how to use `Either` and `Validated` together? `Eit...

# arrow

[Marko Novakovic]

how to use Either and Validated together? Either for fetching and Validated for validating data. I have code that parses some data and then I need to perform some check on that data to determine is it valid or not. parsing function return Either because parsing can fail and I want to validation function to return Validated based on validation logic

[stojan]

something like this should work:

fun fetchData(): Either<Error, Success> = TODO()

fun validateData(s: Success): ValidatedNel<Error, ValidData> = TODO()

fun fetchAndValidate(): Either<Error, ValidData> = 
    fetchData.flatMap { success ->
        validateData(success).toEither()
    }

[stojan]

note: if fetching from network/db you also might wanna do fetchData as suspend

you can also use either {} block instead of flatMap

[stojan]

note: the error sides on both functions must match (have the same supertype)

[Marko Novakovic]

thank you. what if I need different error types? one for fetch error other for validation error?
@stojan

[stojan]

you can convert from one to the other using mapLeft

[Marko Novakovic]

I forgot about that 😄, thanks

[stojan]

or have some sealed class:

sealed class MyErrors {
    object ParseErrors : MyErrors()
    object ValidationError : MyErrors()
}

[stojan]

using object because I'm lazy.... you would typically have more data

[Marko Novakovic]

@stojan this is not working for me. in fold at the end of the chain error is Any

[stojan]

can you share some code?

[stojan]

I'm guessing you have different errors in the Either/Validated, so the only common parent is Any

[Marko Novakovic]

I did and that wasn’t the problem, now I need that error so I used same type for error, still Any

[Marko Novakovic]

changed it to Throwable still nothing

[stojan]

would be great if you could share a minimal example that shows the problem

[Marko Novakovic]

fun List<String>.parseData(): Either<Throwable, List<Data>> = TODO()

fun List<Data>.validate(): ValidatedNel<Throwable, List<Data>> = TODO()

fun main() {
    readData()
        .flatMap(List<String>::parseData)
        .flatMap { data -> data.validate().toEither() }
        .fold(
            { /// it is Any },
            {},
        )

[Marko Novakovic]

if there a way to use different error and still get it recognise types? and is there a reason why I should not do that?

[Marko Novakovic]

and when I have just parseData it still gives me Any as error type if I set it to something other than Throwable

[Marko Novakovic]

for now am doing just flatMap(List<String>::parseData) and folding over Validate inside either’s right

[stojan]

I guess I confused you by using ValidatedNel , which is a typealias for Validated<Nel<E, A>>

so the errors don't match here Throwable from the parseData and List<Throwable> for validate()

[stojan]

if you use Validated instead of ValidatedNel it should work

I used ValidatedNel in the example because it's usually used when you want to collect ALL validation errors in a list

[Marko Novakovic]

and how to make it work with ValidatedNel?

[stojan]

mapLeft

[stojan]

updated:

sealed class Error
data class FetchError(val t: Throwable) : Error()
data class ValidationError(val t: List<Throwable): Error()

fun fetchData(): Either<Error, Success> = TODO()

fun validateData(s: Success): ValidatedNel<Error, ValidData> = TODO()

fun fetchAndValidate(): Either<Error, ValidData> = 
    fetchData.
		.mapLeft<Error> { FetchError(it) }
		.flatMap { success ->
        	validateData(success).toEither()
            	.mapLeft { ValidationError(it) }
    	}