CODEWITHSUNDEEP
validationscalaz
Michael
Michael

Reputation: 42050

How to map on failure of ValidationNel?

I've learned that Validation has methods :-> and <-: to map on the success and failure.

scala> val failure: Validation[String, Unit] = "xxx".failure
failure: scalaz.Validation[String,Unit] = Failure(xxx)

scala> failure.<-:("!!!" + _)
res17: scalaz.Validation[String,Unit] = Failure(!!!xxx)

Unfortunately <-: does not exist for ValidationNel:

scala> val failure: ValidationNel[String, Unit] = "xxx".failureNel
failure: scalaz.ValidationNel[String,Unit] = Failure(NonEmptyList(xxx))

scala> failure.<-:(nel => nel + "!!!")
<console>:15: error: value <-: is not a member of scalaz.ValidationNel[String,Unit]
          failure.<-:(nel => nel + "!!!")

Interesting that it does compile if I define may failure as Validation[NonEmptyList[String], Unit].

Now I wonder if there is another way to use <-: for ValidationNel.

Upvotes: 1

Views: 481

Answers (1)

stew
stew

Reputation: 11366

Right, you won't be able to conjure a Bifunctor instance for ValidationNel, but ValidationNel is really just a type alias for Validation[NonEmptyList[?], ?], and that does, as you know, have a Bifunctor instance. If you coerce the type of your value from ValidationNel to just Validation, things will start to work:

scala> import scalaz._, Scalaz._
import scalaz._
import Scalaz._

scala> val failure: Validation[String, Unit] = "xxx".failure
failure: scalaz.Validation[String,Unit] = Failure(xxx)

scala> failure.<-:("!!!" + _)
res0: scalaz.Validation[String,Unit] = Failure(!!!xxx)

scala> val failure: ValidationNel[String, Unit] = "xxx".failureNel
failure: scalaz.ValidationNel[String,Unit] = Failure(NonEmptyList(xxx))

scala> failure.<-:("!!!" <:: _)
<console>:15: error: value <-: is not a member of scalaz.ValidationNel[String,Unit]
              failure.<-:("!!!" <:: _)
                      ^

scala> val failure2: Validation[NonEmptyList[String], Unit] = failure
failure2: scalaz.Validation[scalaz.NonEmptyList[String],Unit] = Failure(NonEmptyList(xxx))

scala> failure2.<-:("!!!" <:: _)
res2: scalaz.Validation[scalaz.NonEmptyList[String],Unit] = Failure(NonEmptyList(!!!, xxx))

however, you can also just call the leftMap method on Validation:

scala> failure2.leftMap("!!!" <:: _)
res3: scalaz.Validation[scalaz.NonEmptyList[String],Unit] =      Failure(NonEmptyList(!!!, xxx))

scala> failure.leftMap("!!!" <:: _)
res4: scalaz.Validation[scalaz.NonEmptyList[String],Unit] = Failure(NonEmptyList(!!!, xxx))

or use "swapped" to swap failure and success, rightMap the swapped value, then swap them back:

scala> failure.swapped(_ :-> ("!!!" <:: _))
res5: scalaz.Validation[scalaz.NonEmptyList[String],Unit] = Failure(NonEmptyList(!!!, xxx))

Upvotes: 2

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