406 lines
15 KiB
Racket
406 lines
15 KiB
Racket
#lang scribble/doc
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@(require "mz.ss"
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(for-syntax scheme/base)
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scribble/scheme
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(for-label scheme/generator))
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@(define generator-eval
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(lambda ()
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(let ([the-eval (make-base-eval)])
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(the-eval '(require scheme/generator))
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the-eval)))
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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 sequences.})
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@title[#:tag "sequences"]{Sequences}
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@guideintro["sequences"]{sequences}
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A @deftech{sequence} encapsulates an ordered stream of values. The
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elements of a sequence can be extracted with one of the @scheme[for]
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syntactic forms or with the procedures returned by
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@scheme[sequence-generate].
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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{lists (see @secref["pairs"])}
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@item{vectors (see @secref["vectors"])}
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@item{hash tables (see @secref["hashtables"])}
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@item{strings (see @secref["strings"])}
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@item{byte strings (see @secref["bytestrings"])}
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@item{input ports (see @secref["ports"])}
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]
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In addition, @scheme[make-do-sequence] creates a sequence given a
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thunk that returns procedures to implement a generator, and the
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@scheme[prop:sequence] property can be associated with a structure
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type.
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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 the
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sequence of elements from a list does not change the list. For other
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sequence types, each extraction implies a side effect; for example,
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extracting the sequence of bytes from a port cause the bytes to be
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read from the port.
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Inidvidual elements of a sequence typically correspond to single
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values, but an element may also correspond to multiple values. For
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example, a hash table generates two values---a key and its value---for
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each element in the sequence.
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@section{Sequence Predicate and Constructors}
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@defproc[(sequence? [v any/c]) boolean?]{ Return @scheme[#t] if
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@scheme[v] can be used as a sequence, @scheme[#f] otherwise.}
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@defproc*[([(in-range [end number?]) sequence?]
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[(in-range [start number?] [end number?] [step number? 1]) sequence?])]{
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Returns a sequence 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]) sequence?]{
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Returns an infinite sequence of exact integers starting with
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@scheme[start], where each element is one more than the preceeding
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element. @speed[in-naturals "integer"]}
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@defproc[(in-list [lst list?]) sequence?]{
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Returns a sequence equivalent to @scheme[lst].
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@info-on-seq["pairs" "lists"]
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@speed[in-list "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-nonnegative-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 @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]. The
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@scheme[start] and @scheme[stop] values are @emph{not} checked against
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the size of @scheme[vec], so access can fail when an element is
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demanded from the sequence.
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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-nonnegative-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-nonnegative-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?]{ Returns a
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sequence whose elements are read as characters form @scheme[in]
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(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 @scheme[(in-port (lambda (p)
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(read-line p mode)) in)]. Note that the default mode is @scheme['any],
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whereas the default mode of @scheme[read-line] is
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@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 @scheme[(in-port (lambda (p)
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(read-bytes-line p mode)) in)]. Note that the default mode is @scheme['any],
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whereas the default mode of @scheme[read-bytes-line] is
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@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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@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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@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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@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] directly
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as a sequence to get the key and value as separate values for each
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element).}
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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]. @scheme[stop] identifies the value that marks the
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end of the sequence --- this value is not included in the sequence.
