A aliasing of constraints/classes (this is the semantic part that could also be explained by reduction, or by simple mutual implication encodings)
B aliasing of syntax, especially instance definitions (this syntactic part is hard to encode, and simple in terms of syntactic macro expansion)
it just occurred to me that there is a precedence for this kind of translation, in associated types and type functions. defining an AT in a class is equivalent to defining a TF outside the class, and connecting the TF to the class with superclass and instance constraints, right? class C a where type CT a c :: (a,CT a) instance C a where type CT a = .. c = .. -- vs type family CT a type instance CT a = .. class CT a ~ b => C a where c :: (a,CT a) instance CT a ~ b => C a where c = .. though the latter form is not yet supported in GHC (#714). which leads me to a problem i have with ATs, which applies to class aliases as well: although the ATs are written as if they were local to the class, they get lifted out of the class in a naive manner. in particular, they can only refer to their parameters, not to other local definitions/types, and their parameters have to match the class parameters. however, i assume that the restrictions/translations/implementations for class aliases are similar to the those for the implementation of ATs in terms of TFs, which might help? claus