Proposal: Replace __traits and is(typeof(XXX)) with a 'magic namespace'.

[Don]

[I'm moving this from deep inside a TDPL thread, since I think it's 
important]

is(typeof(XXX)) is infamously ugly and unintuitive
__traits(compiles, XXX) is more comprehensible, but just as ugly.

They are giving metaprogramming in D a bad name. I think we need to get 
rid of both of them.

A very easy way of doing this is to replace them with a 'magic 
namespace' -- so that they _look_ as though they're functions in a 
normal module.
Names which have been suggested include 'meta', 'traits', 'scope', 
'compiler'. Personally I think 'meta' is the nicest (and I suggested two 
of the others <g>). This would give us:

meta.compiles(XXX)
meta.isArithmetic; // note, property syntax OK if no arguments
meta.isArithmetic(int*);

Benefits:
* Fewer keywords: __traits -> meta, typeid() -> meta.typeid()
* Get rid of is() expressions, which are the most complicated thing in 
the language.
* Some meta.XXX functions could be defined in runtime library code.
* The existing __traits functions could have more useful return values.
* Retain the flexibility of __traits.

These days, I don't propose anything unless I'm prepared to write a 
patch to implement it. <g>

If nothing is, this should convince everyone that there's NO REASON to 
put up with the ugly metaprogramming syntax we currently have. We can do 
much better. Easily.

======================
PATCH against svn 234 (this is just a quick hack for evaluation, it 
doesn't fix error messages, etc. But it works as described above. It 
allows all existing code to continue to compile).

This allows '__traits' as the magic namespace. The existing __traits 
stuff continues to compile. To allow 'meta' as another synonym for the 
same magic namespace, add this line to lexer.c, line 2960.

     {    "__traits",    TOKtraits    },
+   {    "meta",            TOKtraits    },
     {    "__overloadset", TOKoverloadset    },

(Hmm. Didn't know __overloadset was a keyword. The things you find...)


Index: parse.c
===================================================================
--- parse.c    (revision 234)
+++ parse.c    (working copy)
@@ -4934,6 +4934,20 @@
          Objects *args = NULL;

          nextToken();
+        if (token.value == TOKdot) {
+            // __traits.identifier(args, ...)
+        nextToken();
+        if (token.value != TOKidentifier)
+        {   error("__traits.identifier(args...) expected");
+            goto Lerr;
+        }
+        ident = token.ident;
+        nextToken();
+        if (token.value==TOKlparen)
+        args = parseTemplateArgumentList2();
+            e = new TraitsExp(loc, ident, args);
+        break;
+        }
          check(TOKlparen);
          if (token.value != TOKidentifier)
          {   error("__traits(identifier, args...) expected");