Maybe Functor

In notebook:: FrontEndMasters Hardcore Functional
Created at:: 2017-04-03
Updated:: 2017-07-15
Tags:: Functional Programming JavaScript Fundamentals

Functor

"An object or data structure you can map over"

functions: map

So basically if an object has a map function or method, then it's a functor (except for jQuery which didn't get this right).

Functor: anything with a map method

Them pesky nulls

  var getElement = document.querySelector
var getNameParts = compose(split(' '), getElement)

getNameParts('#full_name')
//=> ['Jonathan', 'Gregory', 'Brandis']

Partial application

(audience question)

Giving a curried function less arguments than it takes by definition.

In the above example, split is partially applied.

Back to above example.

If full_name is empty get('value') will also produce an error, the whole chain will collapse and blow up.

Maybe

This is our first functor. This is an elegant solution to the above null value issue.

Captures a null check
The value inside may not be there
Sometimes has two subclasses Just / Nothing (in Haskell and other languages, here we will only use one)
Sometimes called Option with subclasses Some/None

The above points relate to implementations across different languages.

The idea is that you may or may not have a value in your functor.

Maybe:

  var _Maybe.prototype.map = function(f) {
  return this.val ? Maybe(f(this.val)) : Maybe(null)
}

map(capitalize, Maybe('flamethrower'))
//=> Maybe('Flamethrower')

So if the value is truthy, we run the function on it, if not, we return null and don't even run the passed function.

so:

  map(capitalize, Maybe(null))
// Maybe(null)

We just abstracted out function application!

So wherever you add a maybe it will do a null check and if it's null will stop the chain and just return null.

Another example:

  var firstMatch = compose(first, match(/cat/g))

firstMatch('dogsup')
// Boom!

Blows up...

So we insert a Maybe inside it to protect it:

  var firstMatch = compose(map(first), Maybe, match(/cat/g))

firstMatch('dogsup')
// Maybe(null)

You can drop Maybe anywhere in your composition and "it" will go into it.

Above example: We cannot call first on Maybe, we need to map(first) over it.

Abundancia