Fantasy-land compliant and JIT-optimizable Maybe

maybe.js

// Copyright (c) 2014 Tenor Biel
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

;(function(root, factory) {
  if (typeof define === 'function' && define.amd) {
    define(factory);
  } else if (typeof exports === 'object') {
    module.exports = factory();
  } else {
    root.Maybe = factory();
  }
})(this, function() {
  function Maybe(value) {
    if (value == null) {
      return Nothing();
    } else {
      return Just(value);
    }
  };

  Maybe.of = Maybe.Just = Just;
  Maybe.empty = Maybe.Nothing = Nothing;

  function Just(value) {
    if (!(this instanceof Just)) {
      return new Just(value);
    }

    this.value = value;
  };

  Just.prototype = {
    map: function map(fn) {
      return Just(fn(this.value));
    },

    chain: function chain(fn) {
      return fn(this.value);
    },

    equals: function equals(other) {
      return other instanceof Just && compare(this.value, other.value);
    },

    concat: function concat(other) {
      return this;
    },

    empty: function empty() {
      return Nothing();
    },

    ap: function ap(maybe) {
      return this.map(function(fn) {
        return maybe.map(fn);
      });
    },

    fold: function fold(whenJust, whenNothing) {
      return whenJust(this.value);
    },

    or: function or(value) {
      return this.value;
    },

    toString: function toString() {
      return Just.name + '(' + this.value + ')';
    }
  };

  function Nothing() {
    if (!(this instanceof Nothing)) {
      return new Nothing();
    }

    this.value = null;
  }

  Nothing.prototype = {
    map: function map(fn) {
      return this;
    },

    chain: function chain(fn) {
      return this;
    },

    equals: function equals(other) {
      return other instanceof Nothing;
    },

    concat: function concat(other) {
      return other;
    },

    ap: function ap(other) {
      return this;
    },

    fold: function fold(whenJust, whenNothing) {
      return whenNothing();
    },

    or: function or(value) {
      return value;
    },

    toString: function toString() {
      return Nothing.name;
    }
  };

  function compare(left, right) {
    return left === right ||
      (left != null && right != null) &&
      (typeof left.equals === 'function') &&
      left.equals(right);
  }

  return Maybe;
});