Hi. Let’s say I have my Data class that can be read and then validated by some

fun allErrorsIn(data: Data): List<Error>

- if it returns empty list, all is ok and I want to continue processing, but if there is at least one error, I’d like to switch to

Nel<Error>.left()

I have it done this way:

val incommingData : EitherNel<Error, Data> = readInput()
val validatedData : EitherNel<Error, Data> = incommingData.flatMap { data -> 
  val errors : List<Error> = allErrorsIn(data) // empty list means data is ok
  if (errors.isEmpty) data.right() else errors.toNonEmptyListOrNull()!!
}

but I wonder if there is some nice API that does it prettier?

You could do it all with raise:

fun Raise<Nel<Error>>.validateData(data: Data) { allErrorsIn(Data).toNonEmptyListOrNull()?.let(this::raise) }

val incomingData: EitherNel<Error, Data> = readInput()
val validatedData: EitherNel<Error, Data> = either { data.bind().also { validateData(it) } }

It's even nicer though if you use raise everywhere because as long as you're inside a

Raise<Nel<Error>>

, you can just do:

val validatedData: Data = readInput().also { validateData(it) )

and it's immensely clearer.

🙇 thanks that's what I was looking for