861 lines
34 KiB
Racket
861 lines
34 KiB
Racket
#lang scribble/doc
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@(require "mz.ss"
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(for-syntax racket/base)
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scribble/scheme
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(for-label racket/generator
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racket/mpair))
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@(define (info-on-seq where what)
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@margin-note{See @secref[where] for information on using @|what| as
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sequences.})
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@title[#:style 'toc #:tag "sequences+streams"]{Sequences and Streams}
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@tech{Sequences} and @tech{streams} abstract over iteration of
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elements in a collection. Streams are functional sequences that can be
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used either in a generic way or a stream-specific
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way. @tech{Generators} are closely related stateful objects that can
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be converted to a sequence and vice-versa.
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@local-table-of-contents[]
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@; ======================================================================
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@section[#:tag "sequences"]{Sequences}
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@guideintro["sequences"]{sequences}
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A @deftech{sequence} encapsulates an ordered collection of values.
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The elements of a sequence can be extracted with one of the
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@scheme[for] syntactic forms, with the procedures returned by
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@scheme[sequence-generate], or by converting the sequence into a
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@tech{stream}.
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The sequence datatype overlaps with many other datatypes. Among
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built-in datatypes, the sequence datatype includes the following:
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@itemize[
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@item{strings (see @secref["strings"])}
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@item{byte strings (see @secref["bytestrings"])}
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@item{lists (see @secref["pairs"])}
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@item{mutable lists (see @secref["mpairs"])}
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@item{vectors (see @secref["vectors"])}
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@item{hash tables (see @secref["hashtables"])}
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@item{dictionaries (see @secref["dicts"])}
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@item{sets (see @secref["sets"])}
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@item{input ports (see @secref["ports"])}
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@item{streams (see @secref["streams"])}
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]
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The @scheme[make-do-sequence] function creates a sequence given a
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thunk that returns procedures to implement a sequence, and the
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@scheme[prop:sequence] property can be associated with a structure
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type to implement its implicit conversion to a sequence.
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For most sequence types, extracting elements from a sequence has no
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side-effect on the original sequence value; for example, extracting
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the sequence of elements from a list does not change the list. For
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other sequence types, each extraction implies a side effect; for
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example, extracting the sequence of bytes from a port causes the bytes
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to be read from the port. A sequence's state may either span all uses
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of the sequence, as for a port, or it may be confined to each distinct
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time that a sequence is @deftech{initiate}d by a @racket[for] form,
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@racket[sequence->stream], @racket[sequence-generate], or
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@racket[sequence-generate*]. Concretely, the thunk passed to
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@racket[make-do-sequence] is called to @tech{initiate} the sequence
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each time the sequence is used.
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Individual elements of a sequence typically correspond to single values,
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but an element may also correspond to multiple values. For example, a
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hash table generates two values---a key and its value---for each element
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in the sequence.
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@; ----------------------------------------------------------------------
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@subsection{Sequence Predicate and Constructors}
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@defproc[(sequence? [v any/c]) boolean?]{
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Returns @scheme[#t] if @scheme[v] can be used as a @tech{sequence},
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@scheme[#f] otherwise.}
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@defproc*[([(in-range [end number?]) stream?]
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[(in-range [start number?] [end number?] [step number? 1]) stream?])]{
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Returns a sequence (that is also a @tech{stream}) whose elements are numbers. The single-argument
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case @scheme[(in-range end)] is equivalent to @scheme[(in-range 0 end
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1)]. The first number in the sequence is @scheme[start], and each
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successive element is generated by adding @scheme[step] to the
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previous element. The sequence stops before an element that would be
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greater or equal to @scheme[end] if @scheme[step] is non-negative, or
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less or equal to @scheme[end] if @scheme[step] is negative.
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@speed[in-range "number"]}
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@defproc[(in-naturals [start exact-nonnegative-integer? 0]) stream?]{
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Returns an infinite sequence (that is also a @tech{stream}) of exact integers starting with
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@scheme[start], where each element is one more than the preceding
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element. @speed[in-naturals "integer"]}
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@defproc[(in-list [lst list?]) stream?]{
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Returns a sequence (that is also a @tech{stream}) that is equivalent
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to using @scheme[lst] directly as a sequence.
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@info-on-seq["pairs" "lists"]
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@speed[in-list "list"]}
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@defproc[(in-mlist [mlst mlist?]) sequence?]{
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Returns a sequence equivalent to @scheme[mlst].
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@info-on-seq["mpairs" "mutable lists"]
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@speed[in-mlist "mutable list"]}
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@defproc[(in-vector [vec vector?]
