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

问题

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.

回答1:

@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!)

回答2:

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.

回答3:

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, ?)

来源：https://stackoverflow.com/questions/56551015/how-to-make-method-return-the-same-generic-as-the-input