[Haskell-cafe] Does this function have a name?
parsonsmatt at gmail.com
Fri Apr 7 18:53:39 UTC 2017
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]
On Fri, Apr 7, 2017 at 2:57 AM, Paul Brauner <polux2001 at 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)
> On Fri, Apr 7, 2017 at 10:42 AM Paul Brauner <polux2001 at 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".
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