2 Multiple Values

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2 Multiple Values

(require (planet cce/scheme:7:3/values))

This module provides tools for manipulating functions and expressions that produce multiple values.

(values->list expr)

Produces a list of the values returned by expr.

Example:
> (values->list (values 1 2 3))
'(1 2 3)

(map2 f lst ...)

→

f : (-> A ... (values B C))

lst : (listof A)

Produces a pair of lists of the respective values of f applied to the elements in lst ... sequentially.

Example:
  > (map2 (lambda (x) (values (+ x 1) (- x 1))) (list 1 2 3))
  '(2 3 4)
  '(0 1 2)

(map/values n f lst ...)

→

(listof B_1)

(listof B_n)

n : natural-number/c

f : (-> A ... (values B_1 ... B_n))

lst : (listof A)

Produces lists of the respective values of f applied to the elements in lst ... sequentially.

Example:
  > (map/values
     3
     (lambda (x)
       (values (+ x 1) x (- x 1)))
     (list 1 2 3))
  '(2 3 4)
  '(1 2 3)
  '(0 1 2)

(foldr/values f vs lst ...)

→

B

f : (-> A ... B ... (values B ...))

vs : (list/c B ...)

lst : (listof A)

(foldl/values f vs lst ...)

→

B

f : (-> A ... B ... (values B ...))

vs : (list/c B ...)

lst : (listof A)

These functions combine the values in the lists lst ... using the multiple-valued function f; foldr/values traverses the lists right to left and foldl/values traverses left to right.

Examples:
  (define (add/cons a b c d)
    (values (+ a c) (cons b d)))

  > (foldr/values add/cons (list 0 null)
                  (list 1 2 3 4) (list 5 6 7 8))
  10
  '(5 6 7 8)
  > (foldl/values add/cons (list 0 null)
                  (list 1 2 3 4) (list 5 6 7 8))
  10
  '(8 7 6 5)

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