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): Array[String]
  = commaDelimited.trim.split(',')

def splitByCommaAsSet(commaDelimited: String): Set[String]
  = splitByComma(commaDelimited).toSet

def splitByCommaAsSeq(commaDelimited: String): Seq[String]
  = splitByComma(commaDelimited).toSeq

val foods = "sushi,taco,burrito"
val foodSet = splitByCommaAsSet(foods)
// foodSet: scala.collection.immutable.Set[String] = Set(sushi, taco, burrito)
val foodSeq = splitByCommaAsSeq(foods)
// foodSeq: Seq[String] = List(sushi, taco, burrito)

However, this is quite repetitive. Ideally, I would want to have something like genericSplitByComma[Set](foods) that just returns a Set when I pass Set in, and returns a Seq when I pass Seq.

Luis Miguel Mejía Suárez · Accepted Answer · 2019-06-11T20:41:08.810

18

@KrzysztofAtłasik's answer works great for Scala 2.12.
This is a solution for 2.13. (Not completely sure if this is the best way).

import scala.collection.Factory
import scala.language.higherKinds

def splitByComma[C[_]](commaDelimited: String)(implicit f: Factory[String, C[String]]): C[String] =
  f.fromSpecific(commaDelimited.split(","))
  // Or, as Dmytro stated, which I have to agree looks better.
  commaDelimited.split(",").to(f)

Which you can use like this:

splitByComma[Array]("hello,world!")
// res: Array[String] = Array(hello, world!)

splitByComma[Set]("hello,world!")
// res: Set[String] = Set(hello, world!)

splitByComma[List]("hello,world!")
// res: List[String] = List(hello, world!)

edited Jun 11 '19 at 20:41

answered Jun 11 '19 at 20:26

Luis Miguel Mejía Suárez

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3

Or `commaDelimited.split(",").to(f)`. – Dmytro Mitin Jun 11 '19 at 20:37
2

Happy 2.13! I also wondered. def splitByComma[C](s: String)(implicit f: Factory[String, C]): C = s.split(",").to(f) – som-snytt Jun 11 '19 at 20:37
3

@DmytroMitin Oh, so that was the change of `to[List]` to `to(List)`, because it now receives a factory an every companion object is now a factory, I finally get it :D – Luis Miguel Mejía Suárez Jun 11 '19 at 20:40

Krzysztof Atłasik · Answer 2 · 2020-12-23T14:53:21.780

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.

score 9 · Answer 3 · answered Jun 11 '19 at 20:50

9

There's a simple workaround for this. Not exactly the requested syntax but just as concise and it should be 2.13 compatible.

def simpleSplitByComma(coll :Iterable[String]) =
  coll.flatMap(_.trim.split(","))

simpleSplitByComma(Set("hello,world"))          //res0: Set(hello, world)
simpleSplitByComma(Seq("bellow,world"))         //res1: List(bellow, world)
simpleSplitByComma(Array("fellow,old"))         //res2: ArrayBuffer(fellow, old)
simpleSplitByComma(Stream("end,of,the,world"))  //res3: Stream(end, ?)

answered Jun 11 '19 at 20:50

jwvh

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Now we must wait and see where yhylord places the green check! – som-snytt Jun 11 '19 at 21:15
2

I really like this one and will probably use something like this, but I feel like I have to accept another answer that exactly matches what I asked :) – yhylord Jun 11 '19 at 23:52

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

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