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define-stream
stream-delay
stream*
stream-take
stream-zip
stream-interleave
stream-interleave*
stream-cross-product
stream-cross-product*

6.8 mischief/stream: Streams

 (require mischief/stream) package: mischief

syntax

(define-stream id expr)

Defines id as a lazy stream encapsulating expr.

Examples:
> (define-stream naturals
    (stream-cons 0 (stream-map add1 naturals)))
> naturals

#<delayed>

> (stream-ref naturals 0)

0

> (stream-ref naturals 1)

1

> (stream-ref naturals 2)

2

syntax

(stream-delay expr)

Constructs a lazy stream encapsulating expr.

Examples:
> (define delayed-stream
    (stream-delay '(1 2 3)))
> delayed-stream

#<delayed>

> (stream? delayed-stream)

#t

> (stream-empty? delayed-stream)

#f

> (stream-first delayed-stream)

1

> (stream? (stream-delay 'not-a-stream))

#t

> (stream-empty? (stream-delay 'not-a-stream))

stream-delay: contract violation

  expected: stream?

  given: 'not-a-stream

syntax

(stream* x ... st)

 
  x : any/c
  st : stream?
Creates a lazy stream that contains each x followed by the contents of st.

Examples:
> (define (nonzero-integers-from k)
    (stream* k (- k) (nonzero-integers-from (add1 k))))
> (define nonzero-integers
    (nonzero-integers-from 1))
> (stream-ref nonzero-integers 0)

1

> (stream-ref nonzero-integers 1)

-1

> (stream-ref nonzero-integers 2)

2

> (stream-ref nonzero-integers 3)

-2

> (stream-ref nonzero-integers 4)

3

procedure

(stream-take st n)  list?

  st : stream?
  n : exact-nonnegative-integer?
Produces a list containing the first n elements of st.

Example:
> (stream-take (stream 1 2 3 (error "Oops!")) 3)

#<stream>

procedure

(stream-zip st ...+)  stream?

  st : stream?
Creates a lazy stream containing lists of the respective elements of each st.

Example:
> (stream->list
    (stream-zip (stream 1 2 3) (stream 'a 'b 'c)))

'((1 a) (2 b) (3 c))

procedure

(stream-interleave st ...)  stream?

  st : stream?
Creates a lazy stream that alternates the elements of the given sts.

Example:
> (stream->list
    (stream-interleave (stream 1 2 3) (stream 'a 'b 'c)))

'(1 a 2 b 3 c)

procedure

(stream-interleave* st)  stream?

  st : stream?
Creates a lazy stream from st, which must be a stream of streams. The resulting stream includes all of the elements from the streams in st, even if st is an infinite stream of streams.

Example:
> (stream->list
    (stream-interleave*
      (stream (stream 1 2 3) (stream 'a 'b 'c))))

'(1 2 a 3 b c)

procedure

(stream-cross-product f st1 st2)  stream?

  f : (-> any/c any/c any/c)
  st1 : stream?
  st2 : stream?
Creates a lazy stream whose elements are the results of applying f to every pairwise combination of elements from st1 and st2.

Example:
> (stream->list
    (stream-cross-product
      string-ref
      (stream "cat" "dog")
      (stream 2 1 0)))

'(#\t #\g #\a #\o #\c #\d)

procedure

(stream-cross-product* f st1 st2)  stream?

  f : (-> any/c any/c any/c)
  st1 : stream?
  st2 : stream?
Creates a lazy stream of streams whose elements are the results of applying f to every pairwise combination of elements from st1 and st2.

Example:
> (map stream->list
    (stream->list
      (stream-cross-product*
        string-ref
        (stream "cat" "dog")
        (stream 2 1 0))))

'((#\t #\g) (#\a #\o) (#\c #\d))