Do-notation in JavaScript

operator (>>=) 1 left { $a, $b } => #{ $a.bind($b) }
operator (>>) 1 left { $a, $b } => #{ $a.bind((_) => $b) }

function doM(gen) {
    function step(value) {
        var result = gen.next(value);
        if (result.done) {
            return result.value
        }
        return result.value.bind(step)
    }
    return step()
}

macro (<-) {
    rule infix { $a:ident | $e:expr } => { const $a = yield $e }
}

macro do {
    rule { {$expr ...} } => {
        doM(function*() {
            $expr ...
        }())
    }
}

function Just(value) {
    this.value = value;
}

Just.prototype.bind = function(fn) {
    return fn(this.value)
}

Just.prototype.toString = function() {
    return "Just(" + this.value + ")"
}

function Nothing(value) {}

Nothing.prototype.bind = function(_) {
    return this
}

Nothing.prototype.toString = function() {
    return "Nothing()"
}

function sqrtMaybe(x) {
    return x >= 0 ? new Just(Math.sqrt(x)) : new Nothing()
}

const result = do {
    x <- sqrtMaybe(25)
    y <- sqrtMaybe(36)
    return new Just(x + y)
}

alert(result)   // Just(11)