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.

  1. lazy-seq
    1. API
    2. Examples
      1. Defining a custom lazy-seq
    3. About this egg
      1. Source
      2. Author
      3. Version history
      4. License

API

[procedure] (make-lazy-seq body)

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:

[syntax] (lazy-seq body ...)

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

[procedure] (lazy-seq? seq)

Predicate for checking whether seq is a lazy-seq.

[procedure] (lazy-seq-realized? seq)

Predicate for checking whether seq has already been realized.

[procedure] (lazy-null? seq)

Predicate for checking whether seq is null. Realizes seq.

[procedure] (lazy-seq->list seq)

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

[procedure] (list->lazy-seq list)

Turns list into a realized lazy-seq.

[procedure] (lazy-head seq)

Realizes seq and returns its head.

[procedure] (lazy-tail seq)

Realizes seq and returns its tail.

[procedure] (lazy-length seq)

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

[procedure] (lazy-append seqs ...)

Returns a lazy-seq representing the concatenation of seqs.

[procedure] (lazy-reverse seq)

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.

[procedure] (lazy-take n seq)

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

[procedure] (lazy-drop n seq)

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

[procedure] (lazy-take-while pred? seq)

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

[procedure] (lazy-drop-while pred? seq)

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

[procedure] (lazy-ref n seq)

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

[procedure] (lazy-map proc seqs ...)

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

[procedure] (lazy-filter pred? seq)

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

[procedure] (lazy-each proc seqs ...)

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

[procedure] (lazy-iterate proc x)

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

[procedure] (lazy-repeat x)

Returns an infinite lazy-seq of x.

[procedure] (lazy-repeatedly thunk)

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

[procedure] (lazy-cycle seq)

Returns a lazy-seq which infinitely cycles through seq.

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

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.

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

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.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
 met:
 
 Redistributions of source code must retain the above copyright
 notice, this list of conditions and the following disclaimer.
 
 Redistributions in binary form must reproduce the above copyright
 notice, this list of conditions and the following disclaimer in the
 documentation and/or other materials provided with the distribution.
 
 Neither the name of the author nor the names of its contributors may
 be used to endorse or promote products derived from this software
 without specific prior written permission.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
 INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
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 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
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