CODEWITHSUNDEEP
haskellgenericsghc-generics
user1002430
user1002430

Reputation:

How to create a Mk instance for GHC.Generics.U1?

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?

Upvotes: 1

Views: 67

Answers (1)

Cactus
Cactus

Reputation: 27636

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

Upvotes: 3

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