On Saturday 03 July 2010 10:52:31 pm Felipe Lessa wrote:
I understood your explanation. However, is this an implementation detail/bug or is it an intended feature?
Well, I wouldn't call it a bug. Perhaps it could be called a lack of a feature, because one can imagine such pattern matches being desugared into case statements where instantiation is allowed to happen.... But, the example here is very simple, and I can raise some issues that might convince you that this isn't a simple issue in general, and the GHC devs are somewhat justified in not bothering to sort it out, at your expense. :) First, what if we write: runSomeMonad m@(SomeMonad x) = ... What type does m have? We need to instantiate the argument to do the match on the right, so is m bound to the instantiated value, or the polymorphic value? I guess this comes down to whether the above desugars to: runSomeMonad m = case m of SomeMonad x -> ... or: runSomeMonad v = case v of m@(SomeMonad x) -> ... Second, the original example is uniform regardless of how 's' is instantiated, but that might not be true in general. If we use data families for instance: data family T a :: * data instance T Int = I1 Int | I2 Char data instance T () = U1 Int | U2 data instance T Char = C1 | C2 Int f :: (forall a. T a) -> Int ... Now, how are we going to be allowed to match in this function? f (I1 i) = ... f (I2 c) = ... This, so far, desugars fine into something like: f t = case t :: T Int of I1 i -> ... I2 c -> ... but adding a third case makes the situation trickier: f (U1 i) = ... because that'd require instantiating t to a different type. Of course, you could still desugar that to: f t = case t :: T Int of I1 i -> ... I2 c -> ... _ -> case t :: T () of U1 i -> ... but that third case is dead code. Or, should we allow mixing and matching, and instantiate separately even at the same type: f (I1 i) = ... f (C2 i) = ... f (U1 i) = ... f (I2 i) = ... becomes: f t = case t :: T Int of I1 i -> ... _ -> case t :: T Char of C2 i -> ... _ -> case t :: T () of U1 i -> ... _ -> case t :: T Int of I2 i -> ... although that could be consolidated a bit. That's off the top of my head. There may be other issues. Treating values with polymorphic type as if they were some particular instantiation isn't a simple matter, though. -- Dan