If f2 is legal, why if f3 illegal? For some usage site of f3, the constraint C String b might allow b to be resolved given whatever instances of C are in effect.
Is there a motivation for these behaviors?
Are these sorts of cases discussed in the CHR/FD paper that motivated the coverage condition (which I have yet to read)?
It should be possible to resolve the constraints from the types in the signature. That rules out f1 and f3. For f2, given a type for a, say Int, not just any odd instance for b will do, say instance C Int Bool GHC says: No instance for (C Int b) arising from use of `f2' at Q.hs:30:6-9 Possible fix: add an instance declaration for (C Int b) In the expression: f2 x In the definition of `f': f x = f2 x No, it has to be an instance for all possible types b: instance C Int b So, you can only call f2 with types a for which the instance C a b is completely independent of b. There is no ambiguity. Tom -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers@cs.kuleuven.be