Scala generic functions

Question

I have an abstract class

abstract class Foo {
  def foo(a: Int): Int
  ...
}

// Usage
new Foo {
  def foo(a: Int) = {
    println("Foo")
    a
  }
}

I frequently see a companion object to make this a little less verbose for callers (e.g. the Play framework).

object Foo {
  def apply(f: Int => Int) = new Foo {
    def foo(a: Int) = f(a)
  }
}

// Usage
Foo { a =>
  println("Foo")
  a
}

But suppose I make the method generic

abstract class Foo {
  def foo(a: T): T
  ...
}

// Usage
new Foo {
  def foo(a: T) = {
    println("Foo")
    a
  }
}

Can I still use a companion object, i.e. can I apply generic type parameters to a function, rather than a method or class?

Lee · Accepted Answer · 2014-11-28 22:21:05Z

5

Yes you can do this by emulating rank 2 polymorphism. Based on this article you can do:

trait ~>[F[_],G[_]] {
  def apply[A](a: F[A]): G[A]
}

type Id[A] = A

abstract class Foo {
  def foo[T](a: T): T
}

object Foo {
  def apply(f: Id ~> Id) = new Foo {
    def foo[T](a: T): T = f(a)
  }
}

val fun = new (Id ~> Id) { def apply[T](a: T): T = { println("Foo"); a } }
val foo = Foo(fun)
foo.foo(1)
foo.foo("String")

edited Nov 28, 2014 at 22:21

answered Nov 28, 2014 at 22:11

Lee

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1 Comment

Lee Over a year ago

@PaulDraper - So you want Foo.foo to be generic?

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