#lang scribble/doc
@(require "mz.rkt" scribble/scheme racket/generator racket/list
          (for-syntax racket/base)
          (for-label racket/list))

@(define (generate-c_r-example proc)
  (define (make-it start n)
    (generator ()
      (let loop ([start start]
                 [n n])
        (yield (list* n start))
        (yield (append start (list n)))
        (when (< (length (flatten start)) 8)
          (loop (list* n start) (add1 n))
          (loop (list start n) (add1 n))
          ))))
  (define (example proc)
    (define maker (make-it '() 1))
    (let loop ([value (maker)])
      (with-handlers ([exn? (lambda (e) (loop (maker)))])
        (proc value)
        value)))
  (example proc))

@(define-syntax (defc_r stx)
   (syntax-case stx ()
     [(_ x ... example-arg)
      (let ([xs (map syntax-e (syntax->list #'(x ...)))])
        (let ([name (string->symbol
                     (string-append
                      "c"
                      (apply string-append (map symbol->string xs))
                      "r"))]
              [contract (let loop ([l (reverse xs)])
                          (cond
                            [(null? (cdr l)) 'pair?]
                            [(eq? (car l) 'a) `(cons/c ,(loop (cdr l)) any/c)]
                            [(eq? (car l) 'd) `(cons/c any/c ,(loop (cdr l)))]))]
              [equiv (let loop ([l xs])
                       (cond
                         [(null? l) 'v]
                         [(eq? (car l) 'a) #`(car #,(loop (cdr l)))]
                         [(eq? (car l) 'd) #`(cdr #,(loop (cdr l)))]))])
          (with-syntax ([name name]
                        [contract (let loop ([c contract] [pos 0])
                                    (if (pair? c)
                                      (let* ([a (loop (car c) (add1 pos))]
                                             [b (loop (cdr c) (+ 1 pos (syntax-span a)))]
                                             [span (+ 1 (syntax-span a) (syntax-span b))])
                                        (datum->syntax #'here
                                                       (cons a b)
                                                       (list (syntax-source stx)
                                                             1
                                                             pos
                                                             (add1 pos)
                                                             span)))
                                      (datum->syntax #'here c
                                                     (list (syntax-source stx) 1 pos (add1 pos) 1))))]
                        [example (let ([ex #'example-arg])
                                   (datum->syntax #'here
                                                  (list
                                                   (datum->syntax #'here
                                                                  name
                                                                  (vector (syntax-source ex)
                                                                          (syntax-line ex)
                                                                          (- (syntax-column ex) 2)
                                                                          (- (syntax-position ex) 2)
                                                                          1))
                                                   ex)
                                                  (vector (syntax-source ex)
                                                          (syntax-line ex)
                                                          (- (syntax-column ex) 3)
                                                          (- (syntax-position ex) 3)
                                                          (+ (syntax-span ex) 4))))]
                        [equiv equiv])
            #'(defproc (name [v contract]) any/c
                "Returns " (racket equiv) "." (mz-examples example)))))]))


@title[#:tag "pairs"]{Pairs and Lists}

@guideintro["pairs"]{pairs and lists}

A @deftech{pair} combines exactly two values. The first value is
accessed with the @racket[car] procedure, and the second value is
accessed with the @racket[cdr] procedure. Pairs are not mutable (but
see @secref["mpairs"]).

A @deftech{list} is recursively defined: it is either the constant
@racket[null], or it is a pair whose second value is a list.

A list can be used as a single-valued sequence (see
@secref["sequences"]). The elements of the list serve as elements
of the sequence. See also @racket[in-list].

Cyclic data structures can be created using only immutable pairs via
@racket[read] or @racket[make-reader-graph]. If starting with a pair
and using some number of @racket[cdr]s returns to the starting pair,
then the pair is not a list.

@see-read-print["pair" #:print "pairs"]{pairs and lists}


@; ----------------------------------------
@section{Pair Constructors and Selectors}

@defproc[(pair? [v any/c])
         boolean?]{

Returns @racket[#t] if @racket[v] is a pair, @racket[#f] otherwise.

@mz-examples[
  (pair? 1)
  (pair? (cons 1 2))
  (pair? (list 1 2))
  (pair? '(1 2))
  (pair? '())]}


@defproc[(null? [v any/c])
         boolean?]{

Returns @racket[#t] if @racket[v] is the empty list, @racket[#f]
otherwise.

@mz-examples[
  (null? 1)
  (null? '(1 2))
  (null? '())
  (null? (cdr (list 1)))]}


@defproc[(cons [a any/c] [d any/c])
         pair?]{

Returns a newly allocated pair whose first element is @racket[a] and
second element is @racket[d].

@mz-examples[
  (cons 1 2)
  (cons 1 '())]}


@defproc[(car [p pair?])
         any/c]{

Returns the first element of the pair @racket[p].

@mz-examples[
  (car '(1 2))
  (car (cons 2 3))]}


@defproc[(cdr [p pair?])
         any/c]{

Returns the second element of the pair @racket[p].

@mz-examples[
  (cdr '(1 2))
  (cdr '(1))]}


@defthing[null null?]{

  The empty list.

  @mz-examples[
    null
    '()
    (eq? '() null)]}


@defproc[(list? [v any/c])
         boolean?]{

Returns @racket[#t] if @racket[v] is a list: either the empty list, or a
pair whose second element is a list.  This procedure effectively takes
constant time due to internal caching (so that any necessary traversals
of pairs can in principle count as an extra cost of allocating the
pairs).

@mz-examples[
  (list? '(1 2))
  (list? (cons 1 (cons 2 '())))
  (list? (cons 1 2))]}


@defproc[(list [v any/c] ...)
         list?]{

Returns a newly allocated list containing the @racket[v]s as its
elements.

@mz-examples[
  (list 1 2 3 4)
  (list (list 1 2) (list 3 4))]}


@defproc[(list* [v any/c] ... [tail any/c])
         any/c]{

Like @racket[list], but the last argument is used as the tail of the
result, instead of the final element.  The result is a list only if the
last argument is a list.

@mz-examples[
 (list* 1 2)
 (list* 1 2 (list 3 4))]}


@defproc[(build-list [n exact-nonnegative-integer?]
                     [proc (exact-nonnegative-integer? . -> . any)])
         list?]{

Creates a list of @racket[n] elements by applying @racket[proc] to the
integers from @racket[0] to @racket[(sub1 n)] in order.  If
@racket[_lst] is the resulting list, then @racket[(list-ref _lst _i)] is
the value produced by @racket[(proc _i)].

