I think you're looking at the Monoid instance for First: λ> import Data.Monoid λ> let first f = getFirst . foldMap (First . f) λ> first (\x -> guard (x > 2) *> pure x) [1..10] Just 3 Matt Parsons On Fri, Apr 7, 2017 at 2:57 AM, Paul Brauner <polux2001@gmail.com> wrote:
Oops, there's a free variable in there, let me fix it:
first f [] = Nothing first f (x:xs) = fmap (:xs) (f x) <|> fmap (x:) (first f xs)
Paul
On Fri, Apr 7, 2017 at 10:42 AM Paul Brauner <polux2001@gmail.com> wrote:
Hello Haskell Cafe,
I had to write the following function and I was wondering if it (or its generalization to Alternative or Traversable) was exposed by some library under a different name:
first [] = Nothing first (t:ts) = fmap (:ts) (s t) <|> fmap (t:) (first ts)
Somehow it is to "traverse" as "any" is to "all".
Cheers, Paul
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