Re: [ANN] Scala 2.7.0-RC1

Alex Boisvert wrote:
> I see no mention of type annotations in the language changelog.   Could 
> you elaborate?

(Followups to gmain.comp.lang.scala)

For several months, I worked on optional support in scalac for 
annotations on types.  The support was only turned on if you compiled 
with -Xplug-types.  Now, the option is gone and you always get the support.

This is a pretty good foundation, I think, for the pluggable type 
systems that Gilad Bracha describes.  The work is supported by the 
Hasler Foundation, so hopefully they think it is a pretty good 
foundation, too.  :)

There are two interesting parts to it compared to Java annotations.  One 
is that you can put arbitrary type-checked expressions in your 
annotations, not just constants.  So @dim(Length/Time) is perfectly 
valid.  Second, there is sophisticated support for propagating type 
annotations through the type inference.  Here is a typical example:

   object Foo {
     val x = 3
     val y: Int @GreaterThan(x) = 10
   }

   val z = Foo.y  // inferred type is Int @GreaterThan(Foo.x)

Notice how the @GreaterThan(x) had to be rewritten as 
@GreaterThan(Foo.x).  Let me tell you, that was a long saga to get