GHC doesn't terminate when solving a weird class instance
The following:
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
module Test where
import GHC.TypeLits
data Pet (a :: k) = Pet
{ name :: String
} deriving Show
class SetField (name :: Symbol) s t b | name t -> s b
, name s -> t b where
setField :: b -> s -> t
instance
( SetField "name" (Pet a) (Pet b) String
) => SetField "name" (Pet (a :: k)) (Pet (b :: k')) String where
setField v d = d { name = v }
loop = setField @"name" 'c' (Pet "hi")
makes GHC 9.0.1 and HEAD loop. With 8.10.4 it properly terminates with an error:
TX.hs:27:8: error:
• Couldn't match type ‘[Char]’ with ‘Char’
arising from a functional dependency between:
constraint ‘SetField "name" (Pet a) (Pet b) Char’
arising from a use of ‘setField’
instance ‘SetField "name" (Pet a1) (Pet b1) String’
at TX.hs:(23,3)-(24,60)
• In the expression: setField @"name" 'c' (Pet "hi")
In an equation for ‘loop’: loop = setField @"name" 'c' (Pet "hi")
|
27 | loop = setField @"name" 'c' (Pet "hi")
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Either changing the kind of b
to k
or the instance to
instance
( SetField "name" (Pet a) (Pet b) x
, x ~ String
) => SetField "name" (Pet (a :: k)) (Pet (b :: k')) x where
setField v d = d { name = v }
makes it terminate again.