Hi
I guess it depends on how many extensions one may wish to enable. At the very least we need multi-parameter type classes with functional dependencies (because that's what TypeEq is in any case).
- If we permit no other extension, we need N^2 instances to compare N classes for equality (basically, for each type we should say how it compares to the others). This is not practical except in very limited circumstances.
- If we permit undecidable instances, one may assign numerals to types. This gives us total order and hence comparison on types. In this approach, we only need N instances to cover N types. This is still better than Typeable because the equality is decided and can be acted upon at compile time.
In my particular case whether I act at compile time or run time is unimportant, but obviously this is an important advantage in general. Unfortunately a cost of one instance per type is still higher than I'd like to pay :-)
- If we permit overlapping instances extension, then a few lines of code decide equality for all existing and future types:
class TypeEq x y b | x y -> b instance TypeEq x x HTrue instance TypeCast HFalse b => TypeEq x y b
This is exactly what I was after, but it doesn't seem to work in Hugs - even with overlapping instances and unsafe overlapping instances turned on. *** This instance : TypeEq a b c *** Conflicts with : TypeEq a a HTrue *** For class : TypeEq a b c *** Under dependency : a b -> c Is there any way to modify this to allow it to work under Hugs as well? Thanks very much for your help, Neil