chickadee » lazy-seq

lazy-seq

A lazy sequence implementation inspired by Clojure's. It is an alternative to SRFI 41. See this article for the motivation of creating this egg and a comparison with SRFI 41.

API

(make-lazy-seq body) procedure

Returns a lazy-seq object. body is a thunk which will be called when the sequence is realized. It is expected to return one of the following things:

  • The empty list to signify the end of the sequence
  • A pair with the sequence's head in the car and a lazy-seq representing the sequence's tail in the cdr
  • Another lazy-seq which will be realized recursively when the sequence is realized
(lazy-seq body ...) syntax

Convenience syntax for make-lazy-seq with body ... being the thunk's body.

(lazy-seq? seq) procedure

Predicate for checking whether seq is a lazy-seq.

(lazy-seq-realized? seq) procedure

Predicate for checking whether seq has already been realized.

(lazy-null? seq) procedure

Predicate for checking whether seq is null. Realizes seq.

(lazy-seq->list seq) procedure

Completely realizes seq and returns a list of its elements. Should not be called on infinite sequences.

(list->lazy-seq list) procedure

Turns list into a realized lazy-seq.

(lazy-head seq) procedure

Realizes seq and returns its head.

(lazy-tail seq) procedure

Realizes seq and returns its tail.

(lazy-length seq) procedure

Completely realizes seq and returns its length. Should not be called on infinite sequences.

(lazy-append seqs ...) procedure

Returns a lazy-seq representing the concatenation of seqs.

(lazy-append-map proc seqs ...) procedure

Returns a lazy-seq which is the concatenation of lazy-seqs resulting from applying proc to each element in seqs.

(lazy-reverse seq) procedure

Returns a lazy-seq of the reversed seq. Note that even realizing just the head of the returned lazy-seq will realize seq completely. Infinite sequences can't be reversed.

(lazy-take n seq) procedure

Returns a lazy-seq of the first n elements of seq.

(lazy-drop n seq) procedure

Returns a lazy-seq of all but the first n elements of seq.

(lazy-take-while pred? seq) procedure

Returns a lazy-seq of all leading elements of seq satisfying pred?.

(lazy-drop-while pred? seq) procedure

Returns a lazy-seq of seq without all leading elements satisfying pred?.

(lazy-ref n seq) procedure

Realizes seq up to the nth element and returns that element.

(lazy-map proc seqs ...) procedure

Returns a lazy-seq of applying proc to each element in seqs. Terminates with the sortest of seqs.

(lazy-filter pred? seq) procedure

Returns a lazy-seq of elements from seq satisfying pred?.

(lazy-each proc seqs ...) procedure

Completely realizes seqs and applies proc to each item for its side-effect. Terminates with the sortest of seqs.

(lazy-iterate proc x) procedure

Returns a lazy-seq with a head of x and a tail of applying proc to the preceding element.

(lazy-repeat x) procedure

Returns an infinite lazy-seq of x.

(lazy-repeatedly thunk) procedure

Returns an infinite lazy-seq of the return value of thunk at the time an element is realized.

(lazy-cycle seq) procedure

Returns a lazy-seq which infinitely cycles through seq.

(lazy-numbers #!key (step 1) (start 0) count) procedure

Returns a lazy-seq of numbers starting at start and increasing by step. When count is a positive number the sequence terminates after count elements. Otherwise it is infinte.

(input-port->lazy-seq port read) procedure

Returns a lazy-seq of the results of applying read to port (which must be an input port). The sequence terminates when read returns #!eof. The port is not closed automatically by this procedure.

Examples

Defining a custom lazy-seq

(define odd-numbers
  (let next ((n 0))
    (lazy-seq
      (if (odd? n)
        (cons n (next (+ n 1)))
        (next (+ n 1))))))

(lazy-ref 5 odd-numbers) ; => 11

Note that odd-numbers can be defined more conveniently like this:

(define odd-numbers
  (lazy-filter odd? (lazy-numbers)))

About this egg

Source

The source code is hosted at Bitbucket. Feel free to fork it and send pull requests there.

Author

Moritz Heidkamp

Version history

0.0.3
Add lazy-length, lazy-reverse and lazy-cycle
0.0.2
Add lazy-take-while and lazy-drop-while
0.0.1
Initial release

License

 Copyright (c) 2012, Moritz Heidkamp
 All rights reserved.
 
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 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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