How to make method return the same generic as the input?

Question

I want to split a string delimited by commas and use the result as either a Seq or a Set:

def splitByComma(commaDelimited: String): Arr         


        

Answer 1

There's a method in Scala called to which can transform arbitrary collection to another as long as there is typeclass called CanBuildFrom in scope.

import scala.collection.generic.CanBuildFrom
import scala.languageFeature.higherKinds

def genericSplitByComma[S[_]](s: String)(implicit cbf: CanBuildFrom[Nothing, String, S[String]]): S[String] = {
    s.split(",").to[S]
}

genericSplitByComma[Set]("Hello, hello") //Set(Hello,  hello)
genericSplitByComma[List]("Hello, hello") //List(Hello,  hello)
genericSplitByComma[Array]("Hello, hello") //Array(hello, world!)

We don't need to constrain S[_] because this function won't compile if there is no suitable CanBuildFrom in scope. For example, this will fail:

genericSplitByComma[Option]("Hello, hello")

Below will also fail because our type constructor S[_] accepts only one type argument and the map expects two:

genericSplitByComma[Map]("Hello, hello")

As Luis Miguel Mejía Suárez and Dmytro Mitin noticed, there was major refactor in collections in just-released Scala 2.13, so it will work up to Scala 2.12.