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) from another language and do I in that case need to maintain IO my self?

Does main pulls IO actions (Lazily) as references and then call them? Or it is interpreter job, when it found actions in its way it can call them? Or maybe something else?

Is there good IO monad implementation in different language which can help to deeply understand what happening in main function?

Edit:

Such hGetContents confuses me a lot, and makes me unsure how IO is really implemented.

Ok, let's say I have very simple pure Haskell interpreter witch unfortunately has no IO support and for curiosity I want to add this IO actions to it (unsafeIO tricks also). It is hard to get it from GHC, Hugs or others.

Answer 1

Here is an example of how one could implement the IO monad in Java:

package so.io;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

import static so.io.IOMonad.*;  
import static so.io.ConsoleIO.*;    

/**
 * This is a type containing no data -- corresponds to () in Haskell.
 */
class Unit {
    public final static Unit VALUE = new Unit(); 
}

/**
 * This type represents a function from A to R
 */
interface Function<A,R> {
    public R apply(A argument);
}

/**
 * This type represents an action, yielding type R
 */
interface IO<R> {
    /**
     * Warning! May have arbitrary side-effects!
     */
    R unsafePerformIO();
}

/**
 * This class, internally impure, provides pure interface for action sequencing (aka Monad)
 */
class IOMonad {
    static <T> IO<T> pure(final T value) {
        return new IO<T>() {
            @Override
            public T unsafePerformIO() {
                return value;
            }
        };
    }

    static <T> IO<T> join(final IO<IO<T>> action) {
        return new IO<T>(){
            @Override
            public T unsafePerformIO() {
                return action.unsafePerformIO().unsafePerformIO();
            }
        };
    }

    static <A,B> IO<B> fmap(final Function<A,B> func, final IO<A> action) {
        return new IO<B>(){
            @Override
            public B unsafePerformIO() {
                return func.apply(action.unsafePerformIO());
            }
        };
    }

    static <A,B> IO<B> bind(IO<A> action, Function<A, IO<B>> func) {
        return join(fmap(func, action));
    }
}

/**
 * This class, internally impure, provides pure interface for interaction with stdin and stdout
 */
class ConsoleIO {
    static IO<Unit> putStrLn(final String line) {
        return new IO<Unit>() {
            @Override
            public Unit unsafePerformIO() {
                System.out.println(line);
                return Unit.VALUE;
            }
        };
    };

    // Java does not have first-class functions, thus this:
    final static Function<String, IO<Unit>> putStrLn = new Function<String, IO<Unit>>() {
        @Override
        public IO<Unit> apply(String argument) {
            return putStrLn(argument);
        }
    };

    final static BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

    static IO<String> getLine = new IO<String>() {
            @Override
            public String unsafePerformIO() {
                try {
                    return in.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
        };
}

/**
 * The program composed out of IO actions in a purely functional manner.
 */
class Main {

    /**
     * A variant of bind, which discards the bound value.
     */
    static IO<Unit> bind_(final IO<Unit> a, final IO<Unit> b) {
        return bind(a, new Function<Unit, IO<Unit>>(){
            @Override
            public IO<Unit> apply(Unit argument) {
                return b;
            }
        });
    }

    /**
     * The greeting action -- asks the user for his name and then prints a greeting
     */
    final static IO<Unit> greet = 
            bind_(putStrLn("Enter your name:"), 
            bind(getLine, new Function<String, IO<Unit>>(){
                @Override
                public IO<Unit> apply(String argument) {
                    return putStrLn("Hello, " + argument + "!");
                }
            }));

    /**
     * A simple echo action -- reads a line, prints it back
     */
    final static IO<Unit> echo = bind(getLine, putStrLn);

    /**
     * A function taking some action and producing the same action run repeatedly forever (modulo stack overflow :D)
     */
    static IO<Unit> loop(final IO<Unit> action) {
        return bind(action, new Function<Unit, IO<Unit>>(){
            @Override
            public IO<Unit> apply(Unit argument) {
                return loop(action);
            }
        });
    }

    /**
     * The action corresponding to the whole program
     */
    final static IO<Unit> main = bind_(greet, bind_(putStrLn("Entering the echo loop."),loop(echo)));
}

/**
 * The runtime system, doing impure stuff to actually run our program.
 */
public class RTS {
    public static void main(String[] args) {
        Main.main.unsafePerformIO();
    }
}

This is a runtime system implementing interface to the console I/O together with a small purely functional program which greets the user and then runs an echo loop.

One can't implement the unsafe part in Haskell because Haskell is purely functional language. It is always implemented with lower-level facilities.