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[start exact-nonnegative-integer? 0]
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[stop (or/c exact-integer? #f) #f]
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[step (and/c exact-integer? (not/c zero?)) 1])
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sequence?]{
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Returns a sequence equivalent to @scheme[vec] when no optional
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arguments are supplied.
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@info-on-seq["vectors" "vectors"]
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The optional arguments @scheme[start], @scheme[stop], and
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@scheme[step] are analogous to @scheme[in-range], except that a
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@scheme[#f] value for @scheme[stop] is equivalent to
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@scheme[(vector-length vec)]. That is, the first element in the
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sequence is @scheme[(vector-ref vec start)], and each successive
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element is generated by adding @scheme[step] to index of the previous
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element. The sequence stops before an index that would be greater or
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equal to @scheme[end] if @scheme[step] is non-negative, or less or
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equal to @scheme[end] if @scheme[step] is negative.
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If @racket[start] is not a valid index, or @racket[stop]
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is not in [-1, @racket[(vector-length vec)]] then the @exnraise[exn:fail:contract].
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If @scheme[start] is less than @scheme[stop] and @scheme[step] is
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negative, then the @exnraise[exn:fail:contract:mismatch]. Similarly,
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if @scheme[start] is more than @scheme[stop] and @scheme[step] is
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positive, then the @exnraise[exn:fail:contract:mismatch].
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@speed[in-vector "vector"]}
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@defproc[(in-string [str string?]
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[start exact-nonnegative-integer? 0]
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[stop (or/c exact-integer? #f) #f]
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[step (and/c exact-integer? (not/c zero?)) 1])
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sequence?]{
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Returns a sequence equivalent to @scheme[str] when no optional
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arguments are supplied.
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@info-on-seq["strings" "strings"]
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The optional arguments @scheme[start], @scheme[stop], and
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@scheme[step] are as in @scheme[in-vector].
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@speed[in-string "string"]}
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@defproc[(in-bytes [bstr bytes?]
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[start exact-nonnegative-integer? 0]
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[stop (or/c exact-integer? #f) #f]
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[step (and/c exact-integer? (not/c zero?)) 1])
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sequence?]{
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Returns a sequence equivalent to @scheme[bstr] when no optional
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arguments are supplied.
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@info-on-seq["bytestrings" "byte strings"]
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The optional arguments @scheme[start], @scheme[stop], and
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@scheme[step] are as in @scheme[in-vector].
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@speed[in-bytes "byte string"]}
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@defproc[(in-port [r (input-port? . -> . any/c) read]
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[in input-port? (current-input-port)])
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sequence?]{
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Returns a sequence whose elements are produced by calling @scheme[r]
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on @scheme[in] until it produces @scheme[eof].}
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@defproc[(in-input-port-bytes [in input-port?]) sequence?]{
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Returns a sequence equivalent to @scheme[(in-port read-byte in)].}
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@defproc[(in-input-port-chars [in input-port?]) sequence?]{
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Returns a sequence whose elements are read as characters from
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@scheme[in] (equivalent to @scheme[(in-port read-char in)]).}
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@defproc[(in-lines [in input-port? (current-input-port)]
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[mode (or/c 'linefeed 'return 'return-linefeed 'any 'any-one) 'any])
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sequence?]{
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Returns a sequence equivalent to
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@scheme[(in-port (lambda (p) (read-line p mode)) in)]. Note that the
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default mode is @scheme['any], whereas the default mode of
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@scheme[read-line] is @scheme['linefeed].}
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@defproc[(in-bytes-lines [in input-port? (current-input-port)]
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[mode (or/c 'linefeed 'return 'return-linefeed 'any 'any-one) 'any])
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sequence?]{
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Returns a sequence equivalent to
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@scheme[(in-port (lambda (p) (read-bytes-line p mode)) in)]. Note
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that the default mode is @scheme['any], whereas the default mode of
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@scheme[read-bytes-line] is @scheme['linefeed].}
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@defproc[(in-hash [hash hash?]) sequence?]{
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Returns a sequence equivalent to @scheme[hash].
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@examples[
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(define table (hash 'a 1 'b 2))
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(for ([(key value) (in-hash table)])
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(printf "key: ~a value: ~a\n" key value))
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]
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@info-on-seq["hashtables" "hash tables"]}
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@defproc[(in-hash-keys [hash hash?]) sequence?]{
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Returns a sequence whose elements are the keys of @scheme[hash].
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@examples[
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(define table (hash 'a 1 'b 2))
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(for ([key (in-hash-keys table)])
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(printf "key: ~a\n" key))
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]
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}
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@defproc[(in-hash-values [hash hash?]) sequence?]{
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Returns a sequence whose elements are the values of @scheme[hash].
