Fp-ts: Either: get / extract / unbox

[mustafaekim]

Hi, I'am quite new to FP and as far as I've read, extracting value from any type of monad is not favored. That's ok in the long run, however, for a smooth transition I am expecting to be able to extract the value from the Either monad. Can't I, really ? :)

[gcanti]

You can't, you should use fold, getOrElse or getOrElseL which are safe.

However you can easily define your own unsafe get

const get = <L, A>(fa: Either<L, A>): A =>
  fa.getOrElseL(() => {
    throw new Error('get called on Left')
  })

(I don't recommend it though)

[mustafaekim]

Oh ok. So when do you use isRight() ? Even if you become sure that it is a right, you cant extract the value

Is there any easy-to-follow code examples, like a tutorial or a small project I can understand the mindset?

[gcanti]

isRight (both the method and the function) is defined as a custom type guard so you can do

declare const fa: Either<string, number>
if (fa.isRight()) {
  // const n: number
  const n = fa.value
}

since TypeScript is able to refine fa. IMO it must be used sparingly because that way you are not forced to handle the failing case.

[mustafaekim]

Oh that's interesting, if isRight() returns true, value property is just there.

Well, I've read so many articles/books that favor not to reach the boxed value directly, so I will try my best. Thanks