Report a missing match

Report a missing match

[Don Syme]

Hi,

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...

Thanks,
Don

Re: Report a missing match

[Jean-Yves Moyen]

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.

typically,
let f=
  function
     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:
http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#gardes_partielles

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.

Jym.

Re: Report a missing match

[Luc Maranget]

> 
> 
> Hi,
> 
> 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...
> 
> Thanks,
> Don
> 

Hi,

Conceptually, this is not that difficult to do, since the compiler look for
non-exhautive matches by enumerating possible non-matching values.

In practice, I have to see a little and have no time for that now...

I put this on a personnal TODO list.


--Luc Maranget