In notebook:
FrontEndMasters Hardcore Functional
Created at:
Functional Programming JavaScript Fundamentals

Brian Lonsdorf (the presentators name)

  • Typically used for pure error handling
  • Like Maybe but with an error message embedded
  • Has two subclasses: Left/Right (as opposed to Maybe)
  • Maps the function over a Right, ignores the Left
  // Either

map(function (x) {return x + 1}, Right(2))
//=>  Right(3)

map(function (x) {return x + 1}, Left('some message'))
//=> Left('some message')

So if I run in it with Right it increments, if I run it with Left it does not run at all like with Maybe. But this time we can have an error message in that will be returned.

So I can decide somewhere in my app to return a Left and my app will stop running altogether. And the error message will propagate to the end.

You may or may not have a left or right. There are someways to get out of this, with lowercase either. It's for those cases where you don't want to completely stop your app, but want to take some action. This very rare (2% of the cases)

###Either functor

  // ###Either

var determineAge = function (user) {
  return user.age ? Right(user.age) : Left('couldnt get age')
var yearOlder = compose(map(add(1)), determineAge)

yearOld({age: 22})
// → Right(23)

yearOlder({age: null})
// → Left('couldnt get age')

The above example shows how this system works. He's using an Either library.

Right and Left are subclasses of an either library.

So, it will either run on the right or it will not run on the left. This is the main thing to remember. If you want to stop the execution of your program, return a left, if you want to continue, return a right.

Mentions that Either is not as useful for asynchronous stuff, this is why he doesn't spend so much time on it.