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@scheme[stop] can be a predicate or a value that is tested against the
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results with @scheme[eq?]. Note that you must use a predicate function
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if the stop value is itself a function, or if the @scheme[producer]
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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 form
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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?]{Returns a sequence
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where each element has two values: the value produced by @scheme[seq],
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and a non-negative exact integer starting with @scheme[0]. The
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elements of @scheme[seq] must be single-valued.}
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@defproc[(in-sequences [seq sequence?] ...) sequence?]{Returns a
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sequence that is made of all input sequences, one after the other. The
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elements of each @scheme[seq] must all have the same number of
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values.}
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@defproc[(in-cycle [seq sequence?] ...) sequence?]{Similar to
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@scheme[in-sequences], but the sequences are repeated in an infinite
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cycle.}
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@defproc[(in-parallel [seq sequence?] ...) sequence?]{Returns a
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sequence where each element has as many values as the number of
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supplied @scheme[seq]s; the values, in order, are the values of each
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@scheme[seq]. The elements of each @scheme[seq] must be
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single-valued.}
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@defproc[(stop-before [seq sequence?] [pred (any/c . -> . any)])
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sequence?]{ Returns a sequence that contains the elements of
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@scheme[seq] (which must be single-valued), but only until the last
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element for which applying @scheme[pred] to the element produces
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@scheme[#t], after which the sequence ends.}
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@defproc[(stop-after [seq sequence?] [pred (any/c . -> . any)])
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sequence?]{ Returns a sequence that contains the elements of
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@scheme[seq] (which must be single-valued), but only until the element
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(inclusive) for which applying @scheme[pred] to the element produces
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@scheme[#t], 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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(any/c . -> . any/c)
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(() () #:rest list? . ->* . any/c)
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((any/c) () #:rest list? . ->* . any/c)))])
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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. The generator 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 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 takes the current position and returns a true
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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).}
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@item{The fifth result is 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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@item{The sixth result is like the fourth result, but it takes both
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the current position and the current element values(s) and
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determines a sequence end after the current element is already
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included in the sequence.}
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]
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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
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again. Typically, one of the functions determines the end condition,
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and the other two functions always return @scheme[#t].}
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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 (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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@section{Sequence Generators}
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@defproc[(sequence-generate [seq sequence?]) (values (-> boolean?)
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(-> any))]{
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Returns two thunks to extract elements from the sequence. The first
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returns @scheme[#t] if more values are available for the sequence. The
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second returns the next element (which may be multiple values) from the
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sequence; if no more elements are available, the
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@exnraise[exn:fail:contract].}
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@section{Iterator Generators}
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@defmodule[scheme/generator]
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@defform[(generator body ...)]{ Creates a function that returns a
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value, usually through @scheme[yield], each time it is invoked. When
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the generator runs out of values to yield the last value it computed
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will be returned for future invocations of the generator. Generators
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can be safely nested.
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@examples[#:eval (generator-eval)
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(define g (generator
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(let loop ([x '(a b c)])
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(if (null? x)
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0
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(begin
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(yield (car x))
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(loop (cdr x)))))))
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(g)
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(g)
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(g)
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(g)
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(g)
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]
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To use an existing generator as a sequence, you should use @scheme[in-producer]
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with a stop-value known to the generator.
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@examples[#:eval (generator-eval)
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(define my-stop-value (gensym))
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(define my-generator (generator
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(let loop ([x '(a b c)])
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(if (null? x)
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my-stop-value
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(begin
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(yield (car x))
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(loop (cdr x)))))))
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(for/list ([i (in-producer my-generator my-stop-value)])
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i)
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]}
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@defform[(infinite-generator body ...)]{ Creates a function similar to
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@scheme[generator] but when the last @scheme[body] is executed the function
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will re-execute all the bodies in a loop.
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@examples[#:eval (generator-eval)
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(define welcome
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(infinite-generator
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(yield 'hello)
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(yield 'goodbye)))
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(welcome)
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(welcome)
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(welcome)
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(welcome)
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]}
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@defproc[(in-generator [expr any?] ...) sequence?]{ Returns a generator
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that can be used as a sequence. The @scheme[in-generator] procedure takes care of the
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case when @scheme[expr] stops producing values, so when the @scheme[expr]
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completes, the generator will end.
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@examples[#:eval (generator-eval)
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(for/list ([i (in-generator
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(let loop ([x '(a b c)])
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(when (not (null? x))
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(yield (car x))
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(loop (cdr x)))))])
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i)
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]}
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@defform[(yield expr)]{ Saves the point of execution inside a generator
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and returns a value.}
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@defproc[(sequence->generator [s sequence?]) (-> any?)]{ Returns a generator
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that returns elements from the sequence, @scheme[s], each time the generator
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is invoked.}
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@defproc[(sequence->repeated-generator [s sequence?]) (-> any?)]{ Returns a generator
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that returns elements from the sequence, @scheme[s], similar to
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@scheme[sequence->generator] but looping over the values in the sequence
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when no more values are left.}
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