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@examples[
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(define table (hash 'a 1 'b 2))
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(for ([value (in-hash-values table)])
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(printf "value: ~a\n" value))
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]
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}
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@defproc[(in-hash-pairs [hash hash?]) sequence?]{
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Returns a sequence whose elements are pairs, each containing a key and
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its value from @scheme[hash] (as opposed to using @scheme[hash]
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directly as a sequence to get the key and value as separate values for
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each element).
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@examples[
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(define table (hash 'a 1 'b 2))
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(for ([key+value (in-hash-pairs table)])
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(printf "key and value: ~a\n" key+value))
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]
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}
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@defproc[(in-directory [dir (or/c #f path-string?) #f]) sequence?]{
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Return a sequence that produces all of the paths for files,
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directories, and links with @racket[dir]. If @racket[dir] is not
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@racket[#f], then every produced path starts with @racket[dir] as its
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prefix. If @racket[dir] is @racket[#f], then paths in and relative to
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the current directory are produced.}
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@defproc[(in-producer [producer procedure?] [stop any/c] [args any/c] ...)
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sequence?]{
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Returns a sequence that contains values from sequential calls to
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@scheme[producer]. A @scheme[stop] value returned by
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@racket[producer] marks the end of the sequence (and the
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@racket[stop] value is not included in the sequence); @scheme[stop]
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can be a predicate that is applied to the results of @racket[producer],
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or it can be a value that is tested against the result of
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with @scheme[eq?]. (The @racket[stop] argument must be a predicate
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if the stop value is itself a function or if
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@scheme[producer] returns multiple values.)}
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@defproc[(in-value [v any/c]) sequence?]{
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Returns a sequence that produces a single value: @scheme[v]. This
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form is mostly useful for @scheme[let]-like bindings in forms such as
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@scheme[for*/list].}
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@defproc[(in-indexed [seq sequence?]) sequence?]{
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Returns a sequence where each element has two values: the value
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produced by @scheme[seq], and a non-negative exact integer starting
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with @scheme[0]. The elements of @scheme[seq] must be single-valued.}
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@defproc[(in-sequences [seq sequence?] ...) sequence?]{
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Returns a sequence that is made of all input sequences, one after the
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other. The elements of each @scheme[seq] must all have the same
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number of values.}
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@defproc[(in-cycle [seq sequence?] ...) sequence?]{
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Similar to @scheme[in-sequences], but the sequences are repeated in an
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infinite cycle.}
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@defproc[(in-parallel [seq sequence?] ...) sequence?]{
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Returns a sequence where each element has as many values as the number
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of supplied @scheme[seq]s; the values, in order, are the values of
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each @scheme[seq]. The elements of each @scheme[seq] must be
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single-valued.}
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@defproc[(in-values-sequence [seq sequence?]) sequence?]{
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Returns a sequence that is like @racket[seq], but it combines
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multiple values for each element from @racket[seq] as a list of
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elements.}
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@defproc[(in-values*-sequence [seq sequence?]) sequence?]{
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Returns a sequence that is like @racket[seq], but when an element of
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@racket[seq] has multiple values or a single list value, then the
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values are combined in a list. In other words,
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@racket[in-values*-sequence] is like @racket[in-values-sequence],
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except that non-list, single-valued elements are not wrapped in a
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list.}
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@defproc[(stop-before [seq sequence?] [pred (any/c . -> . any)])
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sequence?]{
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Returns a sequence that contains the elements of @scheme[seq] (which
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must be single-valued), but only until the last element for which
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applying @scheme[pred] to the element produces @scheme[#t], after
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which the sequence ends.}
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@defproc[(stop-after [seq sequence?] [pred (any/c . -> . any)])
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sequence?]{
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Returns a sequence that contains the elements of @scheme[seq] (which
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must be single-valued), but only until the element (inclusive) for
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which applying @scheme[pred] to the element produces @scheme[#t],
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after which the sequence ends.}
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@defproc[(make-do-sequence [thunk (-> (values (any/c . -> . any)
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(any/c . -> . any/c)
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any/c
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(or/c (any/c . -> . any/c) #f)
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(or/c (() () #:rest list? . ->* . any/c) #f)
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(or/c ((any/c) () #:rest list? . ->* . any/c) #f)))])
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sequence?]{
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Returns a sequence whose elements are generated by the procedures and
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initial value returned by the thunk, which is called to @tech{initiate}
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the sequence. The initiated sequence is defined in terms
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of a @defterm{position}, which is initialized to the third result of
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the thunk, and the @defterm{element}, which may consist of multiple
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values.
