small code problem

From: Markus Mottl (knotwell@f5.com)
Date: Sat Jul 03 1999 - 00:30:34 MET DST

From: "<Brad Knotwell" <knotwell@f5.com>
Date: Fri, 2 Jul 1999 15:30:34 -0700 (PDT)
To: caml-list@inria.fr
Subject: small code problem

Hello all--

I'm having a problem with a core dump due to an uncaught exception.
The following snippet of code best illustrates the problem:

let usage = lazy (Printf.printf "Usage: %s file\n" Sys.argv.(0); exit ~-1);;
let filename =
        try
           Sys.argv.(1)
        with Invalid_argument("Array.get") ->
           Lazy.force usage;;

When I compile using the byte-code compiler, it seems to work fine.

         [knotwell@knotwell stock]$ ocamlc junk.ml
         [knotwell@knotwell stock]$ ./a.out
         Usage: ./a.out file
         [knotwell@knotwell stock]$

On the other hand, when I compile with the optimizing compiler, I
receive the following:

        [knotwell@knotwell stock]$ ocamlopt junk.ml
        [knotwell@knotwell stock]$ ./a.out
        Segmentation fault (core dumped)
        [knotwell@knotwell stock]$
When I change the code slightly:

let usage = lazy (Printf.printf "Usage: %s file\n" Sys.argv.(0); exit ~-1);;
let filename =
        try
           Sys.argv.(1)
        with _ -> (* note the pattern matching change *)
           Lazy.force usage;;

I then get correct results:

       [knotwell@knotwell stock]$ ocamlopt junk.ml
       [knotwell@knotwell stock]$ a.out
       Usage: a.out file
       [knotwell@knotwell stock]$

Why do the two compilers exhibit different behaviors with the same code?

As an aside, I don't particular like my use of lazy and force. Is
there a cleaner way do the same thing?

Thanks.

--Brad

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