@mz-examples[
  (build-list 10 values)
  (build-list 5 (lambda (x) (* x x)))]}


@; ----------------------------------------
@section{List Operations}

@defproc[(length [lst list?])
         exact-nonnegative-integer?]{

Returns the number of elements in @racket[lst]. This function takes
time proportional to that length.

@mz-examples[
  (length (list 1 2 3 4))
  (length '())]}


@defproc[(list-ref [lst pair?] [pos exact-nonnegative-integer?])
         any/c]{

Returns the element of @racket[lst] at position @racket[pos], where the
list's first element is position @racket[0].  If the list has
@racket[pos] or fewer elements, then the @exnraise[exn:fail:contract].

The @racket[lst] argument need not actually be a list; @racket[lst] must
merely start with a chain of at least @racket[(add1 pos)] pairs.

This function takes time proportional to @racket[pos].

@mz-examples[
  (list-ref (list 'a 'b 'c) 0)
  (list-ref (list 'a 'b 'c) 1)
  (list-ref (list 'a 'b 'c) 2)
  (list-ref (cons 1 2) 0)
  (eval:error (list-ref (cons 1 2) 1))]}


@defproc[(list-tail [lst any/c] [pos exact-nonnegative-integer?])
         any/c]{

Returns the list after the first @racket[pos] elements of @racket[lst].
If the list has fewer than @racket[pos] elements, then the
@exnraise[exn:fail:contract].

The @racket[lst] argument need not actually be a list; @racket[lst]
must merely start with a chain of at least @racket[pos] pairs.

This function takes time proportional to @racket[pos].

@mz-examples[
  (list-tail (list 1 2 3 4 5) 2)
  (list-tail (cons 1 2) 1)
  (eval:error (list-tail (cons 1 2) 2))
  (list-tail 'not-a-pair 0)]}


@defproc*[([(append [lst list?] ...) list?]
           [(append [lst list?] ... [v any/c]) any/c])]{

When given all list arguments, the result is a list that contains all of
the elements of the given lists in order.  The last argument is used
directly in the tail of the result.

The last argument need not be a list, in which case the result is an
``improper list.''

This function takes time proportional to the length of all arguments
(added together) except the last argument.

@mz-examples[
  (append (list 1 2) (list 3 4))
  (append (list 1 2) (list 3 4) (list 5 6) (list 7 8))]}


@defproc[(reverse [lst list?]) list?]{

Returns a list that has the same elements as @racket[lst], but in
reverse order.

This function takes time proportional to the length of @racket[lst].

@mz-examples[
  (reverse (list 1 2 3 4))]}


@; ----------------------------------------
@section{List Iteration}

@defproc[(map [proc procedure?] [lst list?] ...+)
         list?]{

Applies @racket[proc] to the elements of the @racket[lst]s from the
first elements to the last.  The @racket[proc] argument must accept the
same number of arguments as the number of supplied @racket[lst]s, and
all @racket[lst]s must have the same number of elements.  The result is
a list containing each result of @racket[proc] in order.

@mz-examples[
  (map (lambda (number)
         (+ 1 number))
       '(1 2 3 4))
  (map (lambda (number1 number2)
         (+ number1 number2))
       '(1 2 3 4)
       '(10 100 1000 10000))]}


@defproc[(andmap [proc procedure?] [lst list?] ...+)
          any]{

Similar to @racket[map] in the sense that @racket[proc] is applied to
each element of @racket[lst], but

@margin-note{The @racket[andmap] function is actually closer to
  @racket[foldl] than @racket[map], since @racket[andmap] doesn't
  produce a list.  Still, @racket[(andmap f (list x y z))] is equivalent
  to @racket[(and (f x) (f y) (f z))] in the same way that
  @racket[(map f (list x y z))] is equivalent to
  @racket[(list (f x) (f y) (f z))].}

@itemize[

 @item{the result is @racket[#f] if any application of @racket[proc]
       produces @racket[#f], in which case @racket[proc] is not applied
       to later elements of the @racket[lst]s; and}

 @item{the result is that of @racket[proc] applied to the last elements
       of the @racket[lst]s; more specifically, the application of
       @racket[proc] to the last elements in the @racket[lst]s is in tail
       position with respect to the @racket[andmap] call.}]

If the @racket[lst]s are empty, then @racket[#t] is returned.

@mz-examples[
  (andmap positive? '(1 2 3))
  (eval:error (andmap positive? '(1 2 a)))
  (andmap positive? '(1 -2 a))
  (andmap + '(1 2 3) '(4 5 6))]}


@defproc[(ormap [proc procedure?] [lst list?] ...+)
         any]{

Similar to @racket[map] in the sense that @racket[proc] is applied to
each element of @racket[lst], but

@margin-note{To continue the @racket[andmap] note above,
  @racket[(ormap f (list x y z))] is equivalent to
  @racket[(or (f x) (f y) (f z))].}

@itemize[

 @item{the result is @racket[#f] if every application of @racket[proc]
       produces @racket[#f]; and}

 @item{the result is that of the first application of @racket[proc]
       producing a value other than @racket[#f], in which case
       @racket[proc] is not applied to later elements of the
       @racket[lst]s; the application of @racket[proc] to the last
       elements of the @racket[lst]s is in tail position with respect to
       the @racket[ormap] call.}]

If the @racket[lst]s are empty, then @racket[#f] is returned.

@mz-examples[
  (ormap eq? '(a b c) '(a b c))
  (ormap positive? '(1 2 a))
  (ormap + '(1 2 3) '(4 5 6))]}


@defproc[(for-each [proc procedure?] [lst list?] ...+)
         void?]{

Similar to @racket[map], but @racket[proc] is called only for its
effect, and its result (which can be any number of values) is ignored.

@mz-examples[
  (for-each (lambda (arg)
              (printf "Got ~a\n" arg)
              23)
            '(1 2 3 4))]}


@defproc[(foldl [proc procedure?] [init any/c] [lst list?] ...+)
         any/c]{

Like @racket[map], @racket[foldl] applies a procedure to the elements of
one or more lists.  Whereas @racket[map] combines the return values into
a list, @racket[foldl] combines the return values in an arbitrary way
that is determined by @racket[proc].

If @racket[foldl] is called with @math{n} lists, then @racket[proc] must
take @math{n+1} arguments.  The extra argument is the combined return
values so far.  The @racket[proc] is initially invoked with the first
item of each list, and the final argument is @racket[init].  In
subsequent invocations of @racket[proc], the last argument is the return
value from the previous invocation of @racket[proc].  The input
@racket[lst]s are traversed from left to right, and the result of the
whole @racket[foldl] application is the result of the last application
of @racket[proc].  If the @racket[lst]s are empty, the result is
@racket[init].

