| "Constraints on datatype declarations are a misfeature of Haskell, and | have no useful effect." | | Is this the final conclusion? Yes, it is, I believe. Constraints on data type declarations are a mis-feature. I tried to get them removed, but there was some argument that they could be made useful (see John Hughes's paper "Restricted data types in Haskell", Haskell workshop 1999), and they stayed. Simon Made useful in a way that would solve the problem in this thread, I might add. The paper is at http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps for the curious. John