[tomp]

The problem with this idea, at least compared with current implementations of Haskell and OCaml, is that it requires runtime type information. Right now, Haskell and OCaml both perform erasure, and the only kind of runtime type information used is for the GC, but if you have two ADTs with identical structure (number of branches, payloads) but different names (i.e. they are different types), they will be encoded in the same way. So, in order to tell them apart, you would need some additional type information, which would require changes to compilers and would probably performance characteristics.