On Mon, Dec 15, 2008 at 2:15 PM, Thomas DuBuisson < thomas.dubuis...@gmail.com> wrote:
> 2008/12/15 Mario Blazevic <mblaze...@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
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