monad.js

var Functor = ['map'];
var Applicative = ['maps', 'wrap'].concat(Functor);
var Monad = ['mapcat'].concat(Applicative);

var compliment = function (f) {
	return function (x) {
		return !f(x);
	}
}

Object.prototype.check = function (iface) {
	var candidate = this;
	var isDefined = function(f) { return candidate[f]; };
	if (!iface.every(isDefined)) {
		var type = candidate.constructor.name;
		var absent = iface.filter(compliment(isDefined));
		var msg = type + " does not implement " + absent;
		throw new Error(msg);
	}
}

Object.prototype.implement = function (iface, instance) {
	instance.check(iface);
	for (var i = 0; i < iface.length; i++) {
		var fname = iface[i];
		this.prototype[fname] = instance[fname];
	}
}

Array.implement(Monad, {
	map: Array.prototype.map,
	wrap: function (x) {
		return [x];
	},
	maps: function(Fa2b) {
		return this.map(function (x, i) {
			return Fa2b[i](x);
		});
	},
	mapcat: function(a2b) {
		return this.reduce(function (res, xs) {
			return res.concat(xs.map(a2b));
		}, []);
	}
});

var xs = [[1,2,3],[4,5,6]];
console.log(xs);

var ys = xs.mapcat(function (x) { return "a" + x; });
console.log(ys);