Unlike @racket[foldr], @racket[foldl] processes the @racket[lst]s in
constant space (plus the space for each call to @racket[proc]).

@mz-examples[
  (foldl cons '() '(1 2 3 4))
  (foldl + 0 '(1 2 3 4))
  (foldl (lambda (a b result)
           (* result (- a b)))
         1
         '(1 2 3)
         '(4 5 6))]}


@defproc[(foldr [proc procedure?] [init any/c] [lst list?] ...+)
         any/c]{

Like @racket[foldl], but the lists are traversed from right to left.
Unlike @racket[foldl], @racket[foldr] processes the @racket[lst]s in
space proportional to the length of @racket[lst]s (plus the space for
each call to @racket[proc]).

@mz-examples[
  (foldr cons '() '(1 2 3 4))
  (foldr (lambda (v l) (cons (add1 v) l)) '() '(1 2 3 4))]}


@; ----------------------------------------
@section{List Filtering}

@defproc[(filter [pred procedure?] [lst list?])
         list?]{

Returns a list with the elements of @racket[lst] for which @racket[pred]
produces a true value.  The @racket[pred] procedure is applied to each
element from first to last.

@mz-examples[
  (filter positive? '(1 -2 3 4 -5))]}


@defproc[(remove [v any/c] [lst list?] [proc procedure? equal?])
         list?]{

Returns a list that is like @racket[lst], omitting the first element of
@racket[lst] that is equal to @racket[v] using the comparison procedure
@racket[proc] (which must accept two arguments),
with @racket[v] as the first argument and an element in @racket[lst] as the second argument.
If no element in @racket[lst] is equal to @racket[v] (according to @racket[proc]),
@racket[lst] is returned unchanged.

@mz-examples[
  (remove 2 (list 1 2 3 2 4))
  (remove '(2) (list '(1) '(2) '(3)))
  (remove "2" (list "1" "2" "3"))
  (remove #\c (list #\a #\b #\c))
  (remove "B" (list "a" "A" "b" "B") string-ci=?)
  (remove 5 (list 1 2 3 2 4))]

@history[#:changed "8.2.0.2"
         @elem{Guaranteed that the output is @racket[eq?] to @racket[lst]
               if no removal occurs.}]}


@defproc[(remq [v any/c] [lst list?])
         list?]{

Returns @racket[(remove v lst eq?)].

@mz-examples[
  (remq 2 (list 1 2 3 4 5))
  (remq '(2) (list '(1) '(2) '(3)))
  (remq "2" (list "1" "2" "3"))
  (remq #\c (list #\a #\b #\c))]}


@defproc[(remv [v any/c] [lst list?])
         list?]{

Returns @racket[(remove v lst eqv?)].

@mz-examples[
  (remv 2 (list 1 2 3 4 5))
  (remv '(2) (list '(1) '(2) '(3)))
  (remv "2" (list "1" "2" "3"))
  (remv #\c (list #\a #\b #\c))]}


@defproc[(remove* [v-lst list?] [lst list?] [proc procedure? equal?])
         list?]{

Like @racket[remove], but removes from @racket[lst] every instance of
every element of @racket[v-lst].

@mz-examples[
  (remove* (list 1 2) (list 1 2 3 2 4 5 2))]

@history[#:changed "8.2.0.2"
         @elem{Guaranteed that the output is @racket[eq?] to @racket[lst]
               if no removal occurs.}]}


@defproc[(remq* [v-lst list?] [lst list?])
         list?]{

Returns @racket[(remove* v-lst lst eq?)].

@mz-examples[
  (remq* (list 1 2) (list 1 2 3 2 4 5 2))]}


@defproc[(remv* [v-lst list?] [lst list?])
         list?]{

Returns @racket[(remove* v-lst lst eqv?)].

@mz-examples[
  (remv* (list 1 2) (list 1 2 3 2 4 5 2))]}


@defproc[(sort [lst list?] [less-than? (any/c any/c . -> . any/c)]
               [#:key extract-key (any/c . -> . any/c) (lambda (x) x)]
               [#:cache-keys? cache-keys? boolean? #f])
         list?]{

Returns a list sorted according to the @racket[less-than?] procedure,
which takes two elements of @racket[lst] and returns a true value if the
first is less (i.e., should be sorted earlier) than the second.

The sort is stable; if two elements of @racket[lst] are ``equal''
(i.e., @racket[less-than?] does not return a true value when given the pair in
either order), then the elements preserve their relative order from
@racket[lst] in the output list.  To preserve this guarantee, use
@racket[sort] with a strict comparison functions (e.g., @racket[<] or
@racket[string<?]; not @racket[<=] or @racket[string<=?]).

@margin-note{Because of the peculiar fact that the IEEE-754 number system
specifies that +nan.0 is neither greater nor less than nor equal to any other
number, sorting lists containing this value may produce a surprising result.}

The @racket[#:key] argument @racket[extract-key] is used to extract a
key value for comparison from each list element.  That is, the full
comparison procedure is essentially

@racketblock[
  (lambda (x y)
    (less-than? (extract-key x) (extract-key y)))]

By default, @racket[extract-key] is applied to two list elements for
every comparison, but if @racket[cache-keys?] is true, then the
@racket[extract-key] function is used exactly once for each list item.
Supply a true value for @racket[cache-keys?] when @racket[extract-key]
is an expensive operation; for example, if
@racket[file-or-directory-modify-seconds] is used to extract a timestamp
for every file in a list, then @racket[cache-keys?] should be
@racket[#t] to minimize file-system calls, but if @racket[extract-key]
is @racket[car], then @racket[cache-keys?]  should be @racket[#f].  As
another example, providing @racket[extract-key] as
@racket[(lambda (x) (random))] and @racket[#t] for @racket[cache-keys?]
effectively shuffles the list.}

@mz-examples[
  (sort '(1 3 4 2) <)
  (sort '("aardvark" "dingo" "cow" "bear") string<?)
  (sort '(("aardvark") ("dingo") ("cow") ("bear"))
        #:key car string<?)]


@; ----------------------------------------
@section{List Searching}

@defproc[(member [v any/c] [lst (or/c list? any/c)]
                 [is-equal? (any/c any/c -> any/c) equal?])
         (or/c #f list? any/c)]{

Locates the first element of @racket[lst] that is @racket[equal?] to
@racket[v].  If such an element exists, the tail of @racket[lst]
starting with that element is returned.  Otherwise, the result is
@racket[#f].

