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

Throwing an exception is arguably not pure. If you instead were using a monadic form of error handling, you'd do something like this:

Option(s.map(foo)).
  filter(m => m.count(_ == 1) < 2).
  map{ s =>
    s.map(bar)
     .filter(baz)
     ...
  }

As it is, if you want to compose it within the pipeline, and you don't want to add extra parentheses as is necessary with match, you can use the commonly-enriched tap method:

implicit class TapAnything[A](private val a: A) extends AnyVal {
  def tap[U](f: A => U): A = { f(a); a }
}

Now you can

s.map(...)
 .tap(self => if (self.count(_ == 1) > 1) throw new Exception)
 .map(...)
 ...

(note: the private val + extends AnyVal stuff is just to indicate to the compiler that it's supposed to try to avoid creating an extra object to make the call).