Hi, I'd just been writing some code and an interesting idea for an extension to Haskell's type system sprang into my head. I have no idea if people have played with it, but it looked vaguely useful to me, so I thought I'd see what everyone else thought. Supposing you have these types: type Export = String data SCode = SModule String [Export] | SUnknown It would be useful to specify a function as so: doSomethingToAModule :: SCode(SModule) -> SomeRandomOtherType which would specify, not only that the first argument was of type SCode, but more specifically, that it used the SModule constructor. This would then allow you to safely only write a case for the SModule constructor, and not worry about a runtime error when pattern matching failed on an SUnknown (as this would be checked at compile time). I hope this makes sense What does anyone think of the idea, and is there an obvious flaw in the plan? Thanks Tom Davie