Add toNestedValidated and toNestedValidatedNel

[conniec]

Issue: #1344

This is my first time contributing and am fairly new to these types so any feedback welcome. Would also like to add some examples if the code looks about right.

[codecov-io]

Current coverage is 91.68% (diff: 100%)

Merging #1408 into master will increase coverage by <.01%

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[non]

These seem good to me.

At a minimum we should have tests for both methods. It doesn't look like we have a tutorial for EitherT currently, but if we did have one adding these methods to it would be a good idea.

[travisbrown]

👍, looks good to me with the tests.

[peterneyens]

This looks good, thanks @conniec.

Maybe we can add the following example to toNestedValidatedNel, it shows the stereotypical example of error short circuiting versus error accumulation, but using EitherT and Nested / ValidatedNel. What do you think ?

import cats.data.{Nested, EitherT, ValidatedNel}
import cats.implicits._

type ErrorsOr[x] = ValidatedNel[String, x]
type OptionErrorsOr[x] = Nested[Option, ErrorsOr, x]

val ok = EitherT.right[Option, String, Int](Some(1))
val error = EitherT.left[Option, String, Int](Some("error"))
val none = EitherT[Option, String, Int](None)

val okV: OptionErrorsOr[Int] = ok.toNestedValidatedNel
val errorV: OptionErrorsOr[Int] = error.toNestedValidatedNel
val noneV: OptionErrorsOr[Int] = none.toNestedValidatedNel

(okV |@| okV |@| okV).map(_ + _ + _)
// OptionErrorsOr[Int] = Nested(Some(Valid(3)))

(okV |@| errorV |@| errorV).map(_ + _ + _)
// OptionErrorsOr[Int] = Nested(Some(Invalid(NonEmptyList(error, error))))

(ok |@| error |@| error).map(_ + _ + _)
// EitherT[Option,String,Int] = EitherT(Some(Left(error)))

(okV |@| errorV |@| noneV).map(_ + _ + _)
// OptionErrorsOr[Int] = Nested(None)

(ok |@| error |@| none).map(_ + _ + _)
// EitherT[Option,String,Int] = EitherT(Some(Left(error)))

This might even be something worth adding to the EitherT documentation when it gets written.

[travisbrown]

@peterneyens Maybe we should open an issue for EitherT documentation and update the toNestedValidatedNel example based on what ends up happening there? I'd be happy to see this merged as-is for now.

[peterneyens]

You're right, see #1425.

[conniec]

Thanks for the help everyone!