Stateful function pipeline

Question

The code explains itself.

val s = Seq(1,1,1)
val res: Seq[Int] = s.map(...)
                     .check(count how many 1s, if > 2 throw Exception)
                     .map(...)

I am searching the simple solution to this check function .

• I can use map and closure to count and throw, but I want pure function.
• I can use filter and size or reduce, but it return a value and not resumable with following maps.

How do I make a pure and stateful checking-pipe to the pipeline ?

Answer 1

One solution is to pattern match, so check would become:

> Seq(1, 1) match {
    case ls if (ls.count(_ == 1) <= 2) => ls
    case _ => throw new Exception("oh no!")
  }
List(1, 1): Seq[Int]


> Seq(1, 1, 1) match {
    case ls if (ls.count(_ == 1) <= 2) => ls
    case _ => throw new Exception("oh no!")
  }
java.lang.Exception: oh no!
  $.<init>(Main.scala:177)
  $.<clinit>(Main.scala:-1)

It may be preferable to return an Option type (instead of throwing):

> Seq(1, 1, 1) match {
    case ss if (ss.count(_ == 1) <= 2) => Option(ss)
    case _ => None
  }
None: Option[Seq[Int]]