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The @scheme[thunk] results define the generated elements as follows:
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@itemize[
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@item{The first result is a @scheme[_pos->element] procedure that takes
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the current position and returns the value(s) for the current
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element.}
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@item{The second result is a @scheme[_next-pos] procedure that takes
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the current position and returns the next position.}
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@item{The third result is the initial position.}
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@item{The fourth result is a @racket[_continue-with-pos?] function that
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takes the current position and returns a
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true result if the sequence includes the value(s) for the current
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position, and false if the sequence should end instead of
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including the value(s). Alternatively, the fourth result can be
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@racket[#f] to indicate that the sequence should always include the
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current value(s).}
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@item{The fifth result is a @racket[_continue-with-val?] function that is
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like the fourth result, but it takes the
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current element value(s) instead of the current position.
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Alternatively, the fifth result can be
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@racket[#f] to indicate that the sequence should always include the
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value(s) at the current position.}
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@item{The sixth result is a @racket[_continue-after-pos+val?] procedure
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that takes both
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the current position and the current element value(s) and
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determines whether the sequence ends after the current element is already
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included in the sequence.
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Alternatively, the sixth result can be
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@racket[#f] to indicate that the sequence can always continue
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after the current value(s).}]
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Each of the procedures listed above is called only once per position.
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Among the last three procedures, as soon as one of the procedures
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returns @scheme[#f], the sequence ends, and none are called again.
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Typically, one of the functions determines the end condition, and
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@scheme[#f] is used in place of the other two functions.}
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@defthing[prop:sequence struct-type-property?]{
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Associates a procedure to a structure type that takes an instance of
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the structure and returns a sequence. If @scheme[v] is an instance of
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a structure type with this property, then @scheme[(sequence? v)]
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produces @scheme[#t].
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@let-syntax[([car (make-element-id-transformer
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(lambda (id) #'@schemeidfont{car}))])
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@examples[
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(define-struct train (car next)
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#:property prop:sequence (lambda (t)
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(make-do-sequence
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(lambda ()
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(values train-car
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train-next
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t
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(lambda (t) t)
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(lambda (v) #t)
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(lambda (t v) #t))))))
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(for/list ([c (make-train 'engine
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(make-train 'boxcar
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(make-train 'caboose
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#f)))])
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c)]]}
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@; ----------------------------------------------------------------------
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@subsection{Sequence Conversion}
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@defproc[(sequence->stream [seq sequence?]) stream?]{
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Coverts a sequence to a @tech{stream}, which supports the
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@racket[stream-first] and @racket[stream-rest] operations. Creation
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of the stream eagerly @tech{initiates} the sequence, but the
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stream lazily draws elements from the sequence, caching each element
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so that @racket[stream-first] produces the same result each time
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is applied to a stream.
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In extracting an element from @racket[seq] involves a side-effect,
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then the effect is performed each time that either
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@racket[stream-first] or @racket[stream-rest] is first used to
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access or skip an element.}
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@defproc[(sequence-generate [seq sequence?])
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(values (-> boolean?) (-> any))]{
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@tech{Initiates} a sequence and returns two thunks to extract elements
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from the sequence. The first
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returns @scheme[#t] if more values are available for the sequence.
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The second returns the next element (which may be multiple values)
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from the sequence; if no more elements are available, the
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@exnraise[exn:fail:contract].}
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@defproc[(sequence-generate* [seq sequence?])
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(values (or/c list? #f)
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(-> (values (or/c list? #f) procedure?)))]{
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Like @racket[sequence-generate], but avoids state (aside from any
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inherent in the sequence) by returning a list of values for the sequence's
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first element---or @racket[#f] if the sequence is empty---and a thunk
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to continue with the sequence; the result of the thunk is the same
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as the result of @racket[sequence-generate*], but for the second
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element of the sequence, and so on. If the thunk is called when the
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element result is @racket[#f] (indicating no further values in the sequence),
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the @exnraise[exn:fail:contract].}
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@; ----------------------------------------------------------------------
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@subsection[#:tag "more-sequences"]{Sequence Combinations}
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@note-lib[racket/sequence]
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@defthing[empty-sequence sequence?]{
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A sequence with no elements.}
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@defproc[(sequence->list [s sequence?]) list?]{
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Returns a list whose elements are the elements of @scheme[s],
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each of which must be a single value. If @scheme[s] is infinite, this
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function does not terminate.}
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@defproc[(sequence-length [s sequence?])
