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Where did the exception occur?
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Date: 2000-11-02 (18:28)
From: Pierre Weis <Pierre.Weis@i...>
Subject: Re: Where did the exception occur?
> Maybe a little example in the Ref. Manual or the FAQ would be useful, as
> I'm not the first one to be caught by this problem?

Let me propose this little tutorial for the debugger...

Launching the debugger:

Given the following obviously wrong program written in file
uncaught.ml, (the program raises an uncaught exception Not_found, when
evaluating the expression find_address "INRIA", since "IRIA" (and not
"INRIA") has been added to the list of adresses):

(* file uncaught.ml *)

let l = ref [];;

let find_address name = List.assoc name !l;;

let add_address name adress = l := (name, address) :: ! l;;

add_address "IRIA" "Rocquencourt";;

print_string (find_address "INRIA"); print_newline ();;

If you want to find where and why this exception has been raised, you
have to:

1) compile the program in debug mode:

ocamlc -g uncaught.ml

2) launch the debugger:

ocamldebug a.out

Then the debugger writes a banner and a prompt:

        Objective Caml Debugger version 3.00+14 (2000-09-06)


Finding the cause of a spurious exception:
Type r (for run); you get

(ocd) r
Loading program... done.
Time : 12
Program end.
Uncaught exception: Not_found

Self explanatory, is'nt it ?

So, you want to step backward to set the program counter before the
time the exception is raised; hence type in b as backstep, and you get

(ocd) b
Time : 11 - pc : 15500 - module List
143     [] -> <|b|>raise Not_found

The debugger tells you that you are in module List, inside a pattern
matching on a list that allready chose the [] case and is about to
execute raise Not_found, since the program is stopped just before this
expression (as witnessed by the <|b|> mark).

But, as you know, you want the debugger to tell you which procedure
calls the one from list, and may who calls the procedure that call the
one from list; hence, you want a backtrace of the execution stack:

(ocd) bt
#0  Pc : 15500  List char 3562
#1  Pc : 19128  Uncaught char 221

So the last function called is from module List at charadcter 3562,
that is :

let rec assoc x = function
    [] -> raise Not_found
  | (a,b)::l -> if a = x then b else assoc x l

The function that calls it is in module Uncaught, file uncaught.ml char 221:
print_string (find_address "INRIA"); print_newline ();;
What else ?

If you're developping a program you can compile it with the -g option,
to be ready to debug it if necessary.  Hence, to find your spurious
exception you just need to type ocamldebug a.out, then r, b, and bt
gives you the backtrace.

Help and info in the debugger:

To get more info about the current status of the debugger you can ask
it directly at the toplevel prompt of the debugger; for instance:

(ocd) info breakpoints
No breakpoint.

(ocd) help break
  1      15396  in List, character 3539
break : Set breakpoint at specified line or function.
Syntax: break function-name
break @ [module] linenum
break @ [module] # characternum

Setting break points:
Let's set up a breakpoint and rerun the entire program ((g)oto 0 then (r)un):

(ocd) break @Uncaught 9
Breakpoint 3 at 19112 : file Uncaught, line 9 column 34

(ocd) g 0
Time : 0
Beginning of program.

(ocd) r
Time : 6 - pc : 19112 - module Uncaught
Breakpoint : 1
9 add "IRIA" "Rocquencourt"<|a|>;;

Then, we can step and find what happens when find_address is about to
be called
(ocd) s
Time : 7 - pc : 19012 - module Uncaught
5 let find_address name = <|b|>List.assoc name !l;;

(ocd) p name
name : string = "INRIA"

(ocd) p !l
$1 : (string * string) list = ["IRIA", "Rocquencourt"]

Now we can guess that List.assoc will fail to find "INRIA" in the list...

Debugging under Emacs:
Note also that under Emacs you call the debugger using ESC-x camldebug
a.out. Then emacs will set you directly to the file and character
reported by the debugger, and you can step back and forth using ESC-b
and ESC-s, you can set up break points using CTRL-X space, and so on...

Hope this helps,

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://cristal.inria.fr/~weis/