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index.js

var State = require('fantasy-states'),
    Tuple2 = require('fantasy-tuples').Tuple2,
    
    // Tuple2 Number Number
    initial = Tuple2(0, 1),
    
    // State (Tuple2 Number Number) Number -> State (Tuple2 Number Number) Number
    next = discard(
      	State.modify(function(t) {
            return Tuple2(t._1 + 1, (t._1 + 1) * t._2);
        }),
        State.get.map(snd)
    ),
    result = factorial(5);

// Tuple2 a b -> b
function snd(t) {
    return t._2;
}

// (m a, m b) -> m b
function discard(a, b) {
    return a.chain(function(_) {
        return b;
    });
}

// Array (m a) -> m a
function sequence(actions) {
    var r = actions[0],
       	i;
    for(i = 1; i < actions.length; i++) {
        r = discard(r, actions[i]);
    }
    return r;
}

// (Number, a) -> Array a
function replicate(n, x) {
    var r = [],
        i;
    for(i = 0; i < n; i++) {
        r.push(x); 
    }
    return r;
}

// Number -> Number
function factorial(n) {
    return sequence(replicate(n, next)).eval(initial);
}

// Side-effect!
if(typeof document != 'undefined') {
    document.body.innerHTML = result;
} else {
    console.log(result);
}

