Hi! Probably a basic question, about `either{}` DS...

# arrow

[Gopal S Akshintala]

Hi! Probably a basic question, about either{} DSL. The return type of this is Either<E,A> , so I understand one way the composition trips to left is -- one of the functions inside returns an either in left
I am unable to figure out how to explicitly return an either in left from within the block.
For example:

val result: Either<String, String> = either { // This casts the side-effect function into a suspend function.
            val id = !getSomethingFromDb(true)
            val data = !getSomethingElseFromNetwork(id)
           val either = if (data.isEmpty()) {
               // vvv how to return either on left
               !"SOME_CUSTOM_ERROR".raiseError()
           } else data.right()
           either
        }

I understand for a case like this where I have this requirement on the last step, I can return from outside the block, but just curious how to short-circuit from the middle of the block?

[stojan]

This might help: https://github.com/arrow-kt/frdomain.kt/blob/6fc7d48096a1e718ef4a3040ea1bf951cf8a9f56/arrow-fx/src/main/kotlin/io/arrowkt/frdomain/service/interpreter/AccountService.kt#L76

[Gopal S Akshintala]

Thanks @stojan, but I am not clear on how can I solve when my return type (data in my example) is on right state, but based on it's value, I have to return a custom error so that the output of either{} block is on CUSTOM_ERROR.left()

[Gopal S Akshintala]

I feel this looks better n consistent with the style, but unnecessary wrapping and bind

@Test
    fun `either fx`() = runBlocking {
        val result: Either<String, String> = either { // This casts the side-effect function into a suspend function.
            val id = !getSomethingFromDb(true)
            val data = !getSomethingElseFromNetwork(id)
            !(if (data.isEmpty()) "CUSTOM_ERROR".left() else data.right())
        }
        println(result)
    }

[stojan]

I would argue the unwrapping is unnecessary here

val dataEither = getSomethingElseFromNetwork(id)
!dataEither.filterOrElse({!it.isEmpty()}, {CustomError})

[stojan]

to fail the either block, you need an Either.Left and you have to bind on it 🙂

[Gopal S Akshintala]

ya, but I am trying to go totally imperative without using any functional operations

[stojan]

then you go with your approach 🙂

[Gopal S Akshintala]

ya... thanks @stojan!

[Gopal S Akshintala]

🙂

[stojan]

the either block is sugar for map and flatMap

[stojan]

when you operate on a value, it's map

when you do a bind() it's flatMap

[Gopal S Akshintala]

yes, I guess ur suggestion about filterOrElse can be said consistent in this style, as this comprehension is all about eliminating only the noise of flatMap

[Gopal S Akshintala]

I had something like this before

val validate2Throwable: Validator<Egg, ValidationFailure> = {
    // Awesome DSL from arrow to work with either in an imperative way (Best of both worlds).
    either<ValidationFailure, Boolean> { // This casts the side-effect function into a suspend function.
        throwableRule2(it)
    }.filterOrElse(::identity) { TOO_LATE_TO_HATCH_2 }
}