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sjoerdvisscher/liftedMonoid.hs

Last active April 7, 2020 09:41
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If you have a Functor f with an instance Monoid a => Monoid (f a), f is Applicative!
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liftedMonoid.hs
{-# LANGUAGE QuantifiedConstraints, ScopedTypeVariables #-}
import Data.Monoid
-- Note: fails for instances that don't need the Monoid a or Semigroup a
pureDefault :: forall f a. (Functor f, forall a. Monoid a => Monoid (f a)) => a -> f a
pureDefault a = a <$ (mempty :: f ())
apDefault :: (Functor f, forall a. Semigroup a => Semigroup (f a)) => f (a -> b) -> f a -> f b
apDefault ff fa = (\([f], [a]) -> f a) <$> fmap (\f -> ([f], [])) ff <> fmap (\a -> ([], [a])) fa
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