minified.js

function snd(t){return t._2}function discard(t,n){return t.chain(function(){return n})}function sequence(t){var n,r=t[0];for(n=1;t.length>n;n++)r=discard(r,t[n]);return r}function replicate(t,n){var r,e=[];for(r=0;t>r;r++)e.push(n);return e}function factorial(t){return sequence(replicate(t,next)).eval(initial)}require=function(t,n,r){function e(r,u){if(!n[r]){if(!t[r]){var i="function"==typeof require&&require;if(!u&&i)return i(r,!0);if(o)return o(r,!0);throw Error("Cannot find module '"+r+"'")}var c=n[r]={exports:{}};t[r][0].call(c.exports,function(n){var o=t[r][1][n];return e(o?o:n)},c,c.exports)}return n[r].exports}for(var o="function"==typeof require&&require,u=0;r.length>u;u++)e(r[u]);return e}({"fantasy-states":[function(t,n){n.exports=t("KN4SDL")},{}],KN4SDL:[function(t,n){var r=t("fantasy-tuples").Tuple2,e=t("daggy"),o=e.tagged("run");o.of=function(t){return o(function(n){return r(t,n)})},o.prototype.chain=function(t){var n=this;return o(function(r){var e=n.run(r);return t(e._1).run(e._2)})},o.get=o(function(t){return r(t,t)}),o.modify=function(t){return o(function(n){return r(null,t(n))})},o.put=function(t){return o.modify(function(){return t})},o.prototype.eval=function(t){return this.run(t)._1},o.prototype.exec=function(t){return this.run(t)._2},o.prototype.map=function(t){return this.chain(function(n){return o.of(t(n))})},o.prototype.ap=function(t){return this.chain(function(n){return t.map(n)})},o.StateT=function(t){var n=e.tagged("run");return n.lift=function(t){return n(function(){return t})},n.of=function(e){return n(function(n){return t.of(r(e,n))})},n.prototype.chain=function(){var t=this;return n(function(n){t.run(n)})},n.get=n(function(n){return t.of(r(n,n))}),n.modify=function(e){return n(function(n){return t.of(r(null,e(n)))})},n.put=function(t){return n.modify(function(){return t})},n.prototype.eval=function(t){return this.run(t).chain(function(t){return t._1})},n.prototype.exec=function(t){return this.run(t).chain(function(t){return t._2})},n.prototype.map=function(t){return this.chain(function(r){return n.of(t(r))})},n.prototype.ap=function(t){return this.chain(function(n){return t.map(n)})},n},n!==void 0&&(n.exports=o)},{"fantasy-tuples":1,daggy:2}],2:[function(t,n,r){(function(){(function(t,n){"use strict";"function"==typeof define&&define.amd?define(["exports"],n):r!==void 0?n(r):(t.daggy={},n(t.daggy))})(this,function(t){function n(t){function n(){}return n.prototype=t,new n}function r(t,r){return t instanceof r?t:n(r.prototype)}function e(){function t(){var e,o=r(this,t);if(arguments.length!=n.length)throw new TypeError("Expected "+n.length+" arguments, got "+arguments.length);for(e=0;n.length>e;e++)o[n[e]]=arguments[e];return o}var n=[].slice.apply(arguments);return t._length=n.length,t}function o(t){function r(){throw new TypeError("Tagged sum was called instead of one of its properties.")}function o(n){return function(r){var e,o=t[n],u=[];if(!r[n])throw new TypeError("Constructors given to cata didn't include: "+n);for(e=0;o.length>e;e++)u.push(this[o[e]]);return r[n].apply(this,u)}}function u(t){var e=n(r.prototype);return e.cata=o(t),e}var i;for(i in t)t[i].length?(r[i]=e.apply(null,t[i]),r[i].prototype=u(i)):r[i]=u(i);return r}t.create=n,t.getInstance=r,t.tagged=e,t.taggedSum=o})})()},{}],1:[function(t,n,r){var e=t("daggy"),o=e.tagged("_1","_2"),u=e.tagged("_1","_2","_3"),i=e.tagged("_1","_2","_3","_4"),c=e.tagged("_1","_2","_3","_4","_5");o.prototype.concat=function(t){return o(this._1.concat(t._1),this._2.concat(t._2))},u.prototype.concat=function(t){return u(this._1.concat(t._1),this._2.concat(t._2),this._3.concat(t._3))},i.prototype.concat=function(t){return i(this._1.concat(t._1),this._2.concat(t._2),this._3.concat(t._3),this._4.concat(t._4))},c.prototype.concat=function(t){return c(this._1.concat(t._1),this._2.concat(t._2),this._3.concat(t._3),this._4.concat(t._4),this._5.concat(t._5))},r!==void 0&&(r.Tuple2=o,r.Tuple3=u,r.Tuple4=i,r.Tuple5=c)},{daggy:2}]},{},[]),require=function(t,n,r){function e(r,u){if(!n[r]){if(!t[r]){var i="function"==typeof require&&require;if(!u&&i)return i(r,!0);if(o)return o(r,!0);throw Error("Cannot find module '"+r+"'")}var c=n[r]={exports:{}};t[r][0].call(c.exports,function(n){var o=t[r][1][n];return e(o?o:n)},c,c.exports)}return n[r].exports}for(var o="function"==typeof require&&require,u=0;r.length>u;u++)e(r[u]);return e}({"fantasy-tuples":[function(t,n){n.exports=t("UWk06O")},{}],UWk06O:[function(t,n,r){var e=t("daggy"),o=e.tagged("_1","_2"),u=e.tagged("_1","_2","_3"),i=e.tagged("_1","_2","_3","_4"),c=e.tagged("_1","_2","_3","_4","_5");o.prototype.concat=function(t){return o(this._1.concat(t._1),this._2.concat(t._2))},u.prototype.concat=function(t){return u(this._1.concat(t._1),this._2.concat(t._2),this._3.concat(t._3))},i.prototype.concat=function(t){return i(this._1.concat(t._1),this._2.concat(t._2),this._3.concat(t._3),this._4.concat(t._4))},c.prototype.concat=function(t){return c(this._1.concat(t._1),this._2.concat(t._2),this._3.concat(t._3),this._4.concat(t._4),this._5.concat(t._5))},r!==void 0&&(r.Tuple2=o,r.Tuple3=u,r.Tuple4=i,r.Tuple5=c)},{daggy:1}],1:[function(t,n,r){(function(){(function(t,n){"use strict";"function"==typeof define&&define.amd?define(["exports"],n):r!==void 0?n(r):(t.daggy={},n(t.daggy))})(this,function(t){function n(t){function n(){}return n.prototype=t,new n}function r(t,r){return t instanceof r?t:n(r.prototype)}function e(){function t(){var e,o=r(this,t);if(arguments.length!=n.length)throw new TypeError("Expected "+n.length+" arguments, got "+arguments.length);for(e=0;n.length>e;e++)o[n[e]]=arguments[e];return o}var n=[].slice.apply(arguments);return t._length=n.length,t}function o(t){function r(){throw new TypeError("Tagged sum was called instead of one of its properties.")}function o(n){return function(r){var e,o=t[n],u=[];if(!r[n])throw new TypeError("Constructors given to cata didn't include: "+n);for(e=0;o.length>e;e++)u.push(this[o[e]]);return r[n].apply(this,u)}}function u(t){var e=n(r.prototype);return e.cata=o(t),e}var i;for(i in t)t[i].length?(r[i]=e.apply(null,t[i]),r[i].prototype=u(i)):r[i]=u(i);return r}t.create=n,t.getInstance=r,t.tagged=e,t.taggedSum=o})})()},{}]},{},[]);var State=require("fantasy-states"),Tuple2=require("fantasy-tuples").Tuple2,initial=Tuple2(0,1),next=discard(State.modify(function(t){return Tuple2(t._1+1,(t._1+1)*t._2)}),State.get.map(snd)),result=factorial(5);"undefined"!=typeof document?document.body.innerHTML=result:console.log(result);

page-head.html

<style type='text/css'>html, body { margin: 0; padding: 0; border: 0; }
body, html { height: 100%; width: 100%; }</style>