23 Sep
2008
23 Sep
'08
12:19 p.m.
On Tue, Sep 23, 2008 at 6:07 PM, Wolfgang Jeltsch
Hello,
please consider the following code:
{-# LANGUAGE GADTs, MultiParamTypeClasses, FunctionalDependencies #-}
data GADT a where
GADT :: GADT ()
class Class a b | a -> b
instance Class () ()
fun :: (Class a b) => GADT a -> b fun GADT = ()
I'd expect this to work but unfortunately, using GHC 6.8.2, it fails with the following message:
bear in mind that the only instance you defined is instance Class () () which doesn't involve your GADT at all. Maybe you meant something like: instance Class (GADT a) () Moreover, your fun cannot typecheck, regardless of using MPTC or GADTs. The unit constructor, (), has type () and not b.