On Mon, Dec 15, 2008 at 2:15 PM, Thomas DuBuisson <thomas.dubuisson@gmail.com> wrote:

2008/12/15 Mario Blazevic <mblazevic@stilo.com>

Alexander Dunlap wrote:

The problem is that y is not mentioned in the signature of wrapper.
When you call wrapper x, there could be many different instances of
Container x y with the same x, so GHC doesn't know which version to
call.

I guess I see it now. However, if the explicit 'Container x y =>' context couldn't fix the y to use for instantiation of Container x y, I don't see any way to fix it. And if there is no way to call wrapper in any context, the class declaration itself is illegal and GHC should have reported the error much sooner. Should I create a ticket?

Please do not create a ticket. Such a typeclass is legitimate, but not useful alone or with functional dependencies. It is useful with Type Families though, so celebrate!

Thomas

Ok, now I get to laugh at myself. Caught up in the type family fun, I didn't even notice I obliterated the MPTC issue that started the whole discussion. Slowing down to think, I can't find an example where the original MPTC is any good and it should thus receive a compile time error. Perhaps someone will come along and give a legitimate example.

Thomas