[Haskell-cafe] Aren't type system extensions fun? [Further analysis]

[Darrin Thompson]

On Tue, May 27, 2008 at 3:40 PM, Kim-Ee Yeoh <a.biurvOir4 at asuhan.com> wrote:
> Let's fill in the type variable: (x -> x) -> (Char, Bool) ==>
> forall x. (x -> x) -> (Char, Bool) ==> x_t -> (x -> x) -> (Char, Bool),
> where x_t is the hidden type-variable, not unlike the reader monad.
>
> As you've pointed out, callER chooses x_t, say Int when
> passing in (+1) :: Int -> Int, which obviously would break
>  \f -> (f 'J', f True).
>
> What we want is the callEE to choose x_t since callEE needs to
> instantiate x_t to Char and Bool. What we want is
>  (x_t -> x -> x) -> (Char, Bool).
> But that's just
>  (forall x. x -> x) -> (Char, Bool).
>

Nice. That's the first time any of this really made sense to me. Is it
possible to construct valid argument for that function?

--
Darrin