[Haskell-cafe] Transparent identity instances

[Jafet]

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