Re: [Haskell-cafe] forall vs "=>"

Daryoush Mehrtash wrote:

What is the difference between forall as in:

runST :: (forall s. ST s a) -> a

and the "=>" as in

evalStateT :: Monad m => StateT s m a -> s -> m a

The forall is Rank-2 polymorphism (the argument must be polymorphic in s). The => is for typeclass constraints (restricting the Rank-1 polymorphism on m, but not so far as to make it monomorphic). Or did you have another question in mind? -- Live well, ~wren