How to constrain a class instance

Question

I'm trying to apply a type constraint to an instance of a class that has kind ->.

data Data1 = Data1 String

class Class1 m where
    c1f :: m -> m

instance Class1 Data1 where
    c1f (Data1 v) = Data1 v


data Data2 a = Data2 a

class Class2 m where
    c2f :: a -> m a

instance Class2 Data2 where
    c2f x = Data2 (c1f x)

I see this error message from the last line:

No instance for (Class1 a) arising from a use of ‘c1f’

How do I apply the type constraint (Data 2 must be instance of Class1)?

Answer 1

Are you looking to let each instance of Class2 pick its own constraint? If so, try this:

{-# LANGUAGE ConstraintKinds, TypeFamilies #-}

import Data.Kind

data Data1 = Data1 String

class Class1 m where
    c1f :: m -> m

instance Class1 Data1 where
    c1f (Data1 v) = Data1 v


data Data2 a = Data2 a

class Class2 m where
    type C2c m :: Type -> Constraint
    c2f :: C2c m a => a -> m a

instance Class2 Data2 where
    type C2c Data2 = Class1
    c2f x = Data2 (c1f x)