The @racket[lst] argument need not actually be a list; @racket[lst]
must merely start with a chain of pairs until a matching element is
found. If no matching element is found, then @racket[lst] must be a
list (and not a cyclic list). The result can be a non-list in the case
that an element is found and the returned tail of @racket[lst] is a
non-list.

@mz-examples[
  (member 2 (list 1 2 3 4))
  (member 9 (list 1 2 3 4))
  (member #'x (list #'x #'y) free-identifier=?)
  (member #'a (list #'x #'y) free-identifier=?)
  (member 'b '(a b . etc))]}


@defproc[(memv [v any/c] [lst (or/c list? any/c)])
         (or/c #f list? any/c)]{

Like @racket[member], but finds an element using @racket[eqv?].

@mz-examples[
  (memv 2 (list 1 2 3 4))
  (memv 9 (list 1 2 3 4))]}


@defproc[(memq [v any/c] [lst (or/c list? any/c)])
         (or/c #f list? any/c)]{

Like @racket[member], but finds an element using @racket[eq?].

@mz-examples[
  (memq 2 (list 1 2 3 4))
  (memq 9 (list 1 2 3 4))]}


@defproc[(memf [proc procedure?] [lst (or/c list? any/c)])
         (or/c #f list? any/c)]{

Like @racket[member], but finds an element using the predicate
@racket[proc]; an element is found when @racket[proc] applied to the
element returns a true value.

@mz-examples[
  (memf (lambda (arg)
          (> arg 9))
        '(7 8 9 10 11))]}


@defproc[(findf [proc procedure?] [lst list?])
         any/c]{

Like @racket[memf], but returns the element or @racket[#f] instead of a
tail of @racket[lst] or @racket[#f].

@mz-examples[
  (findf (lambda (arg)
           (> arg 9))
         '(7 8 9 10 11))]}


@defproc[(assoc [v any/c]
                [lst (or/c (listof pair?) any/c)]
                [is-equal? (any/c any/c -> any/c) equal?])
         (or/c pair? #f)]{

Locates the first element of @racket[lst] whose @racket[car] is equal to
@racket[v] according to @racket[is-equal?].  If such an element exists,
the pair (i.e., an element of @racket[lst]) is returned.  Otherwise, the
result is @racket[#f].

The @racket[lst] argument need not actually be a list of pairs;
@racket[lst] must merely start with a chain of pairs contains pairs
until a matching element is found. If no matching element is found,
then @racket[lst] must be a list of pairs (and not a cyclic list).

@mz-examples[
  (assoc 3 (list (list 1 2) (list 3 4) (list 5 6)))
  (assoc 9 (list (list 1 2) (list 3 4) (list 5 6)))
  (assoc 3.5
         (list (list 1 2) (list 3 4) (list 5 6))
         (lambda (a b) (< (abs (- a b)) 1)))]}


@defproc[(assv [v any/c] [lst (or/c (listof pair?) any/c)])
         (or/c pair? #f)]{

Like @racket[assoc], but finds an element using @racket[eqv?].

@mz-examples[
  (assv 3 (list (list 1 2) (list 3 4) (list 5 6)))]}


@defproc[(assq [v any/c] [lst (or/c (listof pair?) any/c)])
         (or/c pair? #f)]{

Like @racket[assoc], but finds an element using @racket[eq?].

@mz-examples[
  (assq 'c (list (list 'a 'b) (list 'c 'd) (list 'e 'f)))]}


@defproc[(assf [proc procedure?] [lst (or/c (listof pair?) any/c)])
         (or/c pair? #f)]{

Like @racket[assoc], but finds an element using the predicate
@racket[proc]; an element is found when @racket[proc] applied to the
@racket[car] of an @racket[lst] element returns a true value.

@mz-examples[
  (assf (lambda (arg)
          (> arg 2))
        (list (list 1 2) (list 3 4) (list 5 6)))]}


@; ----------------------------------------
@section{Pair Accessor Shorthands}

@defc_r[a a '((1 2) 3 4)]
@defc_r[a d '((1 2) 3 4)]
@defc_r[d a '((7 6 5 4 3 2 1) 8 9)]
@defc_r[d d '(2 1)]
@defc_r[a a a '(((6 5 4 3 2 1) 7) 8 9)]
@defc_r[a a d '(9 (7 6 5 4 3 2 1) 8)]
@defc_r[a d a '((7 6 5 4 3 2 1) 8 9)]
@defc_r[a d d '(3 2 1)]
@defc_r[d a a '(((6 5 4 3 2 1) 7) 8 9)]
@defc_r[d a d '(9 (7 6 5 4 3 2 1) 8)]
@defc_r[d d a '((7 6 5 4 3 2 1) 8 9)]
@defc_r[d d d '(3 2 1)]
@defc_r[a a a a '((((5 4 3 2 1) 6) 7) 8 9)]
@defc_r[a a a d '(9 ((6 5 4 3 2 1) 7) 8)]
@defc_r[a a d a '((7 (5 4 3 2 1) 6) 8 9)]
@defc_r[a a d d '(9 8 (6 5 4 3 2 1) 7)]
@defc_r[a d a a '(((6 5 4 3 2 1) 7) 8 9)]
@defc_r[a d a d '(9 (7 6 5 4 3 2 1) 8)]
@defc_r[a d d a '((7 6 5 4 3 2 1) 8 9)]
@defc_r[a d d d '(4 3 2 1)]
@defc_r[d a a a '((((5 4 3 2 1) 6) 7) 8 9)]
@defc_r[d a a d '(9 ((6 5 4 3 2 1) 7) 8)]
@defc_r[d a d a '((7 (5 4 3 2 1) 6) 8 9)]
@defc_r[d a d d '(9 8 (6 5 4 3 2 1) 7)]
@defc_r[d d a a '(((6 5 4 3 2 1) 7) 8 9)]
@defc_r[d d a d '(9 (7 6 5 4 3 2 1) 8)]
@defc_r[d d d a '((7 6 5 4 3 2 1) 8 9)]
@defc_r[d d d d '(4 3 2 1)]


@; ----------------------------------------
@section{Additional List Functions and Synonyms}

@note-lib[racket/list]
@(define list-eval (make-base-eval))
@examples[#:hidden #:eval list-eval
          (require racket/list (only-in racket/function negate))]


@defthing[empty null?]{

  The empty list.