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exact-nonnegative-integer?]{
|
|
Returns the number of elements of @scheme[s] by extracting and
|
|
discarding all of them. If @scheme[s] is infinite, this function
|
|
does not terminate.}
|
|
|
|
@defproc[(sequence-ref [s sequence?] [i exact-nonnegative-integer?])
|
|
any]{
|
|
Returns the @scheme[i]th element of @scheme[s] (which may be
|
|
multiple values).}
|
|
|
|
@defproc[(sequence-tail [s sequence?] [i exact-nonnegative-integer?])
|
|
sequence?]{
|
|
Returns a sequence equivalent to @scheme[s], except that the first
|
|
@scheme[i] elements are omitted.
|
|
|
|
In case @tech[#:key "initiate"]{initiating} @racket[s] involves a
|
|
side effect, the sequence @racket[s] is not @tech{initiate}d
|
|
until the resulting sequence is @tech{initiate}d, at which point the
|
|
first @racket[i] elements are extracted from
|
|
the sequence.}
|
|
|
|
@defproc[(sequence-append [s sequence?] ...)
|
|
sequence?]{
|
|
Returns a sequence that contains all elements of each sequence in the
|
|
order they appear in the original sequences. The new sequence is
|
|
constructed lazily.
|
|
|
|
If all given @racket[s]s are @tech{streams}, the result is also a
|
|
@tech{stream}.}
|
|
|
|
@defproc[(sequence-map [f procedure?]
|
|
[s sequence?])
|
|
sequence?]{
|
|
Returns a sequence that contains @scheme[f] applied to each element of
|
|
@scheme[s]. The new sequence is constructed lazily.
|
|
|
|
If @racket[s] is a @tech{stream}, then the result is also a
|
|
@tech{stream}.}
|
|
|
|
@defproc[(sequence-andmap [f (-> any/c ... boolean?)]
|
|
[s sequence?])
|
|
boolean?]{
|
|
Returns @scheme[#t] if @scheme[f] returns a true result on every
|
|
element of @scheme[s]. If @scheme[s] is infinite and @scheme[f] never
|
|
returns a false result, this function does not terminate.}
|
|
|
|
@defproc[(sequence-ormap [f (-> any/c ... boolean?)]
|
|
[s sequence?])
|
|
boolean?]{
|
|
Returns @scheme[#t] if @scheme[f] returns a true result on some
|
|
element of @scheme[s]. If @scheme[s] is infinite and @scheme[f] never
|
|
returns a true result, this function does not terminate.}
|
|
|
|
@defproc[(sequence-for-each [f (-> any/c ... any)]
|
|
[s sequence?])
|
|
(void)]{
|
|
Applies @scheme[f] to each element of @scheme[s]. If @scheme[s] is
|
|
infinite, this function does not terminate.}
|
|
|
|
@defproc[(sequence-fold [f (-> any/c any/c ... any/c)]
|
|
[i any/c]
|
|
[s sequence?])
|
|
(void)]{
|
|
Folds @scheme[f] over each element of @scheme[s] with @scheme[i] as
|
|
the initial accumulator. If @scheme[s] is infinite, this function
|
|
does not terminate.}
|
|
|
|
@defproc[(sequence-count [f procedure?] [s sequence?])
|
|
exact-nonnegative-integer?]{
|
|
Returns the number of elements in @scheme[s] for which @scheme[f]
|
|
returns a true result. If @scheme[s] is infinite, this function does
|
|
not terminate.}
|
|
|
|
@defproc[(sequence-filter [f (-> any/c ... boolean?)]
|
|
[s sequence?])
|
|
sequence?]{
|
|
Returns a sequence whose elements are the elements of @scheme[s] for
|
|
which @scheme[f] returns a true result. Although the new sequence is
|
|
constructed lazily, if @scheme[s] has an infinite number of elements
|
|
where @scheme[f] returns a false result in between two elements where
|
|
@scheme[f] returns a true result, then operations on this sequence will
|
|
not terminate during the infinite sub-sequence.
|
|
|
|
If @racket[s] is a @tech{stream}, then the result is also a
|
|
@tech{stream}.}
|
|
|
|
@defproc[(sequence-add-between [s sequence?] [e any/c])
|
|
sequence?]{
|
|
Returns a sequence whose elements are the elements of @scheme[s],
|
|
but with @scheme[e] between each pair of elements in @racket[s].
|
|
The new sequence is constructed lazily.
