[Haskell] Problem when mixing type classes and type families
- by Giuseppe Maggiore
Hi! This code compiles fine:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
TypeOperators, TypeSynonymInstances, TypeFamilies #-}
class Sel a s b where
type Res a s b :: *
instance Sel a s b where
type Res a s b = (s -> (b,s))
instance Sel a s (b->(c,a)) where
type Res a s (b->(c,a)) = (b -> s -> (c,s))
but as soon as I add the R predicate ghc fails:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables,
TypeOperators, TypeSynonymInstances, TypeFamilies #-}
class Sel a s b where
type Res a s b :: *
instance Sel a s b where
type Res a s b = (s -> (b,s))
class R a where
type Rec a :: *
cons :: a -> Rec a
elim :: Rec a -> a
instance Sel a s (b->(c,Rec a)) where
type Res a s (b->(c,Rec a)) = (b -> s -> (c,s))
complaining that:
Illegal type synonym family application in instance:
b -> (c, Rec a)
In the instance declaration for `Sel a s (b -> (c, Rec a))'
what does it mean and (most importantly) how do I fix it?
Thanks
