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10.6 Streams

In addition to promises, MIT/GNU Scheme supports a higher-level abstraction called streams. Streams are similar to lists, except that the tail of a stream is not computed until it is referred to. This allows streams to be used to represent infinitely long lists.

procedure: stream object … ¶

Returns a newly allocated stream whose elements are the arguments. Note that the expression (stream) returns the empty stream, or end-of-stream marker.

procedure: list->stream list ¶

Returns a newly allocated stream whose elements are the elements of list. Equivalent to (apply stream list).

procedure: stream->list stream ¶

Returns a newly allocated list whose elements are the elements of stream. If stream has infinite length this procedure will not terminate. This could have been defined by

(define (stream->list stream)
  (if (stream-null? stream)
      '()
      (cons (stream-car stream)
            (stream->list (stream-cdr stream)))))

special form: cons-stream object expression ¶

Returns a newly allocated stream pair. Equivalent to (cons object (delay expression)).

procedure: stream-pair? object ¶

Returns #t if object is a pair whose cdr contains a promise. Otherwise returns #f. This could have been defined by

(define (stream-pair? object)
  (and (pair? object)
       (promise? (cdr object))))

procedure: stream-car stream ¶

procedure: stream-first stream ¶

Returns the first element in stream. stream-car is equivalent to car. stream-first is a synonym for stream-car.

procedure: stream-cdr stream ¶

procedure: stream-rest stream ¶

Returns the first tail of stream. Equivalent to (force (cdr stream)). stream-rest is a synonym for stream-cdr.

procedure: stream-null? stream ¶

Returns #t if stream is the end-of-stream marker; otherwise returns #f. This is equivalent to null?, but should be used whenever testing for the end of a stream.

procedure: stream-length stream ¶

Returns the number of elements in stream. If stream has an infinite number of elements this procedure will not terminate. Note that this procedure forces all of the promises that comprise stream.

procedure: stream-ref stream k ¶

Returns the element of stream that is indexed by k; that is, the kth element. K must be an exact non-negative integer strictly less than the length of stream.

procedure: stream-head stream k ¶

Returns the first k elements of stream as a list. K must be an exact non-negative integer strictly less than the length of stream.

procedure: stream-tail stream k ¶

Returns the tail of stream that is indexed by k; that is, the kth tail. This is equivalent to performing stream-cdr k times. K must be an exact non-negative integer strictly less than the length of stream.

procedure: stream-map procedure stream stream … ¶

Returns a newly allocated stream, each element being the result of invoking procedure with the corresponding elements of the streams as its arguments.