| This compiles just fine, as I would expect. But now let's say I want | to change it to make IntFn a newtype: | | newtype IntFn a = IntFn (a -> Int) | | bar :: (forall a. (FooClass a) => IntFn a) -> Bool | bar (IntFn fn) = foo fn The easiest way is to imagine transforming the function to System F. I'll ignore the (FooClass a) bit since it's irrelevant. We'd get bar = \(x :: forall a. IntFn a). ...what?... We can't do case-analysis on x; it has a for-all type, so we must instantiate it. But at what type? Let's guess some random type 'b': bar = \(x : forall a. IntFn a). case (x b) of IntFn (f :: b -> Int) -> foo f But now we are in trouble. 'foo' itself needs an argument of type (forall a. a->Int). So we need a big lambda bar = \(x : forall a. IntFn a). case (x b) of IntFn (f :: b -> Int) -> foo (/\a. f) But this is obviously wrong because 'b' escapes the /\a. I don't know if I can explain it better than that Simon | | I had expected this to compile too. But instead, I get an error like | this one (using GHC 7.4.1): | | Couldn't match type `a0' with `a' | because type variable `a' would escape its scope | This (rigid, skolem) type variable is bound by | a type expected by the context: FooClass a => a -> Int | The following variables have types that mention a0 | fn :: a0 -> Int (bound at <the line number that implements bar>) | In the first argument of `foo', namely `fn' | In the expression: foo fn | In an equation for `bar': bar (IntFn fn) = foo fn | | I don't think I am grasping why adding the layer of newtype | wrapping/unwrapping, without otherwise changing the types, introduces | this problem. Seems to me like the newtype version would also be type- | safe if GHC allowed it (am I wrong?), and I'm failing to understand why | GHC can't allow it. Is there a simple explanation, or else some | reading that someone could point me to? (-: | | Cheers, | -Matt | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe