Suppose I have the following function:
(define foo
(lambda (n)
(values n (* 2 n))))
I want to call map foo over the list '(1 2 3 4). Hence, I create a map-values function:
(define map-values
(lambda (f xs)
(match xs
['() (values '() '())]
[(cons x xs)
(let-values ([(a b) (f x)]
[(as bs) (map-values f xs)])
(values (cons a as) (cons b bs)))]
)))
Now, I can do the following:
(map-values foo '(1 2 3 4)) => (values '(1 2 3 4) '(2 4 6 8))
However, what if foo returns three values, or four, ... etc.? I'll need to create a new map-values function for each case. Is there a way to generalize map-values so that is can work with any foo that returns multiple values?
This does what you want:
(define (map-values proc lst)
(define (wrap e)
(call-with-values (lambda () (proc e)) list))
(apply values
(apply map list (map wrap lst))))
(define (foo n)
(values n (* 2 n) (/ 1 n)))
(map-values foo '(1 2 3 4))
; ==> (1 2 3 4)
; ==> (2 4 6 8)
; ==> (1 1/2 1/3 1/4)
This is compatible with the standard schemes as well.