Why are so few things @specialized in Scala's standard library?

Question

I\'ve searched for the use of @specialized in the source code of the standard library of Scala 2.8.1. It looks like only a handful of traits and classes use this an

Answer 1

Specialized can be expensive ( exponential ) in both size of classes and compile time. Its not just the size like the accepted answer says.

Open your scala REPL and type this.

import scala.{specialized => sp}
trait S1[@sp A, @sp B, @sp C, @sp D] { def f(p1:A): Unit }

Sorry :-). Its like a compiler bomb.

Now, lets take a simple trait

trait Foo[Int]{ }

The above will result in two compiled classes. Foo, the pure interface and Foo$1, the class implementation.

Now,

trait Foo[@specialized A] { }

A specialized template parameter here gets expanded/rewritten for 9 different primitive types ( void, boolean, byte, char, int, long, short, double, float ). So, basically you end up with 20 classes instead of 2.

Going back to the trait with 5 specialized template parameters, the classes get generated for every combination of possible primitive types. i.e its exponential in complexity.

2 * 10 ^ ^{(no of specialized parameters)}

If you are defining a class for a specific primitive type, you should be more explicit about it such as

trait Foo[@specialized(Int) A, @specialized(Int,Double) B] { }

Understandably one has to be frugal using specialized when building general purpose libraries.

Here is Paul Phillips ranting about it.