Hi, does anybody have experiences with "Type Classes with Functional Dependencies" (LNCS 1782, ESOP 2000, March 200, by Mark P. Jones)? I've tried the example in the above paper: class Collects e ce | ce -> e where empty :: ce insert :: e -> ce -> ce member :: e -> ce -> Bool This works correctly for: instance Eq a => Collects a [a] where instance Collects Char Int where and complains correctly for: instance Collects Char Int where instance Collects Bool Int where with "Functional dependencies conflict between instance declarations" Now to my problem with a context in the class declaration: class Eq ce => Collects e ce | ce -> e where empty :: ce ... This works as above, but correctly complains for: data Stupid = Stupid -- without equality instance Collects Bool Stupid where with "No instance for (Eq Stupid)". However, if I supply an (admittingly stupid) default definition for "empty" within the class declaration: class Eq ce => Collects e ce | ce -> e where empty :: ce empty = error("empty") ... I no longer get the above "No instance for (Eq Stupid)"-Message, and I guess that is a bug (or a feature that I don't understand). When I change the context to "Eq e" everything is fine again: instance Collects Stupid Int where complains as above. I don't know when (even meaningful) default definitions cause that context conditions are not checked. In fact I noticed this error only after I've removed a default definition just to find out that my instance declaration were wrong that seemed to be correct before (but instances for contexts were missing). Are there any other (known) pitfalls with functional dependencies? Regards Christian