如何使用类型别名来定义类型构造函数

Question

Let's say I have some code that uses List

def processList(input: List[Int]): List[Int]

I want to replace list with other collection types, like Vector.

Is there a way to define a type constructor so that I can write something like

type SomeCollection[_] = List[_]

def processList(input: SomeCollection[Int]): SomeCollection[Int]

Now I have written processList in terms of SomeCollection. To change SomeCollection to Vector, I just change the type alias, and everywhere in the codebase where I use SomeCollection, I now use Vector. Like so:

type SomeCollection[_] = Vector[_]

def processList(input: SomeCollection[Int]): SomeCollection[Int]

This way, I only need to change the codebase in one place instead of everywhere.

I do not want to write

type SomeIntCollection = List[Int]

because I have connected the collection to Int type.

Is there a way?

Answer 1

You're pretty close, this can be done along the lines of

type SomeCollection[A] = List[A]

def processList(input: SomeCollection[Int]): SomeCollection[Int] = input.map(_+1)

However, there's better ways to describe the abstraction. In the cats library, there are a variety of typeclasses designed to abstract over the type of operation you want to perform. The above with cats would look something like

import cats._
import cats.implicits._

def process[F[_]: Functor](input: F[Int]): F[Int] = input.map(_+1)

It doesn't lock you into a specific underlying collection, so you're free to use whatever makes the most sense at the call site.