[Javascript] Monads

Posted 2020-08-08

Monads allow you to nest computations. They are a pointed functor that adds mjoin and chain functions to combine other functors. Brian shows a number of code examples of different monads in action.

 

functions: "mjoin", "chain"

 

mjoin:

var setSearchInput = function(x){ return ("#input").val(x); }.toIO()
var getSearchTerm = function(){ return getParam("term", location.search) }.toIO()
var initSearchForm = compose(mjoin, map(setSearchInput),  getSearchTerm)

initSearchForm()
//=> IO(Dom)

runIO(initSearchForm())

"getSearchTerm" return an IO, so in "setSearchInput" we need to use map(), but itself will return another IO, so the result is IO(IO()), then we apply mjoin, the result will be "IO()".

 

chain:

var chain = function(f){
 return compose(mjoin, map(f))
}

 

var setSearchInput = function(x){ return ("#input").val(x); }.toIO()
var getSearchTerm = function(){ return getParam("term", location.search) }.toIO()
var initSearchForm = compose(chain(setSearchInput),  getSearchTerm)

initSearchForm()
//=> IO(Dom)

runIO(initSearchForm())

var sendToServer = httpGet(‘/upload‘)
var uploadFromFile = compose(chain(sendToServer), chain(readFile), askUser)

uploadFromFile(‘what file?‘).fork(logErr, alertSuccess)

 

 

requirejs.config({
  shim: {},
  paths: {
    domReady: ‘https://cdnjs.cloudflare.com/ajax/libs/require-domReady/2.0.1/domReady.min‘,
    ramda: ‘//cdnjs.cloudflare.com/ajax/libs/ramda/0.8.0/ramda.min‘,
    maybe: ‘http://looprecur.com/hostedjs/v2/maybe‘,
    io: ‘http://looprecur.com/hostedjs/v2/io‘,
    future: ‘http://looprecur.com/hostedjs/v2/data.future.umd‘,
    hcjs: ‘http://looprecur.com/hostedjs/v2/hcjs‘
  }
});

require(
  [
    ‘ramda‘,
    ‘maybe‘,
    ‘io‘,
    ‘future‘,
    ‘hcjs‘,
    ‘domReady!‘
  ],
  function (_, Maybe, io, Future) {
    console.clear();


    var runIO = io.runIO;



    // Exercise 1
    // ==========
    // Use safeGet and mjoin or chain to safetly get the street name
    console.log("--------Start exercise 1--------");

    var safeGet = _.curry(function (x, o) {
      return Maybe(o[x]);
    });
    var user = {
      id: 2,
      name: "Albert",
      address: {
        street: {
          number: 22,
          name: ‘Walnut St‘
        }
      }
    };
    function log (x){
      console.log(x.toString());
      return x;
    }

    var ex1 = compose(mjoin, map(safeGet(‘name‘)) ,mjoin, map(safeGet(‘street‘)) ,safeGet(‘address‘));
    var ex1 = compose(chain(safeGet(‘name‘)), chain(safeGet(‘street‘)), safeGet(‘address‘));
    assertEqual(Maybe(‘Walnut St‘), ex1(user));
    console.log("exercise 1...ok!");




    // Exercise 2
    // ==========
    // Use monads to get the href, then purely log it.

    console.log("--------Start exercise 2--------");

    var getHref = function () {
      return location.href;
    }.toIO();
    var pureLog = function (x) {
      console.log(x);
      return x;
    }.toIO();

    var ex2 = compose(chain(pureLog), getHref);

    assertEqual("http://run.jsbin.com/runner", runIO(ex2(null)));
    console.log("exercise 2...ok!");




    // Exercise 3
    // ==========
    // Use monads to first get the Post with getPost(), then pass it‘s id in to getComments().
    console.log("--------Start exercise 3--------");

    var ex3 = compose(chain(compose(getComments ,_.get(‘id‘))) , getPost);
    var ex3 = compose(mjoin, map(compose(getComments, _.get(‘id‘))), getPost)
        
    ex3(13).fork(log, function (res) {
      assertEqual(2, res.length);
      console.log("exercise 3...ok!");
    });




    // HELPERS
    // =====================

    function getPost(i) {
      return new Future(function (rej, res) {
        setTimeout(function () {
          res({
            id: i,
            title: ‘Love them futures‘
          });
        }, 300);
      });
    }

    function getComments(i) {
      return new Future(function (rej, res) {
        setTimeout(function () {
          res(["This class should be illegal", "Monads are like space burritos"]);
        }, 300);
      });
    }

  });