List comprehensions (historical revision 37139)

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List comprehensions
1. The API
2. Requirements
3. Examples
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List comprehensions

List comprehensions are popularized by other functional languages like Standard ML, Miranda, Haskell.

The implementation of this module is inspired by Clojure, in particular the for macro.

Other list-generating procedures, like range, repeat, iterate-times, iterate-until and iterate-while are provided as well.

The API

list-comprehensions

[procedure] (list-comprehensions sym ..)

Documentation procedure. Without argument, returns the list of exported symbols, with argument the documentation of that symbol.

range

[procedure] (range upto)
[procedure] (range from upto)
[procedure] (range from upto step)

creates a list of numbers with given limits from defaults to 0 step defaults to 1

repeat

[procedure] (repeat times)

returns a unary procedure which repeats its only argument a number of times

iterate-times

[procedure] (iterate-times fn times)

returns a unary procedure which iterates the function fn on its only argument a number of times

iterate-while

[procedure] (iterate-while fn ok?)

returns a unary procedure which iterates the function fn on its only argument while the predicate ok? returns true

iterate-until

[procedure] (iterate-until fn ok?)

returns a unary procedure which iterates the function fn on its only argument until the predicate ok? returns true

for

</macro>(for (qualifier ....) item)</macro>

where each qualifier is of the form (var lst fltr ...) whith

var a variable
lst a list from which values are chosen
fltr a filter expression

Creates a new list by binding var to each element of lst in sequence, and if it passes the checks fltr ..., inserts the item expression into the result list. The qualifieres are processed sequentially from left to right, so that filters of a qualifier have access to the variables of qualifiers to its left.

For example

  (for ((x (list #\A #\B))
        (y (list 1 2))
        (z (list #f #t)))
       (list x y z))

produces

  ((#\A 1 #f)
   (#\A 1 #t)
   (#\A 2 #f)
   (#\A 2 #t)
   (#\B 1 #f)
   (#\B 1 #t)
   (#\B 2 #f)
   (#\B 2 #t))

Requirements

None

Examples


(import list-comprehensions)

(range 0) ; -> '()

(range 5) ; -> '(0 1 2 3 4)

(range -5) ; -> '(0 -1 -2 -3 -4)

(range 1 5) ; -> '(1 2 3 4)

(range 5 1) ; -> '(5 4 3 2)

(range 1 5 2) ; -> '(1 3)

((repeat 5) 'x) ; -> '(x x x x x)

((iterate-times add1 5) 1) ; -> '(1 2 3 4 5)

((iterate-until sub1 zero?) 5) ; -> '(5 4 3 2 1)

((iterate-while sub1 positive?) 5) ; -> '(5 4 3 2 1)

(for ((c (string->list "ABCDEFG"))
      (k '(1 2 3 4 5 6 7 8)))
     (list c k))
; -> ((#\A 1) ... (#\A 8) ... (#\G 1) ... (#\G 8))

(for ((c (string->list "ABCDEFG"))
      (k '(1 2 3 4 5 6 7 8) (memv c '(#\A #\G)) (memv k '(1 8))))
     (list c k))
; -> ((#\A 1) (#\A 8) (#\G 1) (#\G 8)) 

(for ((x '(0 1 2 3))) (add1 x)) ; map
; -> '(1 2 3 4)

(for ((x '(0 1 2 3 4 5) (odd? x))) x) ; filter
; -> '(1 3 5)

Copyright (c) 2017-2019, Juergen Lorenz
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Version History

1.0: initial import for chicken-5