id.js

'use strict';

var R = require('ramda');
var T1 = require('transducers.js');
var T2 = require('transducers-js');

function Identity(x) {
  if (!(this instanceof Identity)) {
    return new Identity(x);
  };

  this.runIdentity = function() {
    return x;
  };
}

Identity.prototype.toString = function() {
  return 'Identity(' + this.runIdentity().toString() + ')';
};

Identity.prototype.map = function(f) {
  return Identity(f(this.runIdentity()));
};

Identity.prototype[Symbol.iterator] = function*() {
  yield this.runIdentity();
};

Identity.prototype['@@transducer/init'] = function() {
  return this;
};

Identity.prototype['@@transducer/result'] = function(x) {
  return x;
};

Identity.prototype['@@transducer/step'] = function(x, y) {
  return Identity(y);
};

var i0 = Identity(0);
var i3 = Identity(3);
console.log('%s', R.map(R.add(1), i3));
// The following two lines fail if the iterator spec is not implemented.
console.log('%s', R.into(i0, R.map(R.add(1)), i3));
console.log('%s', T1.into(i0, R.map(R.add(1)), i3));
// This line is always Identity(0);
console.log('%s', T2.into(i0, R.map(R.add(1)), i3));