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

Sat, 08 Apr 2006 11:31:13 -0700

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


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