.js

  var EitherAsync = function (actions, g) { 
    this.cata = function (alg) {
      actions(alg.ok , alg.error)
    }
  }
  
  Promise.prototype.toEither = function () {
    var promise = this;
    var either = new EitherCoyoAsync(function (resolve, reject) {
      promise.then(resolve).catch(reject)
    });
    return either;
  }