Scala equivalent of C#’s extension methods?

Question

In C# you can write:

using System.Numerics;
namespace ExtensionTest {
public static class MyExtensions {
    public static BigInteger Square(this BigInteger n) {
        return n * n;
    }
    static void Main(string[] args) {
        BigInteger two = new BigInteger(2);
        System.Console.WriteLine("The square of 2 is " + two.Square());
    }
}}

How would this simple extension method look like in Scala?

score 74 · Answer 1 · edited May 31 '13 at 23:12

74

The Pimp My Library pattern is the analogous construction:

object MyExtensions {
  implicit def richInt(i: Int) = new {
    def square = i * i
  }
}


object App extends Application {
  import MyExtensions._

  val two = 2
  println("The square of 2 is " + two.square)

}

Per @Daniel Spiewak's comments, this will avoid reflection on method invocation, aiding performance:

object MyExtensions {
  class RichInt(i: Int) {
    def square = i * i
  }
  implicit def richInt(i: Int) = new RichInt(i)
}

edited May 31 '13 at 23:12

Peter Mortensen

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answered Jun 25 '10 at 16:23

Mitch Blevins

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Warning: by using the `new { ... }` syntax, you're actually causing the compiler to infer a structural type for the `richInt` method, meaning that any invocations of `square` will be done using reflection (a performance killer). It would be better to define a `RichInt` class which defines `square` and is returned from the `richInt` method. – Daniel Spiewak Jun 25 '10 at 16:46
4

@Support Add `class RichInt(i: Int) { def square = i * i }` to `MyExtensions` and do `new RichInt(i)` on the `richInt` method. – Daniel C. Sobral Jun 25 '10 at 17:26
@Daniel, great, I have an answer with your comment. – OscarRyz Jun 25 '10 at 17:45
3

@Daniel interesting! I did a small test, the version with the structural type is indeed much slower: http://www.scala-notes.org/2010/06/avoid-structural-types-when-pimping-libraries/ – Jesper Jun 26 '10 at 08:01
@Jesper You can even get a little less sneaky and do the following: `val a = new { def b() { error("") } }; a.b()`. This way, you can inspect the stack trace. – Daniel Spiewak Jun 27 '10 at 20:00

score 67 · Answer 2 · answered Oct 10 '13 at 13:59

Since version 2.10 of Scala, it is possible to make an entire class eligible for implicit conversion

implicit class RichInt(i: Int) {
  def square = i * i
}

In addition, it is possible to avoid creating an instance of the extension type by having it extend AnyVal

implicit class RichInt(val i: Int) extends AnyVal {
  def square = i * i
}

For more information on implicit classes and AnyVal, limitations and quirks, consult the official documentation:

Note that case classes **cannot** be annotated with `implicit` keyword, because they have auto-generated companion object. For more information see documentation links provided by @megri. — falconepl, Jan 31 '15 at 12:35

score 10 · Answer 3 · edited May 23 '17 at 11:47

10

This would be the code after Daniel's comment.

object MyExtensions {
    class RichInt( i: Int ) {
        def square = i * i
    }
    implicit def richInt( i: Int ) = new RichInt( i )

    def main( args: Array[String] ) {
        println("The square of 2 is: " + 2.square )
    }
}

edited May 23 '17 at 11:47

Community

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answered Jun 25 '10 at 16:52

OscarRyz

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score 2 · Answer 4 · answered Jun 25 '10 at 16:24

2

In Scala we use the so-called (by the inventor of the language) Pimp My Library pattern, which is much discussed and pretty easy to find on the Web, if you use a string (not keyword) search.

answered Jun 25 '10 at 16:24

Randall Schulz

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GravityWell · Answer 5 · 2020-08-06T15:18:22.120

Scala 3 now has extension methods. Functionally it seems similar as expected to C# and Kotlin.

https://dotty.epfl.ch/docs/reference/contextual/extension-methods.html
https://github.com/scala/scala
https://github.com/lampepfl/dotty

A recent (as of this post) pull shows the syntax being simplified. Stable version as of this post is still 2.x. But there is a 3.xRC, and I noticed Jetbrains already supports it in Idea, partially I assume.

Scala equivalent of C#’s extension methods?

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