Hi Thomas, this reminded me of GADTs http://www.haskell.org/ghc/docs/6.4/html/users_guide/gadt.html a first attempt is appended below, although you might find it difficult to use in practice, because of the static guarantee. cheers, claus -------------------------------------------------------------- {-# OPTIONS -fglasgow-exts #-} type Export = String data SModule data SUnknown data SCode a where SModule :: String -> [Export] -> SCode SModule SUnknown :: SCode SUnknown doSomethingToAModule :: SCode SModule -> String doSomethingToAModule (SModule s es) = s -- doSomethingToAModule SUnknown = "oops" main = putStrLn $ doSomethingToAModule (SModule "hi" []) -- main = putStrLn $ doSomethingToAModule SUnknown -- type-checks and works as given in ghci-6.4.1 -- adding the second doSomethingToAModule line results in: -- Scode.hs:14:21: -- Inaccessible case alternative: Can't match types `SUnknown' and `SModule' -- When checking the pattern: SUnknown -- In the definition of `doSomethingToAModule': -- doSomethingToAModule SUnknown = "oops" -- Failed, modules loaded: none. -- using the second main alternative results in: -- Scode.hs:17:39: -- Couldn't match `SModule' against `SUnknown' -- Expected type: SCode SModule -- Inferred type: SCode SUnknown -- In the first argument of `doSomethingToAModule', namely `SUnknown' -- In the second argument of `($)', namely `doSomethingToAModule SUnknown' -- Failed, modules loaded: none. --------------------------------------------------------------
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).