Overlapping, undecidable, incoherent -- or worse?

MR K P SCHUPKE k.schupke at imperial.ac.uk
Thu May 20 22:25:20 EDT 2004

I think you should read the GHC manual (assuming it is up to date
and undecidable instances means what it says it does, the difference

Without undecidable instances at least one type in the instance
must not be a type variable. 

With undecidable instances you can have a default-instance (all
type variables) or a class synonym (one instance - all type variables)

Thats it... Neither GHC nor Hugs pay any attention to the 
dependancies when choosing which instance to use. The
dependancies are only considered after the decision has
been irrevocably made. If the dependancies don't hold, the
only option the compiler has is to bail out with a compile
time error.


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