help wanted with type error message

Marcin 'Qrczak' Kowalczyk
Thu, 5 Apr 2001 13:18:48 +0200 (CEST)

On Thu, 5 Apr 2001, Malcolm Wallace wrote:

> And incidentally, nhc98 also supports this particular extension with
> this syntax.  (Hbc supports it too, but with a slightly different
> syntax.)

A propos language extensions. I have a case where multiparameter classes
with functional dependencies are essential. I'm developing a unified
collection interface (this is about a fifth try, but it finally seems
to look well). It would be great if that extension was standard some day.

It's also very helpful to have arbitrary types in instance contexts, not
only type variables (it's not the first time I need it). Unfortunately it
implies -fallow-undecidable-instances in ghc. I assume that the fact that
it's undecidable makes it a poor candidate for general acceptance. So if
some great brain found how to extend the decidable area to cover some
richer instance contexts...

A third extension which is ocasionally useful is not present in any
Haskell implementation. I would like to say
    instance (forall a. Seq (s a) a) => Foo (Bar s)
It would allow to e.g. wrap my collections in Edison's interfaces, not
only vice versa.

Marcin 'Qrczak' Kowalczyk