On Thu, Nov 24, 2011 at 10:06 AM, Evan Laforge
Well yes, but the key feature is that the IDs are arbitrary strings. And they're not knowable at compile time, since they are read from user input...
You can still use phantom types for this, you just need existentials as well. For example, you could define a type for indexed strings/id's as: data IString s = IString String data Exists f = forall x. Exists (f x) read :: String -> Exists IString read stuff = Exists (IString stuff) I assume you'll also want to compare strings/id's, in which case you'll also need (using GADT's): data Equal a b where Refl :: Equal a a equal :: IString s -> IString t -> Maybe (Equal s t) equal (IString x) (IString y) | x == y = Just (unsafeCoerce Refl) equal _ _ = Nothing