  @mz-examples[#:eval list-eval
    empty
    (eq? empty null)]}


@defproc[(cons? [v any/c])
         boolean?]{

The same as @racket[(pair? v)].

@mz-examples[#:eval list-eval
  (cons? '(1 2))]}


@defproc[(empty? [v any/c])
         boolean?]{

The same as @racket[(null? v)].

@mz-examples[#:eval list-eval
  (empty? '(1 2))
  (empty? '())]}


@defproc[(first [lst list?])
         any/c]{

The same as @racket[(car lst)], but only for lists (that are not empty).

@mz-examples[#:eval list-eval
  (first '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(rest [lst list?])
         list?]{

The same as @racket[(cdr lst)], but only for lists (that are not empty).

@mz-examples[#:eval list-eval
  (rest '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(second [lst list?])
         any]{

Returns the second element of the list.

@mz-examples[#:eval list-eval
  (second '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(third [lst list?])
         any]{

Returns the third element of the list.

@mz-examples[#:eval list-eval
  (third '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(fourth [lst list?])
         any]{

Returns the fourth element of the list.

@mz-examples[#:eval list-eval
  (fourth '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(fifth [lst list?])
         any]{

Returns the fifth element of the list.

@mz-examples[#:eval list-eval
  (fifth '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(sixth [lst list?])
         any]{

Returns the sixth element of the list.

@mz-examples[#:eval list-eval
  (sixth '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(seventh [lst list?])
         any]{

Returns the seventh element of the list.

@mz-examples[#:eval list-eval
  (seventh '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(eighth [lst list?])
         any]{

Returns the eighth element of the list.

@mz-examples[#:eval list-eval
  (eighth '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(ninth [lst list?]) any]{

Returns the ninth element of the list.

@mz-examples[#:eval list-eval
  (ninth '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(tenth [lst list?]) any]{

Returns the tenth element of the list.

@mz-examples[#:eval list-eval
  (tenth '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(last [lst list?]) any]{

Returns the last element of the list.

This function takes time proportional to the length of @racket[lst].

@mz-examples[#:eval list-eval
  (last '(1 2 3 4 5 6 7 8 9 10))]}


@defproc[(last-pair [p pair?])
         pair?]{

Returns the last pair of a (possibly improper) list.

This function takes time proportional to the ``length'' of @racket[p].

@mz-examples[#:eval list-eval
  (last-pair '(1 2 3 4))]}


@defproc[(make-list [k exact-nonnegative-integer?] [v any/c])
         list?]{

Returns a newly constructed list of length @racket[k], holding
@racket[v] in all positions.

@mz-examples[#:eval list-eval
  (make-list 7 'foo)]}

@defproc[(list-update [lst list?]
                      [pos (and/c (>=/c 0) (</c (length lst)))]
                      [updater (-> any/c any/c)])
         list?]{

Returns a list that is the same as @racket[lst] except at the specified index.
The element at the specified index is @racket[(updater (list-ref lst pos))].

This function takes time proportional to @racket[pos].

@examples[#:eval list-eval
(list-update '(zero one two) 1 symbol->string)]
@history[#:added "6.3"]{}
}

@defproc[(list-set [lst list?]
                   [pos (and/c (>=/c 0) (</c (length lst)))]
                   [value any/c])
         list?]{

Returns a list that is the same as @racket[lst] except at the specified index.
The element at the specified index is @racket[value].

This function takes time proportional to @racket[pos].

@examples[#:eval list-eval
(list-set '(zero one two) 2 "two")]
@history[#:added "6.3"]{}
}

@defproc[(index-of [lst list?] [v any/c]
                   [is-equal? (any/c any/c . -> . any/c) equal?])
         (or/c exact-nonnegative-integer? #f)]{
Like @racket[member], but returns the index of the first element found
instead of the tail of the list.
                          
@mz-examples[#:eval list-eval
  (index-of '(1 2 3 4) 3)]

@history[#:added "6.7.0.3"]}

@defproc[(index-where [lst list?] [proc (any/c . -> . any/c)])
         (or/c exact-nonnegative-integer? #f)]{
Like @racket[index-of] but with the predicate-searching behavior of
@racket[memf].

@mz-examples[#:eval list-eval
  (index-where '(1 2 3 4) even?)]

@history[#:added "6.7.0.3"]}

@defproc[(indexes-of [lst list?] [v any/c]
                     [is-equal? (any/c any/c . -> . any/c) equal?])
         (listof exact-nonnegative-integer?)]{
Like @racket[index-of], but returns the a list of all the indexes
where the element occurs in the list instead of just the first one.
                          
@mz-examples[#:eval list-eval
  (indexes-of '(1 2 1 2 1) 2)]

@history[#:added "6.7.0.3"]}

@defproc[(indexes-where [lst list?] [proc (any/c . -> . any/c)])
         (listof exact-nonnegative-integer?)]{
Like @racket[indexes-of] but with the predicate-searching behavior of
@racket[index-where].

@mz-examples[#:eval list-eval
  (indexes-where '(1 2 3 4) even?)]

@history[#:added "6.7.0.3"]}

@defproc[(take [lst any/c] [pos exact-nonnegative-integer?])
         list?]{

Returns a fresh list whose elements are the first @racket[pos] elements
of @racket[lst].  If @racket[lst] has fewer than @racket[pos] elements,
the @exnraise[exn:fail:contract].

The @racket[lst] argument need not actually be a list; @racket[lst]
must merely start with a chain of at least @racket[pos] pairs.

This function takes time proportional to @racket[pos].

@mz-examples[#:eval list-eval
  (take '(1 2 3 4 5) 2)
  (take 'non-list 0)]}


@defproc[(drop [lst any/c] [pos exact-nonnegative-integer?])
         any/c]{

Just like @racket[list-tail].}


@defproc[(split-at [lst any/c] [pos exact-nonnegative-integer?])
         (values list? any/c)]{

Returns the same result as

@racketblock[(values (take lst pos) (drop lst pos))]

except that it can be faster, but it will still take time
proportional to @racket[pos].}


@defproc[(takef [lst any/c] [pred procedure?])
         list?]{

Returns a fresh list whose elements are taken successively from
@racket[lst] as long as they satisfy @racket[pred].  The returned list
includes up to, but not including, the first element in @racket[lst] for
which @racket[pred] returns @racket[#f].

The @racket[lst] argument need not actually be a list; the chain of
pairs in @racket[lst] will be traversed until a non-pair is encountered.

