问题
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?
回答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.
来源：https://stackoverflow.com/questions/58293648/how-to-use-a-type-alias-to-define-a-type-constructor