Good morning, regarding `Either`, which is the bes...

# arrow

[Davide Giuseppe Farella]

Good morning, regarding Either, which is the best way to get left?
Example

// ok
assertEquals(expectedResult, either.orNull())

// how?
assertEquals(expectedError, either.<leftOrNull()>)

[Javier]

Reading the documentation I am not finding how to do this.

Known that Kotlin is using the convention `getSomething()`(if it is null, it will crash) and getSomethingOrNull() , should be interesting in having something like getLeft() and getLeftOrNull()

[albertosh]

either.fold(::identity) { throw RuntimeException("I don't want a right") }

[Davide Giuseppe Farella]

@Javier there’s none
@albertosh thanks, I was hoping in something out of the box; I’ll add some extensions for fit our needing

[albertosh]

Search for fold in https://arrow-kt.io/docs/apidocs/arrow-core-data/arrow.core/-either/index.html#either

[albertosh]

Looks like getOrHandle() fits your usecase

[Davide Giuseppe Farella]

Not really, it will still give me a Right 🙂

[simon.vergauwen]

I think leftOrNull() would make sense on Either, we also have it for inclusive or, Ior, so I think it makes sense to also have it for Either.

[Jannis]

For testing you could also use assertEquals(Either.left(error), either) since that won't produce weird assertions if you ever have either = Either.Left(null)

[Davide Giuseppe Farella]

@simon.vergauwen It would be great, as now I added to our project as an extension.

Maybe also *OrThrow variants would make sense, as we have Either.catch { iCanThrow() } , so a way out of exception, but a way in ( not ideal, but most of the products won’t be rewritten from scratch, in order to add Arrow 🙂 )

@Jannis Thanks! That looks nice! But probably I’ll go for the extensions, in order to easily integrate in our project

[stojan]

https://kotest.io/docs/assertions/arrow.html

if you are using Kotest, it has built in arrow assertions..... so you can do either.shouldBeLeft(v)

otherwise I would go with what @Jannis recommended

which is the best way to get left?

to answer this first you need to answer what happens if it's Right , default value? throw? null?

you are using Either because you are not sure if it's a Left or a Right .... if you know it's always the Left value, then you don't need the Either

[Cody Mikol]

This should work for your case I believe

fun <L, R> Either<L, R>.getLeftOrNull() = this.fold({ it }, { null })

[Davide Giuseppe Farella]

you are using Either because you are not sure if it’s a Left  or a Right .... if you know it’s always the Left  value, then you don’t need the Either

But since we have a function for know if it’s left, I don’t see why I can’t get the actual Left.

I understand that the ideal behind it is to go functional, but:
1. isLeft doesn’t sound so functional
2. considering a pre-existent project, where you don’t wanna go functional everywhere ( yet ), the following scenario would be pretty common

Either
  - Left
    - OneErrorType
    - AnotherErrorType
  - Right
    _ Whatever

if (either.isLeft()) {
  when (either.getLeft())
    OneErrorType -> doSomething()
    AnotherErrorType -> doAnotherThing()
  }
}

[simon.vergauwen]

if (either.isLeft()) {
  when (either.getLeft())
    OneErrorType -> doSomething()
    AnotherErrorType -> doAnotherThing()
  }
}

Writing if without else is considered a bad pattern. In this case you would write the following, and here the Kotlin compiler will do smart casting for you. Using Kotlin Contracts we can perhaps also enable this for isLeft but that needs to be investigated.

when(either) {
  is Left -> when(either.value) {
     OneErrorType -> doSomething()
     AnotherErrorType -> doAnotherThing()
  }
  is Right -> noErrorFunction()
}

[Davide Giuseppe Farella]

Writing if without else is considered a bad pattern

It is, I wrote it incomplete just as an example, but what you said is totally true! Thanks 🙂

[Chris Paul]

similar to previous answer you could also do this:

assertEquals(expectedError.left(), either)