Scala: Method overloading over generic types

问题

In C# I can overload methods on generic type as shown in the example below:

// http://ideone.com/QVooD
using System;
using System.Collections.Generic;

public class Test {
  public static void Foo(List<int> ints) {
    Console.WriteLine("I just print");
  }

  public static void Foo(List<double> doubles) {
    Console.WriteLine("I iterate over list and print it.");
    foreach(var x in doubles)
      Console.WriteLine(x);
  }

  public static void Main(string[] args) {
    Foo(new List<int> {1, 2});
    Foo(new List<double> {3.4, 1.2});
  }
}

However if I try to do the same in Scala, it will raise a compile time error that List[Int] and List[Double] erase to the same type due to erasure. I heard Scala's Manifests can be used to work around this, but I don't know how. I didn't find anything helpful in the docs either.

So my question is: How do I use Manifests (or whatever else that works) to overload methods over generic types that erase to same type due to erasure?

回答1:

The Manifest won't really help either becuase those will have the same type after erasure.

What will help having different numbers of arguments (or different types after erasure). I find having different numbers of implicit arguments can transparently solve this problem, and by using scala.Predef.DummyImplicit, you don't even have to import an implicit anywhere.

class Test{
  def foo(ints : List[Int])
  def foo(doubles : List[Double])(implicit i1:DummyImplicit)
  def foo(strings : List[String])(implicit i1:DummyImplicit, i2:DummyImplicit)
}

回答2:

You would not do it like that in Scala. Why try to emulate something that can never work properly given JVM restrictions? Try idiomatic Scala instead:

trait Fooable[T] {
  def foo : Unit
}

object IntListFoo extends Fooable[List[Int]] {
  def foo {
    println("I just print")
  }
}

class DoubleListFoo(val l : List[Double]) extends Fooable[List[Double]] {
  def foo {
    println("I iterate over list and print it.")
    l.foreach { e =>
      println(e)
    }
  }
}

implicit def intlist2fooable(l : List[Int]) = IntListFoo
implicit def doublelist2fooable(l : List[Double]) = new DoubleListFoo(l)

Then, you can execute code like

List(1,2,3,4).foo
List(1.0,2.0,3.0).foo

回答3:

Kinda hackish, and both methods need the same return type (here: Unit)...

def fooInt(list: List[Int]) = println("int")
def fooDouble(list: List[Double]) = println("double")

def foo[N <: AnyVal](list:List[N])(implicit m:ClassManifest[N]) = m.erasure match {
    case c if c == classOf[Int] => fooInt(list.asInstanceOf[List[Int]])
    case c if c == classOf[Double] => fooDouble(list.asInstanceOf[List[Double]])
    case _ => error("No soup for you!")
}

foo(List(1,2,3,4))
//--> int

foo(List(1.0,2.0,3.0))
//--> double

来源：https://stackoverflow.com/questions/4982552/scala-method-overloading-over-generic-types

标签

generics

scala

manifest

reification