7 Streams

(require (planet krhari/pfds:1:3/stream))

Streams are nothing but lazy lists. They are similar to ordinary lists and they provide the same functionality as that of lists. The difference between Streams and lists is that they are lazy in nature and each cell of a Stream is suspended and is forced only when required. Streams have been used in some of the below mentioned data structures. Since each suspention comes with a little overhead, Streams should be used only when there is a good enough reason to do so.

Example:

> (stream 1 2 3 4 5 6)

- : #(struct:Promise ((U Mt (InStream Positive-Fixnum))))

#<promise>

In the above example, the stream obtained will be similar to lists but will lazy in nature. It will have 1 as its first element.

In the above example, (stream-cons 10 (stream 1 2 3 4 5 6)) returns the stream (stream 10 1 2 3 4 5 6).

Examples:

> (stream-car (stream 1 2 3 4 5 6))

- : Positive-Fixnum

1

> (stream-car empty-stream)

stream-car: given stream is empty

In the above example, (stream-cdr strm) returns (stream 2 3 4 5 6).

Examples:

> (define strm1 (stream 1 2 3 4 5 6))

> (define strm2 (stream 51 32 42))

> (stream-append strm1 strm2)

- : #(struct:Promise ((U Mt (InStream Positive-Fixnum))))

#<promise>

In the above example, (stream-append strm1 strm2) returns the stream, (stream 1 2 3 4 5 6 51 32 42).

In the above example, (stream-reverse (stream 1 2 3 4 5 6)) returns (stream 6 5 4 3 2 1).

Examples:

> (drop 3 (stream 1 2 3 4 5 6))

- : #(struct:Promise ((U Mt (InStream Positive-Fixnum))))

#<promise>

> (drop 10 (stream 1 2 3 4 5 6))

drop: Not enough elements to drop

In the above example, (drop 3 (stream 1 2 3 4 5 6)) returns (stream 4 5 6).

In the above example, (take 3 (stream 1 2 3 4 5 6)) returns (stream 1 2 3).