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feat: add .toEitherT to Validated #4554

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feat: add .toEitherT to Validated #4554

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c5ba3b2
feat: add `.toEitherT` to `Validated`
jeengbe Jan 27, 2024
b16951c
fix: polish comment a bit
jeengbe Jan 27, 2024
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24 changes: 24 additions & 0 deletions core/src/main/scala/cats/data/Validated.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -265,6 +265,30 @@ sealed abstract class Validated[+E, +A] extends Product with Serializable {
case Valid(a) => Right(a)
}

/**
* Converts the value to an EitherT[F, E, A]
*
* Example:
* {{{
* scala> import cats.syntax.all._
* scala> given ExecutionContext = ExecutionContext.global
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I believe this is Scala 3 syntax, please replace it with the Scala 2 equivalent (which I assume doc comments are written in)

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Didn't compile it myself, but I think this would do it:

Suggested change
* scala> given ExecutionContext = ExecutionContext.global
* scala> import scala.concurrent.ExecutionContext.Implicits.global

*
* scala> val v1 = "error".invalid[Int]
* scala> val v2 = 123.valid[String]
*
* scala> v1.toEitherT[Future].value
* res0: Future[Either[String, Int]] = Future(Left(error))
*
* scala> v2.toEitherT[Future].value
* res1: Future[Either[String, Int]] = Future(Right(123))
* }}}
*/
def toEitherT[F[_]: Applicative]: EitherT[F, E, A] =
this match {
case Invalid(e) => EitherT.leftT(e)
case Valid(a) => EitherT.rightT(a)
}
Comment on lines +287 to +290
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And perhaps

Suggested change
this match {
case Invalid(e) => EitherT.leftT(e)
case Valid(a) => EitherT.rightT(a)
}
EitherT.fromEither(this.toEither)

is cleaner


/**
* Returns Valid values wrapped in Some, and None for Invalid values
*
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12 changes: 6 additions & 6 deletions docs/datatypes/validated.md
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Expand Up @@ -298,9 +298,9 @@ But, what about if we want _another_ way of combining? We can provide our custom
Cats offers you a nice set of combinators for transforming your `Validated` based approach to an `Either` one and vice-versa.
We've used `.toValidated` in our second example, now let's see how to use `.toEither`.

#### From `Validated` to `Either`
#### From `Validated` to `Either`/`EitherT`

To do this, simply use `.toEither` combinator:
To do this, simply use `.toEither`/`.toEitherT` combinator:

```scala mdoc
// Successful case
Expand All @@ -319,7 +319,7 @@ FormValidatorNec.validateForm(
firstName = "John",
lastName = "Doe",
age = 5
).toEither
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The next sentence says we have an Either. Perhaps it's better to make the EitherT a separate example?

).toEitherT[Future]
```

With this conversion, as you can see, we got an `Either` with a `NonEmptyChain` detailing the possible validation errors or our `RegistrationData` object.
Expand Down Expand Up @@ -591,7 +591,7 @@ validatedMonad.tuple2(Validated.invalidNec[String, Int]("oops"), Validated.inval
```

This one short circuits! Therefore, if we were to define a `Monad` (or `FlatMap`) instance for `Validated` we would
have to override `ap` to get the behavior we want.
have to override `ap` to get the behavior we want.

```scala mdoc:silent:nest
import cats.Monad
Expand Down Expand Up @@ -629,14 +629,14 @@ But then the behavior of `flatMap` would be inconsistent with that of `ap`, and
```scala
// the `<->` operator means "is equivalent to" and returns a data structure
// `IsEq` that is used to verify the equivalence of the two expressions
def flatMapConsistentApply[A, B](fa: F[A], fab: F[A => B]): IsEq[F[B]] =
def flatMapConsistentApply[A, B](fa: F[A], fab: F[A => B]): IsEq[F[B]] =
fab.ap(fa) <-> fab.flatMap(f => fa.map(f))
```

```scala mdoc:silent
import cats.laws._

val flatMapLawsForAccumulatingValidatedMonad =
val flatMapLawsForAccumulatingValidatedMonad =
FlatMapLaws[Validated[NonEmptyChain[String], *]](accumulatingValidatedMonad)

val fa = Validated.invalidNec[String, Int]("oops")
Expand Down
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