Code used while exploring examples for http://davesquared.net/2013/01/yamt.html

messingwithmonads.js

//monads.js
//
exports.bestSellingCategories = function () {
    return [ createCat(42, "MOL", false)
           , createCat(101, "Both kinds", false)
           , createCat(418, "http", true)
           ];
}

createCat = function(id,name,shh) { return { id: id, name: name, isSecret: shh}; }

exports.productsInCategory = function (id) {
    if (id === 42) return ["earl grey", "english breakfast"];
    if (id === 101) return ["country", "western"];
    if (id === 418) return ["teapot"];
    else return [];
}

exports.flatten = function(list) {
    return list.reduce(function (acc, x) {
        return acc.concat(x);
    }, []);
}

exports.bind = function(f, list) {
    return exports.flatten(list.map(f));
}

Array.prototype.bind = (function() {
    return function(f) { return m.flatten(this.map(f)); }
})();

exports.sample = function() {
    return exports.bestSellingCategories().bind(function(category) {
    return exports.productsInCategory(category.id).bind(function (product) {
    return [product + " " + exports.onSpecial(product, category)];
}); }); };

exports.sample2 = function() {
    return exports.bestSellingCategories().map(function(category) {
    return exports.productsInCategory(category.id).map(function (product) {
    return (product + " " + exports.onSpecial(product, category));
}); }); };

unit = function(x) { return [x]; }

exports.onSpecial = function(product,category) {
    if (category.name === "MOL" || product === "western") return "[ON SALE!]";
    return "";
}

exports.inStock = function(product) {
    var isInStock = ['earl grey', 'western', 'teapot'].indexOf(product) >= 0;
    return isInStock ? "[in stock]" : "[sold out]";
}

exports.sequence = function(m, list) {
    return list.reduce(function(acc, monad) {
        return m.bind(function(x) { 
            return m.map(function (ms) { return ms.concat(x); }, acc);
        }, monad);
    }, m.unit([]));
};


exports.lift2 = function(m, f, ma, mb) {
    return m.bind(function(a) {
        return m.bind(function(b) { return m.unit(f(a,b)); }, mb);
    }, ma);
}

exports.OptionMonad = {
    bind: function(f, m) {
              if (m.isEmpty) return m;
              else return f(m.value);
              },
    unit: function(x) { return new exports.Option(x); },
}
/*
    map: function(f, m) { 
        if (m.isEmpty) return m;
        else return new exports.Option(f(m.value));
    }
    */

exports.Option = (function() {
    return function(x) {
        var is_empty = typeof x == 'undefined' || x == null
        return { value: x, isEmpty: is_empty, toString: function() {
            if (is_empty) return "{Empty}";
            else return "{Just " + x + "}";
        } };
    };
})();