[Haskell-cafe] Re: Rank 2 polymorphism in pattern matching?

8 Apr 2006


      On 2006-04-08, C Rodrigues  wrote:
...
This counterintuitive typechecking result came up when I wrote a wrapper 
around runST.  Is there some limitation of HM with respect to type checking 
pattern matching?
data X a b = X (a -> a)
run :: forall a. (forall b. X a b) -> a -> a
-- This definition doesn't pass the typechecker
run (X f) = f
-- But this definition works
run x = (\(X f) -> f) x
Have you tried 
run (X f) x = f x
?


-- 
Aaron Denney
-><-