Re: Report a missing match

From: Jean-Yves Moyen (
Date: Fri Sep 10 1999 - 00:36:36 MET DST

Date: Fri, 10 Sep 1999 00:36:36 +0200 (MET DST)
From: Jean-Yves Moyen <>
To: Don Syme <>
Subject: Re: Report a missing match
In-Reply-To: <39ADCF833E74D111A2D700805F1951EF1801418B@RED-MSG-06>

On Thu, 9 Sep 1999, Don Syme wrote:

> It would be very helpful for me if the Caml compiler could report at least
> one missing match case when it says a match is non-exhaustive. Is this
> feasible to implement fairly painlessly? When working with very large
> datatypes, e.g. 100 constructors, it's very hard to know which case has been
> missed...

I think this can lead to a problem when using gards ('when' cases) in the
pattern matching.

let f=
     n when n >= 0 -> true
   | n when n < 0 -> false;;

is said to have a non exhaustive matching but the pattern matching is
actually exhaustive.

This is because it is sometimes really tricky to look into when-clauses in
order to check if there are exhaustive or not.

This is a bit more explained in the FAQ:

In your case, if your not using gards, I guess a little script may be able
to find which cases are not in the pattern matching.
(Maybe this can even be done using camlp4, I don't know)

Quand on est jeune, on croit que le monde est peuple de vieux cons.
En vieillissant, on s'appercoit qu'il est peuple de jeunes imbeciles.


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