Hi, Does it make sense to declare a transparent identity instance for Functor, Applicative, Monad, etc? For example, I might want to generalize ($) = (<*>) where
($) :: (a -> b) -> a -> b (<*>) :: (Functor f) => f (a -> b) -> f a -> f b
The traditional definition makes Identity a newtype:
newtype Identity a = Identity a instance Applicative Identity where pure a = Identity a (Identity f) <*> (Identity a) = Identity (f a)
But using this instance becomes unwieldy. If using Identity was transparent, eg. if it was a type synonym
{-# LANGUAGE TypeSynonymInstances #-} type Identity a = a instance Applicative Identity where -- something like pure a = a f <*> a = f a
But GHC does not accept type synonym instances unless they are fully applied. Is it sound for such an instance to exist? If so, how might it be defined? -- Jafet