From: Dominik Bathon <dbatml@...>
Date: 2006-01-03T07:54:43+09:00
Subject: Re: Implicit block parameter?

On Mon, 02 Jan 2006 21:32:57 +0100, Ross Bamford  
<rosco@roscopeco.remove.co.uk> wrote:

> Hi,
>
> Probably this has been considered before, but I'll ask anyway.

I have also proposed this in October:

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/162588

And it seems to have been proposed before.

I have actually implemented it for ruby 1.9 lately (it should work similar  
for 1.8). It's just a small patch for eval.c:

--- eval_org.c  2005-12-30 02:17:49.000000000 +0100
+++ eval.c      2006-01-02 23:25:46.000000000 +0100
@@ -4656,6 +4656,7 @@ rb_yield_0(VALUE val, VALUE self, VALUE
      NODE *cnode = ruby_current_node;
      int lambda = flags & YIELD_LAMBDA_CALL;
      int state;
+    VALUE it_val = Qnil;

      rb_need_block();

@@ -4675,8 +4676,10 @@ rb_yield_0(VALUE val, VALUE self, VALUE
      scope_vmode = (flags & YIELD_PUBLIC_DEF) ? SCOPE_PUBLIC :  
block->vmode;
      ruby_block = block->prev;
      if (block->flags & BLOCK_D_SCOPE) {
+       if (avalue && RARRAY(val)->len != 0) it_val = RARRAY(val)->ptr[0];
+       if (!avalue && val != Qundef) it_val = val;
         /* put place holder for dynamic (in-block) local variables */
-       ruby_dyna_vars = new_dvar(0, 0, block->dyna_vars);
+       ruby_dyna_vars = new_dvar(0, it_val, block->dyna_vars);
      }
      else {
         /* FOR does not introduce new scope */
@@ -7596,6 +7599,15 @@ rb_exec_end_proc(void)
      ruby_safe_level = safe;
  }

+static VALUE
+rb_f_it(void)
+{
+    if (ruby_dyna_vars) {
+       if (ruby_dyna_vars->id == 0) return ruby_dyna_vars->val;
+    }
+    return Qnil;
+}
+
  void
  Init_eval(void)
  {
@@ -7650,6 +7662,8 @@ Init_eval(void)
      rb_define_global_function("global_variables", rb_f_global_variables,  
0); /* in variable.c */
      rb_define_global_function("local_variables", rb_f_local_variables, 0);

+    rb_define_global_function("it", rb_f_it, 0);
+
      rb_define_method(rb_mKernel, "send", rb_f_send, -1);
      rb_define_method(rb_mKernel, "__send__", rb_f_send, -1);
      rb_define_method(rb_mKernel, "funcall", rb_f_funcall, -1);


This defines a global function #it that always returns the first block  
argument, it is not assignable. #it is available whether other block  
parameters are there or not. #it defaults to nil (if no arguments are  
given or if not in a block context.

$ cat test_it.rb
p (1..5).to_a.map { -it }

%w[a b c].each { |x| p [x, it] }

p it
$ ./miniruby test_it.rb
[-1, -2, -3, -4, -5]
["a", "a"]
["b", "b"]
["c", "c"]
nil


Dominik