@mz-examples[#:eval list-eval
  (takef '(2 4 5 8) even?)
  (takef '(2 4 6 8) odd?)
  (takef '(2 4 . 6) even?)]}


@defproc[(dropf [lst any/c] [pred procedure?])
         any/c]{

Drops elements from the front of @racket[lst] as long as they satisfy
@racket[pred].

@mz-examples[#:eval list-eval
  (dropf '(2 4 5 8) even?)
  (dropf '(2 4 6 8) odd?)]}


@defproc[(splitf-at [lst any/c] [pred procedure?])
         (values list? any/c)]{

Returns the same result as

@racketblock[(values (takef lst pred) (dropf lst pred))]

except that it can be faster.}


@defproc[(take-right [lst any/c] [pos exact-nonnegative-integer?])
         any/c]{

Returns the @racket[list]'s @racket[pos]-length tail. If @racket[lst]
has fewer than @racket[pos] elements, then the
@exnraise[exn:fail:contract].

The @racket[lst] argument need not actually be a list; @racket[lst]
must merely end with a chain of at least @racket[pos] pairs.

This function takes time proportional to the length of @racket[lst].

@mz-examples[#:eval list-eval
  (take-right '(1 2 3 4 5) 2)
  (take-right 'non-list 0)]}


@defproc[(drop-right [lst any/c] [pos exact-nonnegative-integer?])
         list?]{

Returns a fresh list whose elements are the prefix of @racket[lst],
dropping its @racket[pos]-length tail.  If @racket[lst] has fewer than
@racket[pos] elements, then the @exnraise[exn:fail:contract].

The @racket[lst] argument need not actually be a list; @racket[lst] must
merely end with a chain of at least @racket[pos] pairs.

This function takes time proportional to the length of @racket[lst].

@mz-examples[#:eval list-eval
  (drop-right '(1 2 3 4 5) 2)
  (drop-right 'non-list 0)]}


@defproc[(split-at-right [lst any/c] [pos exact-nonnegative-integer?])
         (values list? any/c)]{

Returns the same result as

@racketblock[(values (drop-right lst pos) (take-right lst pos))]

except that it can be faster, but it will still take time proportional
to the length of @racket[lst].

@mz-examples[#:eval list-eval
  (split-at-right '(1 2 3 4 5 6) 3)
  (split-at-right '(1 2 3 4 5 6) 4)]}


@deftogether[(
  @defproc[(takef-right [lst any/c] [pred procedure?]) any/c]
  @defproc[(dropf-right [lst any/c] [pred procedure?]) list?]
  @defproc[(splitf-at-right [lst any/c] [pred procedure?]) (values list? any/c)]
)]{

Like @racket[takef], @racket[dropf], and @racket[splitf-at], but
combined with the from-right functionality of @racket[take-right],
@racket[drop-right], and @racket[split-at-right].}

@defproc[(list-prefix? [l list?]
                       [r list?]
                       [same? (any/c any/c . -> . any/c) equal?])
         boolean?]{
 True if @racket[l] is a prefix of @racket[r].
@examples[#:eval list-eval
(list-prefix? '(1 2) '(1 2 3 4 5))
]
@history[#:added "6.3"]{}
}

@defproc[(take-common-prefix [l list?] [r list?]
                             [same? (any/c any/c . -> . any/c) equal?])
         list?]{

  Returns the longest common prefix of @racket[l] and @racket[r].

@examples[#:eval list-eval
(take-common-prefix '(a b c d) '(a b x y z))
]
@history[#:added "6.3"]{}
}

@defproc[(drop-common-prefix [l list?] [r list?]
                             [same? (any/c any/c . -> . any/c) equal?])
         (values list? list?)]{

  Returns the tails of @racket[l] and @racket[r] with the common
  prefix removed.

@examples[#:eval list-eval
(drop-common-prefix '(a b c d) '(a b x y z))
]
@history[#:added "6.3"]{}
}

@defproc[(split-common-prefix [l list?] [r list?]
                              [same? (any/c any/c . -> . any/c) equal?])
         (values list? list? list?)]{

  Returns the longest common prefix together with the tails of
  @racket[l] and @racket[r] with the common prefix removed.

@examples[#:eval list-eval
(split-common-prefix '(a b c d) '(a b x y z))
]
@history[#:added "6.3"]{}
}


@defproc[(add-between [lst list?] [v any/c]
                      [#:before-first before-first list? '()]
                      [#:before-last  before-last  any/c v]
                      [#:after-last   after-last   list? '()]
                      [#:splice? splice? any/c #f])
         list?]{

Returns a list with the same elements as @racket[lst], but with
@racket[v] between each pair of elements in @racket[lst]; the last pair
of elements will have @racket[before-last] between them, instead of
@racket[v] (but @racket[before-last] defaults to @racket[v]).

If @racket[splice?] is true, then @racket[v] and @racket[before-last]
should be lists, and the list elements are spliced into the result.  In
addition, when @racket[splice?] is true, @racket[before-first] and
@racket[after-last] are inserted before the first element and after the
last element respectively.

@mz-examples[#:eval list-eval
  (add-between '(x y z) 'and)
  (add-between '(x) 'and)
  (add-between '("a" "b" "c" "d") "," #:before-last "and")
  (add-between '(x y z) '(-) #:before-last '(- -)
               #:before-first '(begin) #:after-last '(end LF)
               #:splice? #t)]}


@defproc*[([(append* [lst list?] ... [lsts (listof list?)]) list?]
           [(append* [lst list?] ... [lsts list?]) any/c])]{
@; Note: this is exactly the same description as the one for string-append*

Like @racket[append], but the last argument is used as a list of
arguments for @racket[append], so @racket[(append* lst ... lsts)] is the
same as @racket[(apply append lst ... lsts)].  In other words, the
relationship between @racket[append] and @racket[append*] is similar to
the one between @racket[list] and @racket[list*].

@mz-examples[#:eval list-eval
  (append* '(a) '(b) '((c) (d)))
  (cdr (append* (map (lambda (x) (list ", " x))
                     '("Alpha" "Beta" "Gamma"))))]}


@defproc[(flatten [v any/c])
         list?]{

Flattens an arbitrary S-expression structure of pairs into a list. More
precisely, @racket[v] is treated as a binary tree where pairs are
interior nodes, and the resulting list contains all of the
non-@racket[null] leaves of the tree in the same order as an inorder
traversal.

@mz-examples[#:eval list-eval
  (flatten '((a) b (c (d) . e) ()))
  (flatten 'a)]}


@defproc[(check-duplicates [lst list?]
                           [same? (any/c any/c . -> . any/c) equal?]
                           [#:key extract-key (-> any/c any/c) (lambda (x) x)]
                           [#:default failure-result failure-result/c (lambda () #f)])
         any]{

Returns the first duplicate item in @racket[lst]. More precisely, it
returns the first @racket[_x] such that there was a previous
@racket[_y] where @racket[(same? (extract-key _x) (extract-key _y))].

