Sorry about the double send, David. I forgot to switch to reply-all in the gmail interface.
On Tue, Jun 14, 2011 at 11:49 AM, <dm-list-haskell-pr...@scs.stanford.edu> wrote: > You absolutely still can use FunctionalDependencies to determine type > equality in GHC 7. For example, I just verified the code below with > GHC 7.02: > > *Main> typeEq True False > HTrue > *Main> typeEq (1 :: Int) (2 :: Int) > HTrue > *Main> typeEq (1 :: Int) False > HFalse > > As always, you have to make sure one of the overlapping instances is > more specific than the other, which you can do by substituting a > parameter c for HFalse in the false case and fixing c to HFalse using > another class like TypeCast in the context. (As contexts play no role > in instance selection, they don't make the instance any more > specific.) > > While I don't have convenient access to GHC 6 right this second, I'm > pretty sure there has been no change for a while, as the HList paper > discussed this topic in 2004. Okay. I don't really write a lot of code like this, so maybe I missed the quirks. In that case, HList has been relying on broken behavior of fundeps for 7 years. :) Because the instance: instance TypeEq a b c violates the functional dependency, by declaring: instance TypeEq Int Int Int instance TypeEq Int Int Char instance TypeEq Int Int Double ... instance TypeEq Int Char Int instance TypeEq Int Char Char ... and adding the constraint doesn't actually affect which instances are being declared, it just adds a constraint requirement for when any of the instances are used. It appears I was wrong, though. GHC doesn't actually fix the instance for such fundeps, and the following compiles and runs fine: class C a b | a -> b where foo :: a -> b foo = error "Yo dawg." instance C a b where bar :: Int bar = foo "x" baz :: Char baz = foo "x" so we're using an instance C String Int and an instance C String Char despite the fact that there's a functional dependency from the first argument to the second. -- Dan _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
