如何为GHC.Generics.U1创建一个Mk实例？

Question

I'm working through content at the blog posting Building data constructors with GHC Generics . My previous question is here .

The posting has the following code to create a Rep :

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
{-# LANGUAGE TypeOperators #-}

import GHC.Generics
import Data.Functor.Compose

class Functor f => Mk rep f | rep -> f where
  mk :: f (rep a)

instance Mk (K1 i c) ((->) c) where
  mk = \x -> K1 x

instance (Mk l fl, Mk r fr) => Mk (l :*: r) (Compose fl fr) where
  mk = Compose (fmap (\l -> fmap (\r -> l :*: r) mk) mk)

instance (Mk f f') => Mk (M1 i c f) f' where
  mk = M1 <$> mk

Is it possible to create a similar instance for U1 ? If so, how?

Answer 1

Since U1 has no structure, you should be able to mk one out of thin air in Identity :

import Control.Monad.Identity

instance Mk U1 Identity where
    mk = pure U1