First Functor Fantasy Follow-up

dependencies.js

var curry = function(fn) {
    var arity = fn.length;
    var f = function(args) {
        return function () {
            var newArgs = (args || []).concat([].slice.call(arguments, 0));
            if (newArgs.length >= arity) {
                return fn.apply(this, newArgs);
            }
            else {return f(newArgs);}
        };
    };

    return f([]);
};

var compose = function(f, g) {
    return function() {
        return f.call(this, (g.apply(this, arguments)));
    };
};

var Maybe = function(val) {
    if (!(this instanceof Maybe)) {return new Maybe(val);}
    this.val = val;
};

var Either = function(left, right) {
    if (!(this instanceof Either)) {return new Either(left, right);}
    this.left = left;
    this.right = right;
};

Functor.js

(function(global) {
    // TODO: Make a tree structure from type objects' optional `parentKey` properties.  Do a depth-first search of
    // this tree instead of the simple linear search of `Object.keys`.
    var types = {};

    global.Functor = function(config) {
        types[config.key] = config;
    };

    global.fmap = curry(function(f, obj) {
        var type = types[obj.constructor.name], idx = -1, keys = Object.keys(types), len = keys.length;
        if (!type || !type.test(obj)) {type = false;}
        while (!type && ++idx < len) {
            type = types[keys[idx]].test(obj) ? types[keys[idx]] : false;
        }
        if (!type) {
            throw new TypeError("fmap called on unregistered type: " + obj)
        }
        return type.fmap(f, obj);
    });
}(this));

usage.js

var makeTypeof = function(type) {
    return function(obj) {
        return type.prototype.isPrototypeOf(obj);
    };
};

Functor({
    key: 'Array',
    test: function(obj) {
        return Object.prototype.toString.call(obj) === '[object Array]';
    },
    fmap: function(f, arr){
        return arr.map(function(x){return f(x);});
    }
});

Functor({
    key: 'Function',
    test: function(fn) {
        return Object.prototype.toString.call(fn) === '[object Function]';
    },
    fmap: function(f, g) {
        return compose(f, g);
    }
});

Functor({
    key: 'Maybe',
    test: makeTypeof(Maybe),
    fmap: function(f, maybe) {
        return (maybe.val == null) ? maybe : Maybe(f(maybe.val));
    }
});

Functor({
    key: 'Either',
    test: makeTypeof(Either),
    fmap: function(f, either) {
        return (either.right == null) ? Either(f(either.left), null) : Either(either.left, f(either.right));
    }
});


var plus1 = function(n) {return n + 1;};
var times2 = function(n) {return n * 2;};

console.log(fmap(plus1, [2, 4, 6, 8])); //=> [3, 5, 7, 9]
console.log(fmap(plus1, Maybe(5))); //=> Maybe(6)
console.log(fmap(plus1, Maybe(null))); //=> Maybe(null)
console.log(fmap(plus1, times2)(3)); //=> 7 (=  3 * 2 + 1)
console.log(fmap(plus1, Either(10, 20))); //=> Either(10, 21)
console.log(fmap(plus1, Either(10, null))); //=> Either(11, null)
console.log(fmap(plus1, {val: 5})); //=> TypeError: fmap called on unregistered type: [object Object]