|
|
|
|
If @racket[s] is a @tech{stream}, then the result is also a
|
|
@tech{stream}.}
|
|
|
|
@; ======================================================================
|
|
@section[#:tag "streams"]{Streams}
|
|
|
|
A @deftech{stream} is a kind of sequence that supports functional
|
|
iteration via @racket[stream-first] and @racket[stream-rest]. The
|
|
@racket[stream-cons] form constructs a lazy stream, but plain lists
|
|
can be used as stream, and functions such as @racket[in-range] and
|
|
@racket[in-naturals] also create streams.
|
|
|
|
@note-lib[racket/stream]
|
|
|
|
@defproc[(stream? [v any/c]) boolean?]{
|
|
Returns @scheme[#t] if @scheme[v] can be used as a @tech{stream},
|
|
@scheme[#f] otherwise.}
|
|
|
|
@defproc[(stream-empty? [s stream?]) boolean?]{
|
|
Returns @racket[#f] if @racket[s] has no elements, @racket[#f]
|
|
otherwise.
|
|
}
|
|
|
|
@defproc[(stream-first [s (and/c stream? (not/c stream-empty?))]) any]{
|
|
Returns the value(s) of the first element in @racket[s].
|
|
}
|
|
|
|
@defproc[(stream-rest [s (and/c stream? (not/c stream-empty?))]) stream?]{
|
|
Returns a stream that is equivalent to @racket[s] without its
|
|
first element.
|
|
}
|
|
|
|
@defform[(stream-cons first-expr rest-expr)]{
|
|
|
|
Produces a lazy stream for which @racket[stream-first] forces the
|
|
evaluation of @racket[first-expr] to produce the first element of the
|
|
stream, and @racket[stream-rest] forces the evaluation of
|
|
@racket[rest-expr] to produce a stream for the rest of the returned
|
|
stream.
|
|
|
|
The first element of the stream as produced by @racket[first-expr]
|
|
must be a single value. The @racket[rest-expr] must produce a stream
|
|
when it is evaluated, otherwise the @exnraise[exn:fail:contract?].}
|
|
|
|
@defproc[(in-stream [s stream?]) sequence?]{
|
|
Returns a sequence that is equivalent to @racket[s].
|
|
@speed[in-stream "streams"]}
|
|
|
|
@defthing[empty-stream stream?]{
|
|
A stream with no elements.}
|
|
|
|
@defproc[(stream->list [s stream?]) list?]{
|
|
Returns a list whose elements are the elements of @scheme[s],
|
|
each of which must be a single value. If @scheme[s] is infinite, this
|
|
function does not terminate.}
|
|
|
|
@defproc[(stream-length [s stream?])
|
|
exact-nonnegative-integer?]{
|
|
Returns the number of elements of @scheme[s]. If @scheme[s] is
|
|
infinite, this function does not terminate.
|
|
|
|
In the case of lazy streams, this function forces evaluation only of
|
|
the sub-streams, and not the stream's elements.}
|
|
|
|
@defproc[(stream-ref [s stream?] [i exact-nonnegative-integer?])
|
|
any]{
|
|
Returns the @scheme[i]th element of @scheme[s] (which may be
|
|
multiple values).}
|
|
|
|
@defproc[(stream-tail [s stream?] [i exact-nonnegative-integer?])
|
|
stream?]{
|
|
Returns a stream equivalent to @scheme[s], except that the first
|
|
@scheme[i] elements are omitted.
|
|
|
|
In case extracting elements from @racket[s] involves a side effect,
|
|
they will not be extracted until the first element is extracted from
|
|
the resulting stream.}
|
|
|
|
@defproc[(stream-append [s stream?] ...)
|
|
stream?]{
|
|
Returns a stream that contains all elements of each stream in the
|
|
order they appear in the original streams. The new stream is
|
|
constructed lazily.}
|
|
|
|
@defproc[(stream-map [f procedure?]
|
|
[s stream?])
|
|
stream?]{
|
|
Returns a stream that contains @scheme[f] applied to each element of
|
|
@scheme[s]. The new stream is constructed lazily.}
|
|
|
|
@defproc[(stream-andmap [f (-> any/c ... boolean?)]
|
|
[s stream?])
|
|
boolean?]{
|
|
Returns @scheme[#t] if @scheme[f] returns a true result on every
|
|
element of @scheme[s]. If @scheme[s] is infinite and @scheme[f] never
|
|
returns a false result, this function does not terminate.}
|
|
|
|
@defproc[(stream-ormap [f (-> any/c ... boolean?)]
|
|
[s stream?])
|
|
boolean?]{
|
|
Returns @scheme[#t] if @scheme[f] returns a true result on some
|
|
element of @scheme[s]. If @scheme[s] is infinite and @scheme[f] never
|
|
returns a true result, this function does not terminate.}
|
|
|
|
@defproc[(stream-for-each [f (-> any/c ... any)]
|
|
[s stream?])
