[Haskell-cafe] Data.Functor.Compose Combinator

Ben Kolera ben.kolera at gmail.com
Wed Mar 20 06:45:57 UTC 2019


I was wondering if this function exists somewhere (i'm not attached to the

(<<$>>) :: (Functor f) => (a -> g b) -> f a -> Compose f g b
(<<$>>) mkG = Compose . fmap mkG

It's handy if you're working with an applicative (e.g Reflex.Dynamic) and
want to layer on top a validation type function that returns a different
applicative that need composing. So I figure that it must exist somewhere!


data Person = Person Text Email

nameTextDyn     :: Dynamic t Text
emailTextDyn    :: Dynamic t Text
validEmail      :: Text -> Validation (NonEmpty Text) Email

validPersonDyn :: Dynamic t (Validation (NonEmpty Text) Person =
  getCompose $ Person
    <$> (pure       <<$>> name)
    <*> (validEmail <<$>> emailTextDyn)

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