Overlapping, undecidable, incoherent -- or worse?

Alex Ferguson abf at cs.ucc.ie
Fri May 21 18:10:11 EDT 2004

On Fri, May 21, 2004 at 11:04:53AM +0100, Simon Peyton-Jones wrote:
> Nothing difficult in principle, but the constraint solver is one of
> the more delicate parts of GHC because GHC's constraint language has
> become so complex.

Well, as my day job is working for a constraints lab, I feel it's my
bounded duty to say, "all the better":  more cross-disciplinary synergy.

For my part I must admit the current restrictions aren't _that_ irksome:
they're just the sort of thing I run into every so often, have to remind
myself they're the way they are, and end up writing a few more instance
declarations by hand than I initially imagined I 'ought' to have to.

On the point of complexity:  it's not immediately obvious to me that
(setting aside the Hugs-style extension to H98 in the form of the
instance decls.) it would cause any blowup here in principle.  Granted
there's a combinatorial issue, but the breadth and depth of alternatives
are necessarily finite, so it seems finitely bound overall, and since HM
typing is D-Exp anyway, I'd be surprised if it made it worse than that
in toto.

Hideousness of implementation's another matter, and certainly I don't
want to be biting the hand that feeds...


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