Re: [Haskell-cafe] Does this function have a name?

Hi Matt, this isn't exactly the same thing I think. My function replaces the first element in the list for which f returns (Just x) by x, and leaves the rest of the list untouched. Paul On Fri, Apr 7, 2017, 20:53 Matt wrote:

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 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 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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