If no duplicate is found, then @racket[failure-result] determines the 
result:

@itemize[

 @item{If @racket[failure-result] is a procedure, it is called
       (through a tail call) with no arguments to produce the result.}

 @item{Otherwise, @racket[failure-result] is returned as the result.}

]

The @racket[same?] argument should be an equivalence predicate such as
@racket[equal?] or @racket[eqv?] or a dictionary.
The procedures @racket[equal?], @racket[eqv?], and @racket[eq?] automatically
use a dictionary for speed.

@examples[#:eval list-eval
(check-duplicates '(1 2 3 4))
(check-duplicates '(1 2 3 2 1))
(check-duplicates '((a 1) (b 2) (a 3)) #:key car)
(check-duplicates '(1 2 3 4 5 6)
                  (lambda (x y) (equal? (modulo x 3) (modulo y 3))))
(check-duplicates '(1 2 3 4) #:default "no duplicates")
]

@history[#:added "6.3"
         #:changed "6.11.0.2" @elem{Added the @racket[#:default] optional argument.}]}

@defproc[(remove-duplicates [lst list?]
                            [same? (any/c any/c . -> . any/c) equal?]
                            [#:key extract-key (any/c . -> . any/c)
                                   (lambda (x) x)])
         list?]{

Returns a list that has all items in @racket[lst], but without duplicate
items, where @racket[same?] determines whether two elements of the list
are equivalent.  The resulting list is in the same order as
@racket[lst], and for any item that occurs multiple times, the first one
is kept.

The @racket[#:key] argument @racket[extract-key] is used to extract a
key value from each list element, so two items are considered equal if
@racket[(same? (extract-key x) (extract-key y))] is true.

@mz-examples[#:eval list-eval
  (remove-duplicates '(a b b a))
  (remove-duplicates '(1 2 1.0 0))
  (remove-duplicates '(1 2 1.0 0) =)]}


@defproc[(filter-map [proc procedure?] [lst list?] ...+)
         list?]{

Like @racket[(map proc lst ...)], except that, if @racket[proc]
returns @racket[#false], that element is omitted from the resulting list. 
In other words, @racket[filter-map] is equivalent to 
@racket[(filter (lambda (x) x) (map proc lst ...))], but more efficient, 
because @racket[filter-map] avoids
building the intermediate list.

@mz-examples[#:eval list-eval
  (filter-map (lambda (x) (and (negative? x) (abs x))) '(1 2 -3 -4 8))]}


@defproc[(count [proc procedure?] [lst list?] ...+)
         exact-nonnegative-integer?]{

Returns @racket[(length (filter-map proc lst ...))], but without building
the intermediate list.

@mz-examples[#:eval list-eval
  (count positive? '(1 -1 2 3 -2 5))]}


@defproc[(partition [pred procedure?] [lst list?])
         (values list? list?)]{

Similar to @racket[filter], except that two values are returned: the
items for which @racket[pred] returns a true value, and the items for
which @racket[pred] returns @racket[#f].

The result is the same as

@racketblock[(values (filter pred lst) (filter (negate pred) lst))]

but @racket[pred] is applied to each item in @racket[lst] only once.

@mz-examples[#:eval list-eval
  (partition even? '(1 2 3 4 5 6))]}


@defproc*[([(range [end real?]) list?]
           [(range [start real?] [end real?] [step real? 1]) list?])]{

Similar to @racket[in-range], but returns lists.

The resulting list holds numbers starting at @racket[start] and whose
successive elements are computed by adding @racket[step] to their
predecessor until @racket[end] (excluded) is reached.  If no starting
point is provided, @racket[0] is used. If no @racket[step] argument is
provided, @racket[1] is used.

Like @racket[in-range], a @racket[range] application can provide better
performance when it appears directly in a @racket[for] clause.

@mz-examples[#:eval list-eval
  (range 10)
  (range 10 20)
  (range 20 40 2)
  (range 20 10 -1)
  (range 10 15 1.5)]

@history[#:changed "6.7.0.4"
         @elem{Adjusted to cooperate with @racket[for] in the same
               way that @racket[in-range] does.}]}


@defproc[(inclusive-range [start real?] [end real?] [step real? 1]) list?]{

Similar to @racket[in-inclusive-range], but returns lists.

The resulting list holds numbers starting at @racket[start] and whose
successive elements are computed by adding @racket[step] to their
predecessor until @racket[end] (included) is reached.
If no @racket[step] argument is provided, @racket[1] is used.

Like @racket[in-inclusive-range], an @racket[inclusive-range] application can provide better
performance when it appears directly in a @racket[for] clause.

@mz-examples[#:eval list-eval
  (inclusive-range 10 20)
  (inclusive-range 20 40 2)
  (inclusive-range 20 10 -1)
  (inclusive-range 10 15 1.5)]

@history[#:added "8.0.0.13"]

}


@defproc[(append-map [proc procedure?] [lst list?] ...+)
         list?]{

Returns @racket[(append* (map proc lst ...))].

@mz-examples[#:eval list-eval
  (append-map vector->list '(#(1) #(2 3) #(4)))]}


@defproc[(filter-not [pred (any/c . -> . any/c)] [lst list?])
         list?]{

Like @racket[filter], but the meaning of the @racket[pred] predicate is
reversed: the result is a list of all items for which @racket[pred]
returns @racket[#f].

@mz-examples[#:eval list-eval
  (filter-not even? '(1 2 3 4 5 6))]}


@defproc[(shuffle [lst list?])
         list?]{

Returns a list with all elements from @racket[lst], randomly shuffled.

@mz-examples[#:eval list-eval
  (shuffle '(1 2 3 4 5 6))
  (shuffle '(1 2 3 4 5 6))
  (shuffle '(1 2 3 4 5 6))]}


@defproc*[([(combinations [lst list?]) list?]
           [(combinations [lst list?] [size exact-nonnegative-integer?]) list?])]{
@margin-note{Wikipedia @hyperlink["https://en.wikipedia.org/wiki/Combination"]{combinations}}
Return a list of all combinations of elements in the input list
(aka the @index["powerset"]{powerset} of @racket[lst]).
If @racket[size] is given, limit results to combinations of @racket[size] elements.

