FutureMonadExample.js

class Future {
    slots = [];
    completed = false;
    value;

    ready(slot) {
        if (this.completed) {
            slot(this.value);
        } else {
            this.slots.push(slot);
        }
    }

    complete(val) {
        // ensure immutability
        if (this.completed) {
            throw "Can't complete an already completed future!";
        }
        this.value = val;
        this.completed = true;

        // notify subscribers
        this.slots.forEach(slot => slot(val));

        this.slots = null;
    }

    fmap(fn) {
        const fut = new Future();
        this.ready(function (val) {
            fut.complete(fn(val));
        });
        return fut;
    }

    static unit(val) {
        const fut = new Future();
        fut.complete(val);
        return fut;
    }

    static delay(v, millis) {
        const fut = new Future();
        fut.complete(val);
        return fut;
    }
}

// example with readFile ...
const fs = require(fs);
function readFileF(file, options) {
    const f = new Future();
    fs.readFile(file, options, function (err, data) {
        if (err) throw err;
        f.complete(data);
        return f;
    });
}

logF(readFileF('test.txt', { encoding: 'utf8' }));

// a simple log utility
function logF(f) {
    f.ready(v => console.log(v));
}

logF(Future.unit('hi now'));