On Thu, Dec 08, 2005 at 09:23:22AM -0500, David Roundy wrote:
data EqContext a b = EqContext { safe_coerce :: f(a,b) -> f(b,a) }
where f(a,b) is a function of two types that returns a type, so the value of f(a,b) might be (Patch a b) or (Patch x b) or something like that.
But I'm not sure if this is possible in Haskell, and if it is, then it definitely requires some sort of tricky extension that I'm not familiar with...
I'm a bit confused, but if you have: sc :: p a b -> p a c sc = unsafeCoerce# then (with Either standing in for Patch, PatchList etc and some concrete types as parameters to simplify things) sc (undefined :: Either Int Char) :: Either Int Bool is well-typed but sc (undefined :: Either Int Char) :: Either String Char isn't. Is that what you want? Thanks Ian