<cannot fetch remote object> - WHY ?

<cannot fetch remote object> - WHY ?

[Sorin Stratulat]

Hello !

I started to debug large OCaml code and apparently the only acceptable
solution to do this is to use the debugger. Unfortunately, I found
difficulties to convince the debugger to print objects.

Let's take a simple example (took from the manual):

(***** File essai.ml *******)

class point =
   object 
     val mutable x = 0
     method get_x = x
     method move d = x <- x + d
   end;;

let my_p = new point;;

(****************************)   

In order to print the value x of my_p, I separately defined a printer function as

(***** File essai_printer.ml *******)

open Format

let my_print_point s =   
  open_box 0; 
  print_string  ("The point is situated at" ^ string_of_int (s#get_x)); 
  print_space (); 
  close_box ();
  print_newline ()
;;

(****************************)   


let my_print_point s =   
  open_box 0; 
  print_string  ("The point is situated at" ^ string_of_int (s#get_x)); 
  print_space (); 
  close_box ();
  print_newline ()
;;


Here are the steps I followed:

1. Compile essai.ml and essai_printer.ml
	ocamlc -c -g essai.ml
	ocamlc -c -g essai_printer.ml

2. Create the executable file "essai"
	ocamlc -custom -g -o essai essai.cmo

3. Launch the debugger on the file "essai"

4. load the printer (I show also the answer of the debugger)
(ocd) load_printer "essai_printer.cmo"
File ./essai_printer.cmo loaded
5. install the printer function my_print_point
(ocd) install_printer Essai_printer.my_print_point
6. ....put a breakpoint immediately after the definition of my_p

7. print my_p
(ocd) p my_p
my_p : point = <cannot fetch remote object>

------------------------------------------------------------------

Can somebody tell me the "solution" to print the value "x" of my_p ?


Thank you,

Sorin

French version:

Bonjour !

Je suis en train de deboguer de gros fichiers en OCaml et je suis
contraint d'utiliser le debogueur. Malheureusement, je n'arrive pas a
afficher des objets.

Voici un tres simple exemple (pris du manuel):
(***** File essai.ml *******)

class point =
   object 
     val mutable x = 0
     method get_x = x
     method move d = x <- x + d
   end;;

let my_p = new point;;

(****************************)   

Pour afficher la valeur x de my_p, j'ai defini separement une fonction
d'impression

(***** File essai_printer.ml *******)

open Format

let my_print_point s =   
  open_box 0; 
  print_string  ("The point is situated at" ^ string_of_int (s#get_x)); 
  print_space (); 
  close_box ();
  print_newline ()
;;

(****************************)   

Les etapes que j'ai suivi sont les suivantes:

1. Compiler essai.ml et essai_printer.ml
	ocamlc -c -g essai.ml
	ocamlc -c -g essai_printer.ml
2. Creer le fichier executable "essai":
	ocamlc -custom -g -o essai essai.cmo
3. Lancer le debogueur sur le fichier "essai":
4. Charger le "printer"
(ocd) load_printer "essai_printer.cmo"
File ./essai_printer.cmo loaded
5. Installer la fonction d'impression my_print_point
(ocd) install_printer Essai_printer.my_print_point
6. ... fixer un breakpoint juste apres la definition de my_p

7. imprimer my_p
(ocd) p my_p
my_p : point = <cannot fetch remote object>

------------------------------------------------------------------

Et la question:

Quelle est la solution pour imprimer la valeur x de my_p ?

Merci d'avance,

Sorin

-- 
e-mail : Sorin.Stratulat@loria.fr
www    : http://www.loria.fr/~stratula
Project: PROTHEO
Office : A201                             | Home:
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Re: <cannot fetch remote object> - WHY ?

[Xavier Leroy]

> Unfortunately, I found
> difficulties to convince the debugger to print objects.

> 5. install the printer function my_print_point
> (ocd) install_printer Essai_printer.my_print_point

> 7. print my_p
> (ocd) p my_p
> my_p : point = <cannot fetch remote object>

User-defined printing functions run in the memory space of the
debugger, which is distinct from that of the debugged program.
Structued I/O (output_value/input_value) is used to communicate the
value being printed between the two.  Unfotunately, structured I/O is
currently unable to deal with objects and functions, hence the error
you've observed.

> Can somebody tell me the "solution" to print the value "x" of my_p ?

The only solution I can see right now is put a breakpoint in one of
the methods of my_p's class; then, "x" is in scope and "print x"
should show its value.

- Xavier Leroy