[nem-en] Hygienic macros and lexical scoping

[Kamil Skalski]

Hello!

This week we have implemented property of hygiene and lexical scoping of 
global symbols in macros and quotations. Full description of these features 
is (or will be) in http://nemerle.org/metaprogramming.pdf paper and 
http://nemerle.org/macros.html tutorial.

In short:
- hygiene
  Symbols introduced by macro expansion does not conflict with symbols from   
other macro expansions and top level code. For example with macro definition

 macro m(ex) <[ def f () { printf ("x") }; $ex; f() ]>

code

 def f () {printf ("y") }; m(f()); 

would be transformed  to
 def f () { printf ("y") };  def f_34 () { printf ("x") }; f (); f_34 ()

Thus symbol "f" introduced by macro is "renamed" to avoid capture of other 
symbols.

- lexical scoping of global symbols
 Global symbols used in macros (quotations) are searched in namespaces 
imported in place where they are defined, not in place where macro is 
expanded. For example 
using System.RegularExpressions;
macro m () { <[ Regex ("a+") ]> }

used in code, where System.RegularExpressions are not imported, like

def x = m ();

expands to full name visible at macro definition site

def x = System.RegularExpressions.Regex ("a+");


This stuff is still under testing, but all features seems to work very good.

Kamil Skalski