[Haskell-cafe] Rank N type tutorial?

Anyone know of a good source for learning about higher ranked types? I'm not quite sure why this is illegal...

foo :: Integer -> (forall a. Show a => a) foo 2 = ["foo"] foo x = x

...while this is just fine...

bar :: Integer -> (forall a. Show a => a->b) -> b bar 2 k = k ["bar"] bar x k = k x

Thanks, Greg Buchholz