There is a function in camlp4 that checks for this:
Ast.is_irrefut_patt
Check that you have a recent ocaml. It used to be bugged (it was fixed
in mid september).

Cheers

On Wed, Feb 27, 2008 at 9:32 PM, David Teller
<David.Teller@univ-orleans.fr> wrote:
>    Dear list,
>
>   I'm currently working on a little Camlp4 extension which has to often
>  generate pattern-matching clauses depending on user code -- and deal
>  with match failures accordingly.
>
>  Now, I guess
>
>  1. I can wrap the user's pattern-matching inside a try...with, catch any
>  Match_failure and deal with it. However, ensuring that the Match_failure
>  is the right one and that I'm not catching some other error in the code,
>  all this while performing bindings satisfactorily will require numerous
>  contorsions.
>
>  2. I can add a catch-all clause " _ -> deal_with_error ". While the
>  semantics of this rewriting are exactly what I need, the compiler tends
>  to print "Warning U: this match case is unused" whenever the user has
>  already taken care of all cases. I would need to find a way to
>  deactivate the warning for this specific clause. As I haven't found any
>  way of doing that directly, I've been thinking about adding a "when
>  True" to the second-to-last clause if that clause doesn't already have a
>  "when", but I'd be glad to hear about better solutions.
>
>  Thanks in advance,
>   David
>
>  --
>  David Teller
>   Security of Distributed Systems
>   http://www.univ-orleans.fr/lifo/Members/David.Teller
>   Angry researcher: French Universities need reforms, but the LRU act brings liquidations.
>
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