writer-test.js

var Tuple2 = require('fantasy-tuples').Tuple2,
    daggy = require('daggy'),
    Writer = daggy.tagged('run');

Writer.of = function(x, y) {
  return Writer(function() {
    return Tuple2(x, y.concat(y.empty()));
  });
};

Writer.prototype.chain = function(f) {
  var writer = this;
  return Writer(function() {
    var result = writer.run();
    var t = f(result._1).run();
    return Tuple2(t._1, result._2.concat(t._2));
  });
};

Writer.prototype.tell = function(y) {
  var writer = this;
  return Writer(function(a) {
    var result = writer.run();
    return Tuple2(result._1, result._2.concat(y));
  });
}

Writer.prototype.map = function(f) {
  return this.chain(function(a) {
    return Writer.of(f(a));
  });
};

Writer.prototype.ap = function(a) {
  return this.chain(function(f) {
    return a.map(f);
  });
};

//////////////////////////////////////////////////
// Unofficial Semi-Tests
//////////////////////////////////////////////////

require('pointfree-fantasy').expose(global);

var add = curry (function(x, y) {
  return Writer.of(x + y, ["Adding " + y + " to " + x + "."]);
});

w = Writer.of(1, ["this is a test"])
  .chain(add(3))
  .chain(add(5))
  .chain(add(8))
  .tell(["End Test"])

console.log(w.run());

//  Output:
//  { _1: 17,
//    _2:
//       [ 'this is a test',
//         'Adding 1 to 3.',
//         'Adding 4 to 5.',
//         'Adding 9 to 8.'
//         'End Test' ] }

Sorry didn't catch the of issue at first. of is like haskell's return. So here, it should be separate from the constructor. It's probably best to do: new Writer(1, ["this is a test"]). Otherwise, it's looking awesome

Ah! Okay cool. Thanks for your help!