[Hugs-users] FunDeps: divergent improvement from overlapping instances

[Anthony Clayden]

> I'm now working on the instance sequencing routines, to ignore those
> divergent Dependent positions when deciding overlap ordering.

Ok. Done. So this works:

>* class     TypeEq a b e  | a b -> e
*>* instance TypeEq a a TTrue
*>* instance TypeEq a b TFalse*


In case of multiple FunDeps for an instance, improvement must use all
FunDeps for a given instance, but not go on to improve from a less
specific instance -- if one of the FunDeps is Full.


Note a Full FunDep needn't actually deliver any improvement (change of
types) for this 'don't go on' to apply -- it's sufficient if some
other improvement has already got the Wanted to match the instance.
(In effect this is selecting the instance -- although Hugs doesn't
'officially' do so until a later step.)


Getting the preference sequence of instances was tricky -- especially
in case of multiple FunDeps: in comparing two instance heads, they
might be apart going by some FunDeps, but overlapping going by others.
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