the type declared for
the specified field when the record-type descriptor was created.
\var{obj} is stored in the specified field of the record.


%----------------------------------------------------------------------------
\entryheader
\formdef{record-field-mutable?}{\categoryprocedure}{(record-field-mutable? \var{rtd} \var{field-id})}
\returns \scheme{#t} if the specified field is mutable, otherwise \scheme{#f}
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor, \var{field-id} must be
a symbol or field ordinal, i.e., a nonnegative exact integer less than
the number of fields of the given record type.

Any field declared immutable is immutable.
In addition,
the compiler is free to treat a field as immutable if it can prove that
the field is never assigned.
In making this determination, the compiler is free to ignore the
possibility that a mutator might be created from a record-type
descriptor obtained by calling \scheme{record-type-descriptor} on an
instance of the record type.


%----------------------------------------------------------------------------
\entryheader
\formdef{record-type-name}{\categoryprocedure}{(record-type-name \var{rtd})}
\returns the name of the record-type represented by \var{rtd}
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor.

The name is a always a string.
If a gensym is provided as the record-type name in a
\scheme{define-record} form or \scheme{make-record-type} call, the result
is the ``pretty'' name of the gensym (see~\ref{desc:gensym}).

\schemedisplay
(record-type-name (make-record-type "empty" '())) ;=> "empty"

(define-record #{point bdhavk1bwafxyss1-b} (x y))
(define p (type-descriptor #{point bdhavk1bwafxyss1-b}))
(record-type-name p) ;=> "point"
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{record-type-symbol}{\categoryprocedure}{(record-type-symbol \var{rtd})}
\returns the generated symbol associated with \var{rtd}
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor.

\schemedisplay
(define e (make-record-type "empty" '()))
(record-type-symbol e) ;=> #{empty bdhavk1bwafxyss1-e}

(define-record #{point bdhavk1bwafxyss1-b} (x y))
(define p (type-descriptor #{point bdhavk1bwafxyss1-b}))
(record-type-symbol p) ;=> #{point bdhavk1bwafxyss1-b}
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{record-type-field-names}{\categoryprocedure}{(record-type-field-names \var{rtd})}
\returns a list of field names of the type represented by \var{rtd}
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor.
The field names are symbols.

\schemedisplay
(define-record triple ((immutable x1) (mutable x2) (immutable x3)))
(record-type-field-names (type-descriptor triple)) ;=> (x1 x2 x3)
\endschemedisplay

%----------------------------------------------------------------------------
\entryheader
\formdef{record-type-field-decls}{\categoryprocedure}{(record-type-field-decls \var{rtd})}
\returns a list of field declarations of the type represented by \var{rtd}
\listlibraries
\endentryheader

\noindent
\var{rtd} must be a record-type descriptor.
Each field declaration has the following form:

\schemedisplay
(\var{class} \var{type} \var{field-name})
\endschemedisplay

\noindent
where \var{class}, \var{type}, and \var{field-name} are as described
under \scheme{make-record-type}.

\schemedisplay
(define-record shape (x y))
(define-record circle shape (radius))

(record-type-field-decls
  (type-descriptor circle)) ;=> ((mutable ptr x)
                            ;==  (mutable ptr y)
                            ;==  (mutable ptr radius))
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{record?}{\categoryprocedure}{(record? \var{obj})}
\returns \scheme{#t} if \var{obj} is a record, otherwise \scheme{#f}
\formdef{record?}{\categoryprocedure}{(record? \var{obj} \var{rtd})}
\returns \scheme{#t} if \var{obj} is a record of the given type, otherwise \scheme{#f}
\listlibraries
\endentryheader

\noindent
If present, \var{rtd} must be a record-type descriptor.

A record is ``of the given type'' if it is an instance of the record
type or one of its ancestors.
The predicate generated by \scheme{record-predicate} for a
record-type descriptor \var{rtd} is equivalent to the following.

\schemedisplay
(lambda (x) (record? x \var{rtd}))
\endschemedisplay


%----------------------------------------------------------------------------
\entryheader
\formdef{record-type-descriptor}{\categoryprocedure}{(record-type-descriptor \var{rec})}
\returns the record-type descriptor of \var{rec}
\listlibraries
\endentryheader

\noindent
\var{rec} must be a record.
This procedure is intended for use in the definition of portable printers
and debuggers.
For records created with \scheme{make-record-type},
it may not be the same as the descriptor returned by
\scheme{make-record-type}.
See the comments about field accessibility and mutability under
\scheme{record-field-accessible?} and
\scheme{record-field-mutable?} above.

This procedure is equivalent to the Revised$^6$ Report \scheme{record-rtd}
procedure.

\schemedisplay
(define rtd (make-record-type "frob" '(blit blat)))
rtd ;=> #<record type frob>
(define x ((record-constructor rtd) 1 2))
(record-type-descriptor x) ;=> #<record type frob>
(eq? (record-type-descriptor x) rtd) ;=> \var{unspecified}
\endschemedisplay