Jeremy Yallop wrote:
Why does compiling the following program give an error?
{-# LANGUAGE TypeFamilies, RankNTypes #-}
type family TF a
identity :: (forall a. TF a) -> (forall a. TF a) identity x = x
GHC 6.10.3 gives me:
Couldn't match expected type `TF a1' against inferred type `TF a' In the expression: x In the definition of `identity': identity x = x
The error message is slightly better in GHC head: Couldn't match expected type `TF a1' against inferred type `TF a' NB: `TF' is a type function, and may not be injective In the expression: x In the definition of `identity': identity x = x Dan Doel already explained how the lack of injectivity leads to a type checking error. FWIW, the same code would work with a data family, because data families are injective. regards, Bertram