Re: [Haskell-cafe] fundeps => type family

Ryan, That did it! Thanks a lot for your help. Tad On Sun, Apr 3, 2011 at 1:14 PM, Ryan Ingram wrote:

On Sun, Apr 3, 2011 at 1:00 PM, Tad Doxsee 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)