[Haskell-cafe] exporting Constructors as pattern-only

All, is there any way in Haskell to export a *data* type so that importing modules can pattern match, but not use the constructors to build anything? My use case is an AST with invariant - I want the convenience of pattern matching with the safety of having to build using functions exported by the model rather than the constructors directly. e.g given data T = T1 Bool | T2 Int | TT T T t1 :: Bool -> T t2 :: Int -> T tt :: T -> T -> T from outside I can write f(T1 False) = tt (t1 True) (t2 42) but not f(T1 False) = TT (T1 True) (T2 42) ? Regards, Andrew Butterfield School of Computer Science & Statistics Trinity College Dublin 2, Ireland