Simple monads in JavaScript with `do` syntax

_readme.md

Demo: http://jsbin.com/ademeYuT/1/edit

monads.js

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 trim = function(s) {
  return s.trim();
};

var replace = function(regex, replacement) {
  return function(s) {
    return s.replace(regex, replacement);
  };
};

var doM = function(f) {
  var unit = /return (.+)$/
    , backcall = /(\w+) <- (.+)/
    , end = '';
  return Function(f
    .toString()
    .split(/[\n;]/)
    .slice(1,-1)
    .map(trim)
    .filter(Boolean)
    .reduce(function(a, b) {
      return a +';\n'+ b
        .replace(unit, 'return this.unit($1)')
        .replace(backcall, function(_, a, b) {
          end += '});';
          return 'return '+ b +'.bind(function('+ a +'){';
        });
    }, '')
    .split(/\n/)
    .slice(1)
    .map(replace(/\{;$/, '{'))
    .join('') + end
  )();
};

// 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

This is genius!