We will see a peculiar example of a pure function. This function contained a side-effect, but we dubbed it pure by wrapping its action in another function. Here's another example of this:

// getFromStorage :: String -> (_ -> String)
const getFromStorage = key => () => localStorage[key];

'localStorage' is a side effect, but we wrap into a function call, so what 'getFromStorage' return is not a value but a function. And this returned function is waiting to be called, before calling, we can do all kind of function mapping of composion on it, that's the beautity.

 

Let's see how to define a IO functor for this side-effect code:

class IO {
  static of(x) {
    return new IO(() => x);
  }

  constructor(fn) {
    this.$value = fn;
  }

  map(fn) {
    return new IO(compose(fn, this.$value));
  }

  inspect() {
    return `IO(${this.$value})`;
  }
}

IO will take a function as input. 

 

Example of IO:

( Note: the reuslt shown in comment is not actual result, the actual result is wrapped in {$value: [Function]}, just for now, we show the result which should be in the end.)

// ioWindow :: IO Window
const ioWindow = new IO(() => window);

ioWindow.map(win => win.innerWidth);
// IO(1430)

ioWindow
  .map(prop('location'))
  .map(prop('href'))
  .map(split('/'));
// IO(['http:', '', 'localhost:8000', 'blog', 'posts'])

 

There is some library already define the IO functor for us, we don't need to write one for our own, for example, Async from Crocks.js, or Data.Task from Folktale.

Here we are using 'Data.Task' as an example:

// -- Node readFile example ------------------------------------------

const fs = require('fs');

// readFile :: String -> Task Error String
const readFile = filename => new Task((reject, result) => {
  fs.readFile(filename, (err, data) => (err ? reject(err) : result(data)));
});

readFile('metamorphosis').map(split('\n')).map(head);
// Task('One morning, as Gregor Samsa was waking up from anxious dreams, he discovered that
// in bed he had been changed into a monstrous verminous bug.')


// -- jQuery getJSON example -----------------------------------------

// getJSON :: String -> {} -> Task Error JSON
const getJSON = curry((url, params) => new Task((reject, result) => {
  $.getJSON(url, params, result).fail(reject);
}));

getJSON('/video', { id: 10 }).map(prop('title'));
// Task('Family Matters ep 15')


// -- Default Minimal Context ----------------------------------------

// We can put normal, non futuristic values inside as well
Task.of(3).map(three => three + 1);
// Task(4)

 

To run the Task, we need to call 'fork(rej, res)':

// -- Pure application -------------------------------------------------
// blogPage :: Posts -> HTML
const blogPage = Handlebars.compile(blogTemplate);

// renderPage :: Posts -> HTML
const renderPage = compose(blogPage, sortBy(prop('date')));

// blog :: Params -> Task Error HTML
const blog = compose(map(renderPage), getJSON('/posts'));


// -- Impure calling code ----------------------------------------------
blog({}).fork(
  error => $('#error').html(error.message),
  page => $('#main').html(page),
);

 

Example of Either & IO:

// validateUser :: (User -> Either String ()) -> User -> Either String User
const validateUser = curry((validate, user) => validate(user).map(_ => user));

// save :: User -> IO User
const save = user => new IO(() => ({ ...user, saved: true }));

const validateName = ({ name }) => (name.length > 3
  ? Either.of(null)
  : left('Your name need to be > 3')
);

const saveAndWelcome = compose(map(showWelcome), save);

const register = compose(
  either(IO.of, saveAndWelcome),
  validateUser(validateName),
);

 

More detail