|
|
(void)]{
|
|
Applies @scheme[f] to each element of @scheme[s]. If @scheme[s] is
|
|
infinite, this function does not terminate.}
|
|
|
|
@defproc[(stream-fold [f (-> any/c any/c ... any/c)]
|
|
[i any/c]
|
|
[s stream?])
|
|
(void)]{
|
|
Folds @scheme[f] over each element of @scheme[s] with @scheme[i] as
|
|
the initial accumulator. If @scheme[s] is infinite, this function
|
|
does not terminate.}
|
|
|
|
@defproc[(stream-count [f procedure?] [s stream?])
|
|
exact-nonnegative-integer?]{
|
|
Returns the number of elements in @scheme[s] for which @scheme[f]
|
|
returns a true result. If @scheme[s] is infinite, this function does
|
|
not terminate.}
|
|
|
|
@defproc[(stream-filter [f (-> any/c ... boolean?)]
|
|
[s stream?])
|
|
stream?]{
|
|
Returns a stream whose elements are the elements of @scheme[s] for
|
|
which @scheme[f] returns a true result. Although the new stream is
|
|
constructed lazily, if @scheme[s] has an infinite number of elements
|
|
where @scheme[f] returns a false result in between two elements where
|
|
@scheme[f] returns a true result, then operations on this stream will
|
|
not terminate during the infinite sub-stream.}
|
|
|
|
@defproc[(stream-add-between [s stream?] [e any/c])
|
|
stream?]{
|
|
Returns a stream whose elements are the elements of @scheme[s],
|
|
but with @scheme[e] between each pair of elements in @racket[s].
|
|
The new stream is constructed lazily.}
|
|
|
|
@defthing[prop:stream struct-type-property?]{
|
|
|
|
Associates three procedures to a structure type to implement stream
|
|
operations for instances of the structure type.
|
|
|
|
The property value must be a vector of three procedures: a
|
|
@racket[stream-empty?] implementation, a @racket[stream-first]
|
|
implementation, and a @racket[stream-rest] implementation. The
|
|
procedures are applied only to instances of the structure type that
|
|
has the property value.}
|
|
|
|
@; ======================================================================
|
|
@section{Generators}
|
|
|
|
A @deftech{generator} is a procedure that returns a sequence of
|
|
values, incrementing the sequence each time that the generator is
|
|
called. In particular, the @racket[generator] form implements a
|
|
generator by evaluating a body that calls @racket[yield] to return
|
|
values from the generator.
|
|
|
|
@defmodule[racket/generator]
|
|
|
|
@(define generator-eval
|
|
(let ([the-eval (make-base-eval)])
|
|
(the-eval '(require racket/generator))
|
|
the-eval))
|
|
|
|
|
|
@defproc[(generator? [v any/c]) boolean?]{
|
|
Return @scheme[#t] if @scheme[v] is a @tech{generator},
|
|
@scheme[#f] otherwise.}
|
|
|
|
|
|
@defform[(generator formals body ...+)]{
|
|
Creates a @tech{generator}, where @racket[formals] is like the
|
|
@racket[formals] of @racket[case-lambda] (i.e., the
|
|
@racket[_kw-formals] of @racket[lambda] restricted to non-optional
|
|
and non-keyword arguments).
|
|
|
|
For the first call to a generator, the arguments are bound to the
|
|
@racket[formals] and evaluation of @racket[body] starts. During the
|
|
@tech{dynamic extent} of @racket[body], the generator can return
|
|
immediately using the @racket[yield] function. A second call to the
|
|
generator resumes at the @racket[yield] call, producing the
|
|
arguments of the second call as the results of the @racket[yield],
|
|
and so on. The eventual results of @racket[body] are supplied to an
|
|
implicit final @racket[yield]; after that final @racket[yield],
|
|
calling the generator again returns the same values, but all such
|
|
calls must provide 0 arguments to the generator.
|
|
|
|
@examples[#:eval generator-eval
|
|
(define g (generator ()
|
|
(let loop ([x '(a b c)])
|
|
(if (null? x)
|
|
0
|
|
(begin
|
|
(yield (car x))
|
|
(loop (cdr x)))))))
|
|
(g)
|
|
(g)
|
|
(g)
|
|
(g)
|
|
(g)]}
|
|
|
|
@defproc[(yield [v any/c] ...) any]{
|
|
Returns @racket[v]s from a generator, saving the point of execution
|
|
inside a generator (i.e., within the @tech{dynamic extent} of a
|
|
@racket[generator] body) to be resumed by the next call to the
|
|
generator. The results of @racket[yield] are the arguments
|
|
that are provided to the next call of the generator.
