[L-lang-devel] New Form library added

[Matthieu Lemerre]

I just added support for Forms from L into CVS.  Forms are L's
representation of abstract syntax trees; and are used to manipulate L
code by itself.  This is the way expanders, macros ... work.

Forms are manipulated through a templating mechanism, like in the XML
library, and a bit like the Lisp `/,/,@ operators.

Here are some exerpts of the test file:

  let Form f3 = Form (3 + 4);

The most basic way to create a new form is to use Form(<L code>),
where <L code> can be either a L statement or a L expression.

  let Form f2 = Form (let Int i = $f3;);

To create non-constant Forms, you can inserts $<expression> in your
code, where expression must return a Form.  In this exemple, we insert
f3 in the code; so this has the same effect than of typing

  let Form f2 = Form (let Int i = 3 + 4;);

For instance, a while macro transformer function could be written as
follow:

  Form test_while (Form condition, Form body)
  {
    let Form while_form = Form (
       loop {
         if(!($condition))
         break;
         $body;
       }
                              );
  
    while_form
  }


  test_while (Form (i >= 0),
              Form (i = i - 1;));



The second other operator is $@<expression>, that inserts a list of
Forms (instead of a single form, like $ does).

  let List (Form) fl = list (f2, f3);
  let Form test_form_list = Form ({let Int i = 3;
                                    {$f; address@hidden;
                                    i = 4;
                                    address@hidden (f3, f2); });


So to compare with Common Lisp same facilities, Form is like `, $ like
, and $@ like ,@.

Have fun,

Matthieu