Hey new to the channel and arrow. Let's say that I have ValidatedNel<Error, ValidatedNel<Error...

# arrow

[Derek Towriss]

Hey new to the channel and arrow. Let's say that I have ValidatedNel<Error, ValidatedNel<Error, T>>. How can I convert that to ValidatedNel<Error, T>?

[pakoito]

oooooh, we went through this in another channel a couple of days ago

[pakoito]

let’s see if I can explain myself better now 😄

[pakoito]

Validated doesn’t form a monad, so it doesn’t have flatMap or flatten to do the operation automatically

[pakoito]

the best you can do is fold

[pakoito]

myNested.fold({ it.invalidNel() }, ::identity)

[pakoito]

the errors propagate if there are any, otherwise apply the new nested operation

[pakoito]

you can never get errors from both the top-level and the nested operation together, because one precedes the other

[pakoito]

if they don’t, use ValidatedNel.applicative().mapN(valNel1, valNel2) { /* non-validated */ }

[Derek Towriss]

Thanks you! That worked. I just had to make one small change which was:

myNested.fold({it.invalid()}, ::identity)

[pakoito]

😄