"Scott J." wrote:

As far a I know sets can implemented by implementing a list of anything(a list of all types) The sets Haskell does have are AFAIK sets of elements of the same type: these are not general sets.

Ah. That's a static typing issue. However, I don't think it's really an issue. In axiomatic set theory, every set must be a subset of some other set, except for powersets and unions. Unions are implementable using Either, and powersets are implementable using Set (i.e., powerset :: Set a -> Set (Set a)).

Scott

Jon Cast