Michel Pastor writes:
 > 
 > I have a problem with a little piece of code but
 > I attached the whole thing since i cannot reproduce
 > the "bug" in reduced environement with the same values.
 > I'm using the num library for doing exact maths.
 > 
 > [...]
 >
 > If I try to catch the exception like that the process
 > wont even go beyond the "try with" block
 > I tested printing something instead of trying to exit
 > but nothing is writed to the console and the cpu is at 100%
 > All I can do is to kill the process.
 > But if I don't try catch the Failure all works fine.
 > I don't think doing unsafe things since there is no C code,
 > no black magic, only some maths.

This is  a ``syntax'' problem: you  are not parsing the  code the same
way the ocaml compiler does. When you insert the try/with like this:

 > (************************)
 > (*  HERE IS THE PROBLEM *)
 > (************************)
 > 
 > let draw_curve color =
 >   begin
 >     set_color color;
 >     let index = ref !x_min in
 >     let ival = (!x_max -/ !x_min) // num_of_int (graph_width * 2) in
 >     while !index <=/ !x_max do
 >       let yval = poly_apply poly !index in
 >      (* uncomment the "try with" block to trigger the bug *)
 >      try 
 >      begin
 > 	print_endline ("ici " ^ string_of_num yval);
 > 	let (graph_x, graph_y) = scale_point (!index, yval) in
 > 	  begin
 > 	    printf "%d %d\n" graph_x graph_y;
 > 	    fill_circle graph_x graph_y 1;
 > 	  end;
 > 	print_endline "la";
 > 	end;
 >      with _ -> failwith "ici maintenant";
 >       print_endline "et encore la";
 >       index := !index +/ ival;
 >     done
 >   end

then the three  lines "failwith ...; print_endline ...;  index := ..."
are  all  part  of the  exception  handler.   Since  in your  code  no
exception is  raised, these lines  are never evaluated,  including the
update of the index, which explains the infinite loop.

If you really  need this try/with, you need to  rewrite your code this
way (for instance):

	while ... do
	  ...
	  begin 
	    try
	      ...
	    with _ ->
              ...
	  end;
	  ...
        done

To  avoid  such pitfalls,  it  is convenient  to  use  an editor  with
automatic indentation of  the code which is consistent  with the ocaml
syntax, such as the Tuareg mode under Emacs. At some point, you become
so  familiar with ocaml  syntax that  you don't  need it  anymore, but
meanwhile it helps.

Best regards,
-- 
Jean-Christophe