At Tue, 14 Jun 2011 19:52:00 -0700 (PDT), oleg@okmij.org wrote:
Dan Doel wrote:
class C a b | a -> b where foo :: a -> b foo = error "Yo dawg."
instance C a b where
The instance 'C a b' blatantly violates functional dependency and should not have been accepted. The fact that it was is a known bug in GHC. The bug keeps getting mentioned on Haskell mailing lists about every year. Alas, it is still not fixed. Here is one of the earlier messages about it:
http://www.haskell.org/pipermail/haskell-cafe/2007-March/023916.html
But Oleg, isn't what you are complaining about *exactly* the lifting of the coverage condition, which is one of the explicit points of -XUndecidableInstances? Are you advocating two separate switches for lifting Paterson vs. Coverage? What about the following code--do you think this should be illegal, too? {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE UndecidableInstances #-} class C a b c | a -> b where instance C (Maybe a) (Maybe b) (Maybe b) where David