GHCJS-style imports for asynchronous functions

[ion1]

GHCJS supports

foreign import javascript safe "foo($1, $c);"
  foo :: A -> IO B

where $c stands for a continuation/callback function such that

do { b <- foo a; {-rest-} }

translates into an equivalent of

foo(a, function (b) { /*rest*/ });

instead of

var b = foo(a); /*rest*/

making asynchronous code more convenient to write. It would be nice if Haste had that.

[valderman]

Since Haste's IO monad is synchronous, this would be hard to support with the normal FFI. However, it would probably be quite possible to implement something like this for Haste.Foreign. Will look into it.