How to use Validation?

[Heiko Seeberger]

Hi,

I wonder how to use Validation "the right way":

• How should I extract a Success?
• How should I extract a Failure?

Thanks,

Heiko

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[Robert Wills]

I spent a bit of time on this question last week in response to some
comments by Jason Zaugg.

From what I can tell the main things to tools to use are map, fold and lift.

When you want to get something out of a validation regardless of
whether it is a success or a failure
(analogous to option's 'getOrElse') use fold:

scala> val s = 1.success[String]
s: scalaz.Validation[String,Int] = Success(1)

scala> val f = "fail".fail[Int]
f: scalaz.Validation[java.lang.String,Int] = Failure(fail)

scala> f.fold( x => "", _.toString)
res38: java.lang.String =

scala> s.fold( x => "", _.toString)
res39: java.lang.String = 1

Or you could just convert to an option:
scala> s.toOption.getOrElse("")
res34: Any = 1

If you need to modify the result validation if it's a success:
scala> s.map( _ + 1)
res40: scalaz.Validation[String,Int] = Success(2)

Similarly for a failure:
scala> s.fail.map( _ + ":").validation
res41: scalaz.Validation[java.lang.String,Int] = Success(1)

scala> f.fail.map( _ + ":").validation
res42: scalaz.Validation[java.lang.String,Int] = Failure(fail:)

Lastly use lift for wrapping either the success or failure in something else:
scala> s.lift[Option, Int]
res43: scalaz.Validation[String,Option[Int]] = Success(Some(1))

scala> f.fail.lift[Option, String]
res45: scalaz.Validation[Option[String],Int] = Failure(Some(fail))

Using these methods rather than pattern matching does seem to make for
cleaner code.

Hope that helps,
Rob

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[Debasish Ghosh]

Many examples in scalaz/example/src/main/scala/scalaz/example/ExampleValidation.scala .. also have a look at Tony Morris Presentation on non-breaking error handling at http://applicative-errors-scala.googlecode.com/svn/artifacts/0.6/pdf/index.pdf

Thanks.

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