Don't use the > data (context) => type = constructors syntax, it doesn't do what you want.
All it does is add the context to the constructor B while not providing it to any of the functions that use it. A better solution is > data Bar a = forall b. Foo a b => B a b or, equivalently, using GADT syntax: > data Bar a where > B :: Foo a b => a -> b -> Bar a Pattern matching on B will bring the Foo a b context into scope which will fix b via the functional dependency. However, I prefer this way of solving the problem: > class Foo a where > type FooVal a > ... > > data Bar a = B a (FooVal a) -- ryan 2009/2/16 Louis Wasserman <wasserman.lo...@gmail.com>: > Is there a way of exploiting functional dependencies in the following > fashion? > > class Foo a b | a -> b where... > > data Foo a b => Bar a = B a b > > This is not ambiguous, because the functional dependency ensures a unique b > if one exists. Can this be done without mentioning b as a type variable in > Bar? > > Louis Wasserman > wasserman.lo...@gmail.com > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
