Overlapping, undecidable, incoherent -- or worse?
MR K P SCHUPKE
k.schupke at imperial.ac.uk
Thu May 20 20:34:18 EDT 2004
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),,,
IMHO, things should either stay as they are (only instance heads supported)
or it should switch to full Prolog style backtracking.
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.
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