io.js

// IO a ~ () -> a
class IO {
    constructor(op) {
        // IO a -> a
        this.run = op;
    }

    // a -> IO a
    static returnIO(a) {
        return new IO(() => a);
    }

    // IO a -> (a -> IO b) -> IO b
    bind(k) {
        return new IO(() => k(this.run()).run());
    }
}

const putStrLn = text => new IO(() => console.log(text));

const getDateTime = new IO(Date.now);

IO.returnIO("Hello, world !")
    .bind(putStrLn)
    .bind(_ => getDateTime)
    .bind(putStrLn)
    .run();