f :: forall a b. C a b => T a -> Bool f T1 = True f T2 = (op :: a -> b) 3
as that results in the counter-intuitive
Couldn't match expected type `Bool' against inferred type `b' `b' is a rigid type variable bound by the type signature for `f' at C:\Users\claus\Desktop\tmp\fd-local.hs:17:14 In the expression: (op :: a -> b) 3 In the definition of `f': f T2 = (op :: a -> b) 3
Which seems to be a variant of Oleg's example?
If it is, it is a simpler variant, because it has a workaround: f :: forall a b. C a b => T a -> Bool f T1 = True f T2 = (op :: C a b1 => a -> b1) 3 While playing around with Oleg's example, I also found the following two pieces (still ghc-6.12.3): -- first, a wonderful error message instead of expected simplification Prelude> (id :: (forall b. b~Bool=>b->b) -> (forall b. b~Bool=>b->b)) <interactive>:1:1: Couldn't match expected type `forall b. (b ~ Bool) => b -> b' against inferred type `forall b. (b ~ Bool) => b -> b' Expected type: forall b. (b ~ Bool) => b -> b Inferred type: forall b. (b ~ Bool) => b -> b In the expression: (id :: (forall b. (b ~ Bool) => b -> b) -> (forall b. (b ~ Bool) => b -> b)) In the definition of `it': it = (id :: (forall b. (b ~ Bool) => b -> b) -> (forall b. (b ~ Bool) => b -> b)) -- second, while trying the piece with classic, non-equality constraints Prelude> (id :: (forall b. Eq b=>b->b) -> (forall b. Eq b=>b->b)) <interactive>:1:0: No instance for (Show ((forall b1. (Eq b1) => b1 -> b1) -> b -> b)) arising from a use of `print' at <interactive>:1:0-55 Possible fix: add an instance declaration for (Show ((forall b1. (Eq b1) => b1 -> b1) -> b -> b)) In a stmt of an interactive GHCi command: print it Note that the second version goes beyond the initial problem, to the missing Show instance, but the error message loses the Eq constraint on b! -- it is just the error message, the type is still complete Prelude> :t (id :: (forall b. Eq b=>b->b) -> (forall b. Eq b=>b->b)) (id :: (forall b. Eq b=>b->b) -> (forall b. Eq b=>b->b)) :: (Eq b) => (forall b1. (Eq b1) => b1 -> b1) -> b -> b I don't have a GHC head at hand, perhaps that is doing better? Claus