Haskell – Aplicativos em GHCi

A execução :i Applicativeem ghci produz o seguinte:

class Functor f => Applicative (f :: * -> *) where
  pure :: a -> f a
  (<*>) :: f (a -> b) -> f a -> f b
  GHC.Base.liftA2 :: (a -> b -> c) -> f a -> f b -> f c
  (*>) :: f a -> f b -> f b
  (<*) :: f a -> f b -> f a

Além disso, lista as instâncias disponíveis da typeclass Applicative:

instance Applicative (Either e) -- Defined in ‘Data.Either’
instance Applicative [] -- Defined in ‘GHC.Base’
instance Applicative Maybe -- Defined in ‘GHC.Base’
instance Applicative IO -- Defined in ‘GHC.Base’
instance Applicative ((->) a) -- Defined in ‘GHC.Base’
instance Monoid a => Applicative ((,) a) -- Defined in ‘GHC.Base’

Mais informações sobre aplicativos: http://notes.maciejsmolinski.com/2018/02/17/applicatives-by-example/