JavaScriptでMaybeモナド - Douglas Crockfordの実装とECMA Script 6のgeneratorを使った実装の比較

douglas_maybe.js

// Douglas CrockfordのMaybeモナド
// 
// http://www.youtube.com/watch?v=b0EF0VTs9Dc
// 上記のyoutubeより（時間にして21:50, 26:50, 27:15のコード）

function MONAD(modifier) {
  var prototype = Object.create(null);
  return function unit(value) {
    var monad = Object.create(prototype);
    monad.bind = function (func, args) {
      return func.apply(undefined, [value].concat(Array.prototype.slice.apply(args || [])));
    };
    if (typeof modifier === 'function') {
      modifier(monad, value);
    }
    return monad;
  };
}

var maybe = MONAD(function (monad, value) {
  if (value === null || value === undefined) {
    monad.is_null = true;
    monad.bind = function () {
      return monad;
    };
  }
});


// example 1
var monad = maybe(1);
monad.bind(function (value) {
  monad = maybe(value + 2);
  monad.bind(function (value) {
    console.log(value); // 3
  });
});

// example 2
monad = maybe(null);
monad.bind(function (value) {
  monad = maybe(value + 2);
  monad.bind(function (value) {
    console.log(value); // unreachable
  });
});

generator_maybe.js

// ECMA Script 6 のgeneratorを使ったMaybeモナド
// 
// 実装は https://github.com/nakamura-to/mflow より
// node v0.11.4で --harmony-generators オプションをつけて稼動確認

function maybe(fn) {
  var gen = fn();
  var ret = gen.next();
  while (!ret.done) {
    if (ret.value == null) return null;
    ret = gen.next(ret.value);
  }
  return ret.value;
}


// example 1
maybe(function * () {
  var value = yield 1;
  value = yield value + 2;
  console.log(value); // 3
});

// example 2
maybe(function * () {
  var value = yield null;
  value = yield value + 2;
  console.log(value); // unreachable
});