XPost: comp.lang.scheme

Rob St. Amant wrote:

I've lately found a use for a function that calls a predicate on each
element of a list and returns all non-null results. (I bring this up
here because it has a SOME-like flavor).

(defun all-such-that (predicate list &key key)
"Return non-nil PREDICATE results for elements in LIST."
(loop for elt in list
as value = (if key
(funcall predicate (funcall key elt))
(funcall predicate elt))
when value
collect value))

Gauche Scheme

(define (all-such-that predicate lst :optional (key values))
(filter-map predicate (map key lst)))

(all-such-that (^n (and (odd? n) n)) '(1 2 3 4 5) square)
===>
(1 9 25)

Better:

(define (all-such-that predicate lst :optional (key values))
(filter-map (^x (and (predicate x) x)) (map key lst)))

(all-such-that odd? '(1 2 3 4 5) square)
===>
(1 9 25)

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XPost: comp.lang.scheme

Frode Vatvedt Fjeld wrote:

What about (find-if #'identity my-list)? That seems to work.
Anything better?

For an obfuscation contest, I think (find-if #'null my-list :key
#'not) would do well :) Otherwise, the function "some" is often
used here. I like to define "true" as an alias for the "identity"
function for such uses.

Also, loop is often useful:

(loop for x in my-list thereis x)

Gauche Scheme

Using "+" for "identity".

gosh> (any + '(#f #f "DD"))
"DD"
gosh> (any + '(#f #f))
#f

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