var fluent = function(f) {

  return function() {

    f.apply(this, arguments);

    return this;

  };

};

​

var Maybe = {

  unit: fluent(function(x) {

    this.x = x;

  }),

  bind: function(f) {

    if (this.isNothing) {

      return this.nothing();

    } else if (this.isJust) {

      return f.call(this, this.x);

    }

  },

  nothing: fluent(function(x) {

    this.x = null;

    this.isNothing = true;

    this.toString = function() {

      return 'Nothing';

    };

  }),

  just: fluent(function(x) {

    this.x = x;

    this.isJust = true;

    this.toString = function() {

      return 'Just: '+ this.x;

    };

  }),

  valueOf: function() {

    return this.x;

  }

};

​

var id = function(x) {

  return function() {

    return x;

  }

};

​

var trim = function(s) {

  return s.trim();

};

​

var replace = function(regex, replacement) {

  return function(s) {

    return s.replace(regex, replacement);

  };

};

​

var test = function(regex) {

  return function(s) {

    return regex.test(s);

  };

};

​

var doM = function(f) {

  var lines, unit, end;

  lines = f.toString().split(/[\n;]/).map(trim).filter(Boolean).slice(1);

  unit = lines.slice(-2).shift().replace(/return (.+)/, 'return this.unit($1)');

  lines[lines.length-2] = unit;

  lines = lines.map(replace(/(\w+) <- (.+)/, 'return $2.bind(function($1){'));

  end = lines.map(test(/\)\{$/)).filter(Boolean).map(id('})'));

  lines.pop();

  lines.push.apply(lines, end);

  lines = lines.join(';\n').replace(/\{;/g,'{');

  return Function(lines +';')();

};

​

// Example

// ----------------------------------------

​

var add = function(x, y) {

  if (x == null || y == null) {

    return Maybe.nothing();

  }

  return Maybe.just(x + y);

};

​

// Success

var result = doM(function() {

  x <- add(1, 2);

  y <- add(x, 2);

  z <- add(x, y);

  return x * y * z;

});

​

console.log(result.toString()); //=> Just 120

​

// Fail

var result = doM(function() {

  x <- add(1, 2);

  y <- add(x, null);

  z <- add(x, y);

  return x * y * z;

});

​

console.log(result.toString()); //=> Nothing