monad.js

// IO a -> a
function unsafePerformIO (io) {
    var tuple = io({"memo" : "This is the real world!!"});
    var newWorld = tuple[0];
    var value = tuple[1];
    return value;
}

// IO a -> (a -> IO b) -> IO b
function bind (io1, f) {
    return function (world) {
        var tuple = io1(world);
        var newWorld = tuple[0];
        var value = tuple[1];
        var io2 = f(value);
        return io2(newWorld);
    };
}

// String -> IO ()
function alertIO (message) {
    return function (world) {
        alert(message);

        var newWorld = world;
        newWorld["alerted"] = "DONE";

        return [newWorld, null];
    };
}

// IO Date
function dateIO (world) {
    return [world, new Date()];
}

// (Date -> IO ()) -> IO ()
var doItWithDate = function(f) {
    return bind(dateIO, f);
};

// IO ()
var main = doItWithDate(alertIO);

unsafePerformIO(main);