On Mon, Jul 30, 2007 at 05:31:40AM -0400, Dan Licata wrote:
With the functional dependency, you can't work with the view datatypes at all. Once you write
type Typ data TypView = Unit | Arrow Typ Typ
instance View Typ TypView where view = ...
you're no longer allowed to take apart a TypView at all!
Exactly. And I'm 100% convinced it's a non-issue, since all the heavyweight view proposals don't allow you to manipulate view objects *at all*. My approach is no worse.
E.g. you can't write
outUnit :: TypView -> Bool outUnit Unit = True outUnit _ = False
because the implicit application of the view function will mean that outUnit must consume a Typ.
What would you use it for, anyway? TypView objects don't exist anywhere except internally in case-expressions.
Personally, I'd rather have special syntax in the pattern (-> pat) than have these global effects on what you can do with certain types.
You can only declare the instance for TypView in the same module as TypView itself, since otherwise it would conflict with the implicit instance. Therefore, providing an instance is no more "global" than just renaming the type. Stefan