Haskell: actual IO monad implementation, in different language?

Question

How is IO monad actually implemented?in sense of, what would be the actual implementation of the main function?

How would I call haskell function (IO) f

Answer 1

In fact "IO a" is just "() -> a" in an impure language (where functions can have side effect). Let's say you want to implement IO in SML :

structure Io : MONAD =
struct
  type 'a t = unit -> 'a
  return x = fn () => x
  fun (ma >>= g) () = let a = ma ()
                      in g a ()
  executeIo ma = ma ()
end