How can I validate Option values with Cats validation?

Question

I'm trying to change code that uses cats validation, something like:

  case class Example(text: String, image: String)
  case class ValidExample(text: String, image: String)

  import cats.data.Validated._
  import cats.implicits._

  def validText(text: String) = if (text.nonEmpty) text.valid else invalid(-1)
  def validImage(image: String) = if (image.endsWith(".png")) image.valid else invalid(-1)
  val e = Example("test", "test.png")
  (validText(e.text), validImage(e.image)).mapN(ValidExample)

Which works fine.

But my change requires the image field to be an Option, like:

  case class Example(text: String, image: Option[String])
  case class ValidExample(text: String, image: Option[String])

  import cats.data.Validated._
  import cats.implicits._

  def validText(text: String) = if (text.nonEmpty) text.valid else invalid(-1)
  def validImage(image: String) = if (image.endsWith(".png")) image.valid else invalid(-1)
  val e = Example("test", Some("test.png"))
  (validText(e.text), e.image.map(validImage)).mapN(ValidExample)

With this the mapN fails because the types suddenly differ, it says:

value mapN is not a member of (cats.data.Validated[Int,String], Option[cats.data.Validated[Int,String]])

I want it to only validate if the value exists. So it should be part of the validation result if value is present but ignore the field otherwise. I know there are some ways of combining validations but in my real code that would be much more involved than something like this.

Is there a way to do this in a simple manner? I haven't been able to find anything about it in the docs or in search.

Thanks for help!

Answer 1

The answer is traverse, as usual :):

scala> val exampleWithImage = Example("test", Some("test.png"))
exampleWithImage: Example = Example(test,Some(test.png))

scala> val exampleWithoutImage = Example("test", None)
exampleWithoutImage: Example = Example(test,None)

scala> val badExampleWithImage = Example("test", Some("foo"))
badExampleWithImage: Example = Example(test,Some(foo))

scala> exampleWithImage.image.traverse(validImage)
res1: cats.data.Validated[Int,Option[String]] = Valid(Some(test.png))

scala> exampleWithoutImage.image.traverse(validImage)
res2: cats.data.Validated[Int,Option[String]] = Valid(None)

scala> badExampleWithImage.image.traverse(validImage)
res3: cats.data.Validated[Int,Option[String]] = Invalid(-1)

So it looks like traverse on Option does what you want: it validates the contents of Some, and ignores None (i.e. passes it through as valid).

So you could write the following, replacing map with traverse:

scala> (validText(e.text), e.image.traverse(validImage)).mapN(ValidExample)
res4: cats.data.Validated[Int,ValidExample] = Valid(ValidExample(test,Some(test.png)))

And you're done.