Scalaz use applicative builder with validations and a list of validations

Question

I'm working with Scalaz validations and I've run into a situation like this (note this is heavily simplified from my actual code, but the idea is the same)

Given:

case class Foo(bar: Int)

val x1: Validation[String, Foo] = Foo(1).success
val x2: Validation[String, Foo] = Foo(2).success
val x3: Validation[String, Foo] = Foo(3).success

val l1 = List(x1, x2)

I would like to be able to do something along the lines this:

(x3 |@| l1) { (x1, x2, x3) => /*do something with all of my Foo's*/ }

Of course if there were any errors, either in the list or outside of the list I'd like them to accumulate as they normally would.

I know the above syntax does not work, but any advice on how to achieve the result I'm looking for would be appreciated.

I'm not sure I understand exactly what you're asking, but you're aware that l1.sequenceU will give you a validation containing a list of x1 and x2? — Travis Brown
Ah, I didn't realize that. Just tried it and that does the trick. Thanks. — Bryan Cosgrove

Travis Brown Travis Brown · Accepted Answer · 2015-04-22T23:30:06

If you have a List[F[A]] and F has an applicative functor instance, you can turn the list inside out in effect with sequenceU to get a F[List[A]]:

scala> l1.sequenceU
res0: scalaz.Validation[String,List[Foo]] = Success(List(Foo(1), Foo(2)))

Or:

scala> (x3 |@| l1.sequenceU) {
  case (third, rest) => // do something with the values
}

It's also worth noting that if you find yourself writing things of the xs.map(f).sequenceU, you can use xs.traverseU(f) instead—it's exactly equivalent except you don't build an intermediate list.

Scalaz use applicative builder with validations and a list of validations

1 Answers