Seeing as we are taling about type class extensions, can you see any problems with the following... class X x instance Int instance Float instance x Here we have overlapping instances... (bad), but if we look at the cases there is one which will match 'x' but never any of the others... that is when the overloading is unresolved... like in: show (read y) suppose we replace X with class X x y | x -> y instance Int Int instance Float Float instance x Int What we mean is for 'x' to catch anything that does not match (not Int or Float)... but this is broken because the programs meaning can change when extra instances are added... But considering above, the 'unresolved condition' is included in x, as well as all the overlapping cases... so is it safe to say: class X x y | x -> y instance Int Int instance Float Float instance (_|_) Int Where (_|_) is some symbol that represents "no match is possible" or a failure of the overloading resolution... This _cannot_ overlap with the other instances, and is distinct (the meaning does not change if instances are added)... This could be used to force resolution in unresolvable cases (much like Integrals default to Integer is ambiguous)... Any thoughts? Keean.