Hi, On 7/29/08, Bryan Donlan <[EMAIL PROTECTED]> wrote: > Hi, > > Is there any theoretical reason that functional dependencies can't be used > to resolve a polymorphic type to a concrete type? For example: > >> -- compile with -fglasgow-exts >> >> class DeriveType a b | a -> b >> >> data A = A >> data B = B >> >> instance DeriveType A B >> > >> simpleNarrow :: DeriveType A b => b -> B >> simpleNarrow = id > > Since 'b' is uniquely determined by the fundep in DeriveType, it seems that > this ought to work; ie, since the only type equation satisfying DeriveType A > b > is B -> B, it should reduce to that before trying to fit its type against > its > body.
According to the theory of functional dependencies this function should type check but there is a bug in the current implementation (or you may view it as an incompleteness---the compiler is not smart enough to infer that "b" in this case is really "B" while checking the signature). Iavor _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
