From: Joel VanderWerf <vjoel@...>
Date: 2004-05-03T08:00:10+09:00
Subject: Re: instance_eval from C ext

Mauricio Fern�ndez wrote:
> On Mon, May 03, 2004 at 03:10:24AM +0900, Joel VanderWerf wrote:
> 
>>Any ideas, anyone? Is this something you can do in ruby, but not using 
>>the C API directly?
>>
>>Joel VanderWerf wrote:
>>
>>>I'm stumped... how do I call instance_eval from a C extension?
> 
> 
> Take a look at 
> http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/86105
> 
> you might find it useful.

Well, rb_iterate() is what I was trying before, but apparently it does 
not arrange for rb_block_given_p() to be true, and so 
rb_obj_instance_eval() bails out. I've adapted your example to show how 
that is the case in both your code and mine.

The output (ruby-1.9.0 snapshot) is:

foo2, block given = 0
call_gsub, block given = 0
block2, block given = 0
Inside block, with string: h.
block2, block given = 0
Inside block, with string: e.
block2, block given = 0
Inside block, with string: l.
block2, block given = 0
Inside block, with string: l.
block2, block given = 0
Inside block, with string: l.
"HELLo, worLd!"
instance_eval_proc, block given = 0
my_instance_eval, block given = 0
example.rb:11:in `instance_eval_proc': block not supplied (ArgumentError)
         from example.rb:11


So I'm still looking for a solution to my problem, which is: how to 
execute a Proc in the context of a given self object, from C code.

In other words, given the following working ruby code:

   class Object
     def instance_eval_proc(pr)
       instance_eval(&pr)
     end
   end

   obj = [1,2,3]
   pr = proc { reverse }

   p obj.instance_eval_proc(pr) # ==> [3, 2, 1]

how would you rewrite #instance_eval_proc in C?

== extconf.rb ==

require "mkmf"

create_makefile("example")

== example.rb ==

require "example.so"

b = "hello, world!"
CTest.foo2(b, /[ehl]/)
p b


obj = [1,2,3]
pr = proc { reverse }

p obj.instance_eval_proc(pr)

== example.c ==


#include <ruby.h>
#include <stdio.h>

VALUE mCTest;

static
VALUE
block2(VALUE str)
{
   printf("block2, block given = %d\n", rb_block_given_p());
   Check_Type(str, T_STRING);
   printf("Inside block, with string: %s.\n", RSTRING(str)->ptr);
   return rb_funcall(str, rb_intern("upcase"), 0);
}

static
VALUE
call_gsub(VALUE args)
{
  VALUE str, re;
  printf("call_gsub, block given = %d\n", rb_block_given_p());
  str = rb_ary_entry(args, 0);
  re = rb_ary_entry(args, 1);

  return rb_funcall(str, rb_intern("gsub!"), 1, re);
}

static
VALUE
foo2(VALUE self, VALUE str, VALUE re)
{
   VALUE args;
   printf("foo2, block given = %d\n", rb_block_given_p());
   args = rb_ary_new();
   rb_ary_push(args, str);
   rb_ary_push(args, re);

   return rb_iterate(call_gsub, args, block2, str);
}

static VALUE my_instance_eval(VALUE obj)
{
   printf("my_instance_eval, block given = %d\n", rb_block_given_p());
   return rb_obj_instance_eval(0, 0, obj);
}

static VALUE call_block(VALUE arg1, VALUE block)
{
   printf("call_block, block given = %d\n", rb_block_given_p());
   return rb_funcall(block, rb_intern("call"), 0);
}

static VALUE instance_eval_proc(VALUE self, VALUE pr)
{
   printf("instance_eval_proc, block given = %d\n", rb_block_given_p());
   return rb_iterate(my_instance_eval, self, call_block, pr);
}

void
Init_example()
{
  mCTest = rb_define_module("CTest");
  rb_define_singleton_method(mCTest, "foo2", foo2, 2);
  rb_define_method(rb_cObject, "instance_eval_proc", instance_eval_proc, 1);
}