Overlapping, undecidable, incoherent -- or worse?

Alex Ferguson abf at cs.ucc.ie
Thu May 20 22:15:30 EDT 2004

On Thu, May 20, 2004 at 07:34:18PM +0100, MR K P SCHUPKE wrote:
> that is not the case with "-fallow-undecidable-instances" ... as far as
> I understand it , ghc never considers the dependancies when selecting an
> instance. If you don't think so you will need to show me an example where
> it clearly does... as I haven't seen one yet (but just because I haven't
> seen it doesn't mean its not possible),,,

Am I to understand you're asserting that GHC "undecidable-instances" are
in fact decidable?  Hugs explicitly documents its type system as being
undecidable, and I'd presumed this was the 'make GHC behave that way
too' flag.

> I mean, given two instances like:
> 	instance a b c
> 	instance d e f
> there is no way to tell between them... if you said choose a in preferenc
> 'a' would be chosen all the time. 

If you were to indicate one instance declaration was preferred, and both
_could otherwise yield a suitable instance_, then that would be
prefered.  (Though if one were to 'search' in the preference order of the
rules, one wouldn't need to find more than one in any event.)

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