Ryan, That did it! Thanks a lot for your help.
Tad On Sun, Apr 3, 2011 at 1:14 PM, Ryan Ingram <ryani.s...@gmail.com> wrote: > > > On Sun, Apr 3, 2011 at 1:00 PM, Tad Doxsee <tad.dox...@gmail.com> wrote: >> >> "Equality constraints ... enable a simple translation of programs >> using functional dependencies into programs using family >> synonyms instead. >> >> So I tried: >> >> class (T s ~ a) => ShapeC a s where >> type T s :: * >> draw :: s -> String >> copyTo :: s -> T s -> T s -> s >> >> but got a compile error: >> >> Alas, GHC 7.0 still cannot handle equality superclasses: T s ~ a >> >> So my question is, how does one convert the above code to use type >> families instead of functional dependencies? Is one technique >> preferable over another? > > Sadly the documentation assumes the feature that you show is missing. That > said, you don't need that feature for the simple FD you have. > > Just do > > class ShapeC s where > type T s :: * > draw :: s -> String > copyTo :: s -> T s -> T s -> s > > > This code should work: > > data ShapeD a = forall s. (ShapeC s, a ~ T s) => MkShapeD s > > instance ShapeC (ShapeD a) where > type T (ShapeD a) = a > draw (MkShapeD s) = draw s > copyTo (MkShapeD s) x y = MkShapeD (copyTo s x y) > > > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
