Re: [Haskell-cafe] Existential question

Now, what we can do with kl1? We can feed it an integer, say 1, and obtain function f of the type s -> (s,Bool) for an _unknown_ type s. Informally, that type is different from any concrete type. We can never find the Bool result produced by that function since we can never have any concrete value s. The only applications of f that will type check are \s -> f s f undefined both of which are useless to obtain f's result.

That's not true. We can tie the knot: let (s, o) = f s in o