Scala Examples

I find learning by example super helpful. I love when documentation has lots of examples I can copy/paste and tweak.

With that in mind, I'm going to collect some examples here that don't really fit into a large post or note. Maybe they'll be useful.

Iterator#collect

collect is such a handy method, but I always seem to forget about it. If you'd like to filter and map a collection in a type safe manner, check out collect!

Consider a List[(String, Option[Int])]:

val xs = List(
  ("hi", Some(1)),
  ("bye", None)
)

You'd like to manipulate only the values where the Option is defined:

xs.collect {
  case (msg, Some(n)) => (msg, n*100)
}
// res0: List[(String, Int)] = List(
//   ("hi", 100)
// )

Stream#scanChunks

A lot of stateful methods in fs2's Stream are implemented using scanChunks for efficiency. It's similar to scan in that it takes a starting state, and function to compute the next state and values. But it operates on chunks and maintains the chunk sizes.

Here we implement a function similar to zipWithIndex but the index is one the left and starts at 1.

import fs2.{Pure, Stream}

def iZip[F[_], A](s: Stream[F, A]): Stream[F, (Int, A)] =
  s.scanChunks(1) { (index, chunk) =>
    var idx = index
    val out = chunk.map { elem =>
      val res = (idx, elem)
      idx += 1
      res
    }
    (idx, out) // new state and value
  }

val s: Stream[Pure, String] =
  Stream("hi", "hello", "bye!")

e.g.

iZip(s).toList
// res1: List[(Int, String)] = List(
//   (1, "hi"),
//   (2, "hello"),
//   (3, "bye!")
// )