1. List comprehensions
    1. The API
      1. list-comprehensions
      2. range
      3. repeat
      4. iterate-times
      5. iterate-while
      6. iterate-until
      7. collect
    2. Requirements
    3. Examples
  2. Last update
  3. Author
  4. License
  5. Version History

List comprehensions

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

Here, it is implemented with the macro collect, which is inspired by Clojure's for macro.

Other list-generating procedures, like range, repeat, iterate-times, iterate-until and iterate-while are provided as well, in a curried and an uncurried version. In the sequel, only the uncurried version is documented, the curried version is supplied to be used with map and friends.

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)
[procedure] (repeat times x)

produces a list of length times where all items are x

iterate-times

[procedure] (iterate-times fn k)
[procedure] (iterate-times fn k start)

produces a list where the items are constructed by successively applying fn k times to the last value, starting with start.

iterate-while

[procedure] (iterate-while fn ok?)
[procedure] (iterate-while fn ok? start)

produces a list where the items are constructed by successively applying fn to the last value, starting with start, as long as the value passes the ok? test.

iterate-until

[procedure] (iterate-until fn ok?)
[procedure] (iterate-until fn ok? start)

produces a list where the items are constructed by successively applying fn to the last value, starting with start, until the value passes the ok? test.

collect

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

where item is an expression with variables to be bound in the qualifiers. Each qualifier is of the form (var lst fltr ...) whith

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

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

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)

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

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

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

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

Last update

Jul 05, 2019

Author

Juergen Lorenz

License

Copyright (c) 2017-2019, Juergen Lorenz
All rights reserved.
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Version History

1.2
macro for renamed collect
1.1
curried and uncurried versions, syntax of for changed
1.0
initial import for chicken-5