[Haskell] Quantified class constraints (& back-chaining)

[Stefan O'Rear]

On Wed, Aug 01, 2007 at 05:29:19PM -0700, Conal Elliott wrote:
> Sadly, this solution runs into the problem of instance selection based only
> on head-matching, not back-chaining into constraints.  For instance, I'd
> like also to use the following "conflicting" declaration.
> 
>     instance (Applicative f, Monoid a) => Monoid (f a) where
>       mempty  = pure mempty
>       mappend = liftA2 mappend

No quotes - [] is both Applicative and Monoid.  Should [String]
["ab","cd"] `mappend` ["ef","gh"] give ["ab","cd","ef","gh"] or
["abef","abgh","cdef","cdgh"]?

> What's the state of thinking & doing with regard to universally quantified
> class constraints?
> 
> Note that hereditary Harrop formulas do include universally quantified
> goals.  Less ambitiously, I think GHC's type-checker already deals with
> universally-quantified variables, so perhaps quantified constraints are not
> a great reach (just guessing).

It's something I've wanted... Got a link for hereditary Harrop formulas
so I can add them to my to-implement-when-Qhc-is-good-enough list?
Google isn't telling me much about them except how to add support for
constaints, which isn't terribly helpful.

Stefan
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