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

The IO monad is basically implemented as a state transformer (similar to State), with a special token RealWorld. Each IO operation depends on this token and passes it when it finishes. unsafeInterleaveIO introduces a second token, so that a new IO operation can start, while the other one is still doing its work.

Usually, you don't have to care about the implementation. If you want to call IO-functions from other languages, GHC cares about removing the IO wrapper. Consider this small snippet:

printInt :: Int -> IO ()
printInt int = do putStr "The argument is: "
                  print int

foreign export ccall printInt :: Int -> IO ()

This generates a symbol to call printInt from C. The function becomes:

extern void printInt(HsInt a1);

Where HsInt is just a (depending on your platform) typedefd int. So you see, the monad IO has been removed completely.