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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