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In addition to promises, MIT 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.
Returns a newly allocated stream whose elements are the arguments. Note
that the expression (stream) returns the empty stream, or
end-of-stream marker.
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-first stream
Returns the first element in stream. stream-car is
equivalent to car. stream-first is a synonym for
stream-car.
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.
The following are supported for compatibility with old code. Please do
not use these for new code. The variable the-empty-stream is
bound to the end-of-stream marker; use (stream) in new code.
head is a synonym for stream-car. tail is a
synonym for stream-cdr. empty-stream? is a synonym for
stream-null?.
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