Cannot derive well-kinded instance [GHC-62016]

This error means that the mentioned type class in the deriving clause does not accept the data type you are defining as argument.

More concretely, when GHC finds a declaration of the form

data T ... = C ... | D ...
  deriving Klass

there’s a small procedure which tries to drop type arguments from T until it matches the kind expected by Klass. If this procedure fails, the error message appears.

Examples

Deriving Functor for a type with an argument of non-Type kind

Error message

Main.hs:5:12: error: [GHC-62016]
    • Cannot derive well-kinded instance of form ‘Functor (AppliedToInt ...)’
        Class ‘Functor’ expects an argument of kind ‘* -> *’
    • In the data declaration for ‘AppliedToInt’

Explanation

The Functor type class applies to type constructors of exactly the kind Type -> Type. However, the kind of AppliedToInt is (Type -> Type) -> Type instead. There is thus a mismatch between the expected and given kinds.

Main.hs

Before

{-# language DeriveFunctor #-}
module Main where

data AppliedToInt f = AppliedToInt (f Int) 
  deriving Functor

After

Deriving Functor for a ground type

Error message

Main.hs:5:12: error: [GHC-62016]
    • Cannot derive well-kinded instance of form ‘Functor (Pet ...)’
        Class ‘Functor’ expects an argument of kind ‘* -> *’
    • In the data declaration for ‘Pet’

Explanation

The Functor type class applies to type constructors; that is, data types which take one other type as a parameter. The Pet data type defined in the code does not take any type parameters. As a result, GHC cannot write the Functor instance for you.

Main.hs

Before

{-# language DeriveFunctor #-}
module Main where

data Pet = Cat | Dog 
  deriving Functor

After