@mz-examples[#:eval list-eval
  (combinations '(1 2 3))
  (combinations '(1 2 3) 2)]}


@defproc*[([(in-combinations [lst list?]) sequence?]
           [(in-combinations [lst list?] [size exact-nonnegative-integer?]) sequence?])]{
@index["in-powerset"]{Returns} a sequence of all combinations of elements in the input list,
 or all combinations of length @racket[size] if @racket[size] is given.
Builds combinations one-by-one instead of all at once.

@mz-examples[#:eval list-eval
  (time (begin (combinations (range 15)) (void)))
  (time (begin (in-combinations (range 15)) (void)))]}


@defproc[(permutations [lst list?])
         list?]{

@index["rearrangements"]{Returns} a list of all permutations of the input list.  Note that this
function works without inspecting the elements, and therefore it ignores
repeated elements (which will result in repeated permutations).
Raises an error if the input list contains more than 256 elements.

@mz-examples[#:eval list-eval
  (permutations '(1 2 3))
  (permutations '(x x))]}


@defproc[(in-permutations [lst list?])
         sequence?]{

@index["in-rearrangements"]{Returns} a sequence of all permutations of the input list.  It is
equivalent to @racket[(in-list (permutations l))] but much faster since
it builds the permutations one-by-one on each iteration.
Raises an error if the input list contains more than 256 elements.}


@defproc[(argmin [proc (-> any/c real?)] [lst (and/c pair? list?)])
         any/c]{

Returns the first element in the list @racket[lst] that minimizes the
result of @racket[proc].  Signals an error on an empty list.
See also @racket[min].

@mz-examples[#:eval list-eval
  (argmin car '((3 pears) (1 banana) (2 apples)))
  (argmin car '((1 banana) (1 orange)))]}


@defproc[(argmax [proc (-> any/c real?)] [lst (and/c pair? list?)])
         any/c]{

Returns the first element in the list @racket[lst] that maximizes the
result of @racket[proc].  Signals an error on an empty list.
See also @racket[max].

@mz-examples[#:eval list-eval
  (argmax car '((3 pears) (1 banana) (2 apples)))
  (argmax car '((3 pears) (3 oranges)))]}

@defproc[(group-by [key (-> any/c any/c)]
                   [lst list?]
                   [same? (any/c any/c . -> . any/c) equal?])
         (listof list?)]{

Groups the given list into equivalence classes, with equivalence being
determined by @racket[same?]. Within each equivalence class, @racket[group-by]
preserves the ordering of the original list. Equivalence classes themselves are
in order of first appearance in the input.

@examples[#:eval list-eval
(group-by (lambda (x) (modulo x 3)) '(1 2 1 2 54 2 5 43 7 2 643 1 2 0))
]
@history[#:added "6.3"]{}
}

@defproc[(cartesian-product [lst list?] ...)
         (listof list?)]{

Computes the n-ary cartesian product of the given lists.

@examples[#:eval list-eval
(cartesian-product '(1 2 3) '(a b c))
(cartesian-product '(4 5 6) '(d e f) '(#t #f))
]
@history[#:added "6.3"]{}
}

@defproc[(remf [pred procedure?]
               [lst list?])
         list?]{
Returns a list that is like @racket[lst], omitting the first element of @racket[lst]
for which @racket[pred] produces a true value.

@examples[
#:eval list-eval
(remf negative? '(1 -2 3 4 -5))
]
@history[#:added "6.3"]{}
}

@defproc[(remf* [pred procedure?]
                [lst list?])
         list?]{
Like @racket[remf], but removes all the elements for which @racket[pred]
produces a true value.

@examples[
#:eval list-eval
(remf* negative? '(1 -2 3 4 -5))
]
@history[#:added "6.3"]{}
}


@close-eval[list-eval]


@; ----------------------------------------
@section{Immutable Cyclic Data}

@defproc[(make-reader-graph [v any/c])
         any/c]{

Returns a value like @racket[v], with placeholders created by
@racket[make-placeholder] replaced with the values that they contain,
and with placeholders created by @racket[make-hash-placeholder] with an
immutable hash table.  No part of @racket[v] is mutated; instead, parts
of @racket[v] are copied as necessary to construct the resulting graph,
where at most one copy is created for any given value.

Since the copied values can be immutable, and since the copy is also
immutable, @racket[make-reader-graph] can create cycles involving only
immutable pairs, vectors, boxes, and hash tables.

Only the following kinds of values are copied and traversed to detect
placeholders:

@itemize[

 @item{pairs}

 @item{vectors, both mutable and immutable}

 @item{boxes, both mutable and immutable}

 @item{hash tables, both mutable and immutable}

 @item{instances of a @techlink{prefab} structure type}

 @item{placeholders created by @racket[make-placeholder] and
       @racket[make-hash-placeholder]}]

Due to these restrictions, @racket[make-reader-graph] creates exactly
the same sort of cyclic values as @racket[read].

@mz-examples[
  (let* ([ph (make-placeholder #f)]
         [x (cons 1 ph)])
    (placeholder-set! ph x)
    (make-reader-graph x))]}


@defproc[(placeholder? [v any/c])
         boolean?]{

Returns @racket[#t] if @racket[v] is a placeholder created by
@racket[make-placeholder], @racket[#f] otherwise.}


@defproc[(make-placeholder [v any/c])
         placeholder?]{

Returns a placeholder for use with @racket[placeholder-set!]  and
@racket[make-reader-graph]. The @racket[v] argument supplies the
initial value for the placeholder.}


@defproc[(placeholder-set! [ph placeholder?] [datum any/c])
         void?]{

Changes the value of @racket[ph] to @racket[v].}


@defproc[(placeholder-get [ph placeholder?])
         any/c]{

Returns the value of @racket[ph].}


@defproc[(hash-placeholder? [v any/c])
         boolean?]{

Returns @racket[#t] if @racket[v] is a placeholder created by
@racket[make-hash-placeholder], @racket[#f] otherwise.}


@defproc[(make-hash-placeholder [assocs (listof pair?)])
         hash-placeholder?]{

Like @racket[make-immutable-hash], but produces a table placeholder
for use with @racket[make-reader-graph].}


@defproc[(make-hasheq-placeholder [assocs (listof pair?)])
         hash-placeholder?]{

Like @racket[make-immutable-hasheq], but produces a table placeholder
for use with @racket[make-reader-graph].}


@defproc[(make-hasheqv-placeholder [assocs (listof pair?)])
         hash-placeholder?]{

Like @racket[make-immutable-hasheqv], but produces a table placeholder
for use with @racket[make-reader-graph].}