|
|
|
|
When not in the @tech{dynamic extent} of a @racket[generator],
|
|
@racket[infinite-generator], or @racket[in-generator] body,
|
|
@racket[yield] raises @racket[exn:fail] after evaluating its
|
|
@racket[expr]s.
|
|
|
|
@examples[#:eval generator-eval
|
|
(define my-generator (generator () (yield 1) (yield 2 3 4)))
|
|
(my-generator)
|
|
(my-generator)]
|
|
|
|
@examples[#:eval generator-eval
|
|
(define pass-values-generator
|
|
(generator ()
|
|
(let* ([from-user (yield 2)]
|
|
[from-user-again (yield (add1 from-user))])
|
|
(yield from-user-again))))
|
|
|
|
(pass-values-generator)
|
|
(pass-values-generator 5)
|
|
(pass-values-generator 12)]}
|
|
|
|
@defform[(infinite-generator body ...+)]{
|
|
Like @scheme[generator], but repeats evaluation of the @racket[body]s when
|
|
the last @racket[body] completes without implicitly @racket[yield]ing.
|
|
|
|
@examples[#:eval generator-eval
|
|
(define welcome
|
|
(infinite-generator
|
|
(yield 'hello)
|
|
(yield 'goodbye)))
|
|
(welcome)
|
|
(welcome)
|
|
(welcome)
|
|
(welcome)]}
|
|
|
|
@defform[(in-generator body ...+)]{
|
|
Produces a @tech{sequence} that encapsulates the @tech{generator} formed by
|
|
@racket[(generator () body ...+)]. The values produced by the
|
|
generator form the elements of the sequence.
|
|
|
|
@examples[#:eval generator-eval
|
|
(for/list ([i (in-generator
|
|
(let loop ([x '(a b c)])
|
|
(when (not (null? x))
|
|
(yield (car x))
|
|
(loop (cdr x)))))])
|
|
i)]
|
|
|
|
To use an existing generator as a sequence, use @scheme[in-producer]
|
|
with a stop-value known for the generator.
|
|
|
|
@examples[#:eval generator-eval
|
|
(define my-stop-value (gensym))
|
|
(define my-generator (generator ()
|
|
(let loop ([x '(a b c)])
|
|
(if (null? x)
|
|
my-stop-value
|
|
(begin
|
|
(yield (car x))
|
|
(loop (cdr x)))))))
|
|
|
|
(for/list ([i (in-producer my-generator my-stop-value)])
|
|
i)]}
|
|
|
|
@defproc[(generator-state [g generator?]) symbol?]{
|
|
Returns a symbol that describes the state of the generator.
|
|
|
|
@itemize[
|
|
@item{@scheme['fresh] --- The generator has been freshly created and
|
|
has not been called yet.}
|
|
@item{@scheme['suspended] --- Control within the generator has been
|
|
suspended due to a call to @scheme[yield]. The generator can
|
|
be called.}
|
|
@item{@scheme['running] --- The generator is currently executing.}
|
|
@item{@scheme['done] --- The generator has executed its entire
|
|
body and will continue to produce the same result as from
|
|
the last call.}]
|
|
|
|
@examples[#:eval generator-eval
|
|
(define my-generator (generator () (yield 1) (yield 2)))
|
|
(generator-state my-generator)
|
|
(my-generator)
|
|
(generator-state my-generator)
|
|
(my-generator)
|
|
(generator-state my-generator)
|
|
(my-generator)
|
|
(generator-state my-generator)
|
|
|
|
(define introspective-generator (generator () ((yield 1))))
|
|
(introspective-generator)
|
|
(introspective-generator
|
|
(lambda () (generator-state introspective-generator)))
|
|
(generator-state introspective-generator)
|
|
(introspective-generator)]}
|
|
|
|
@defproc[(sequence->generator [s sequence?]) (-> any)]{
|
|
Converts a @tech{sequence} to a @tech{generator}. The generator
|
|
returns the next element of the sequence each time the generator is
|
|
invoked, where each element of the sequence must be a single
|
|
value. When the sequence ends, the generator returns @|void-const|
|
|
as its final result.}
|
|
|
|
@defproc[(sequence->repeated-generator [s sequence?]) (-> any)]{
|
|
Like @scheme[sequence->generator], but when @racket[s] has no further
|
|
values, the generator starts the sequence again (so that the
|
|
generator never stops producing values).}
|
|
|
|
@close-eval[generator-eval]
|