scalaz using reduceLeft for applicative builder

Question

I am little confused about |@| magic in scalaz. Here is my code:

def isThree(x: Int): Validation[NonEmptyList[String], Int] = if (x!= 3){("failed: %d" format x).wrapNel.failure} else {x.success}

println((isThree(6) |@| isThree(7) |@| isThree(13) ) {_ + _ + _})

output: Failure(NonEmptyList(failed: 6, failed: 7, failed: 13)) This is output is what I want.

Here is my questions:

assume I have sequence of Validation, I want to use applicative builder to chain them together.

Seq(isThree(13), isThree(15)).reduceLeft(_ |@| _) why compilation failed due to type not matching ?
It is similar to first question, if I use bracket :

println((isThree(6) |@| (sThree(7) |@| isThree(13)) ) {_ + _ + _}), it still has compilation errors.

Also, I know I can fix the first one by using <* instead of |@|, but I am still confused why is that, it looks not convenient to use.

Many thanks in advance.

It would be nice to know what exact error message you get. In your 2. you have a typo. `sThree(7)` — Kigyo, Apr 25 '14 at 03:00
Is there some reason you want to use the applicative builder syntax instead of just `vals.sequenceU.map(_.sum)` or `vals.suml`? — Travis Brown, Apr 25 '14 at 11:48
Or even `List(6, 7, 13).traverseU(isThree).map(_.sum)`, which may be better than building up the list of validations in the first place. — Travis Brown, Apr 25 '14 at 11:49

score 2 · Accepted Answer · answered Apr 25 '14 at 03:06

This works: |@| result type is not ValidationNel (Applicative) but ApplicativeBuilder, you need yo apply it first to some function

  import scalaz._, Scalaz._

  val x1: ValidationNel[String, Int] = 1.successNel
  val x2: ValidationNel[String, Int] = 2.successNel
  val x3: ValidationNel[String, Int] = 3.successNel

  println((x1 |@| x2 |@| x3)(_ + _ + _))

  println((x1 :: x2 :: x3 :: Nil).reduceLeft((l, r) => (l |@| r)(_ + _)))

scalaz using reduceLeft for applicative builder

1 Answers1