Ggtcf

This is a simple question.

Why was not the method related to Int resided in Int?

Instead Scala bothers to put related methods into RichInt and rely on implicit conversion so as to have them work like methods of Int.

Why bother?

Scala doesn't exist in a vacuum. It was specifically designed to be hosted in an ecosystem / on a platform which was mostly designed for another language: the Java platform, the .NET platform, the ECMAScript platform, Cocoa, etc.

This means that in some cases compromises had to be made, in order to make Scala operate seamlessly, efficiently, and with high performance with the ecosystem, libraries and language of the host platform. That's why it has null, why it has classes (it could get by with just traits, and allow traits to have constructors), why it has packages (because they can be cleanly mapped to Java packages or .NET namespaces), why it doesn't have proper tail calls, doesn't have reified generics, etc. It's even why it has curly braces, not to make it easier to integrate with Java, but to make it easier to integrate with the brains of Java developers.

scala.Int is a fake class, it represents a native platform integer (primitive int in Java, System.Int32 in .NET, etc.) Being fake, it can't really have any methods other than the operations provided by the host environment.

The alternative would be to have all operations in the Int class and have the compiler know the difference between which methods are native and which aren't. But that's a special case, it makes more sense to concentrate efforts on making "enrich-my-library" fast in general, so that all programmers can benefit from those optimizations instead of spending time, money and resources on optimizations that only apply to twelve or so classes.

The question is why not model Int richly and then optimize, for example, that it has an unboxed representation and that some operations are provided natively?

The answer must surely be that the compiler is still not very good at these optimizations.

scala> 42.isWhole
res1: Boolean = true

scala> :javap -prv -
[snip]
 9: getstatic #26 // Field scala/runtime/RichInt$.MODULE$:Lscala/runtime/RichInt$;
 12: getstatic #31 // Field scala/Predef$.MODULE$:Lscala/Predef$;
 15: bipush 42
 17: invokevirtual #35 // Method scala/Predef$.intWrapper:(I)I
 20: invokevirtual #39 // Method scala/runtime/RichInt$.isWhole$extension:(I)Z
 23: putfield #17 // Field res1:Z
 26: return 

or under -optimize

 9: getstatic #26 // Field scala/runtime/RichInt$.MODULE$:Lscala/runtime/RichInt$;
12: getstatic #31 // Field scala/Predef$.MODULE$:Lscala/Predef$;
15: astore_1 
16: bipush 42
18: invokevirtual #35 // Method scala/runtime/RichInt$.isWhole$extension:(I)Z
21: putfield #17 // Field res0:Z
24: return