Re: [Caml-list] ocaml 4.03 and warning 52 : argument of this constructor should not be matched against a constant pattern

[Gabriel Scherer <gabriel.scherer@gmail.com>]

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If a single call to the binding code can return several of Failure
payloads, then yes, I guess you cannot escape using specific text on the
warning text. You can either disable the warning locally using a @warning
attribute (if your backward-compatibility requirement do not go back before
ppx attributes), or move the string-testing logic to inside the
pattern-matching clause (a guard would also work). Note that the
string-testing solution *can* be made robust if the library/binding exports
identifiers containing the value of the error messages, and you test
against them.

Note that since Benoît Vaugon's optimization work in 4.02.0
pattern-matching on several literal strings is sensibly faster than a
series of string-equality test, so in performance-critical code it makes
sense to use pattern-matching on string -- but it does not need to be part
of the try-handler patterns directly, it can be only in the (Failure err ->
...) clause.

Thanks for your comments (and William's) on the documentation. Feel free to
add additional comments on the related ticket William created, PR#7245
  http://caml.inria.fr/mantis/view.php?id=7245

On Wed, Apr 27, 2016 at 1:17 PM, Adrien Nader <adrien@notk.org> wrote:

> On Wed, Apr 27, 2016, Gabriel Scherer wrote:
> > We now have a section of the reference manual (which has been updated to
> > 4.03) on Warnings, and this particular warning is documented there:
> >   http://caml.inria.fr/pub/docs/manual-ocaml/comp.html#s:comp-warnings
> >
> > Feedback (for example as Mantis tickets) on which warnings are confusing
> > and would deserve additional documentation -- or even patches to provide
> > this documentation -- are warmly welcome.
>
> Thanks for the documentation update. I've found the warning fairly
> confusing and this makes it clear.
>
> As a small note, in a library I use (I won't name it because I haven't
> checked if there were a version more recent than a couple years), C
> bindings use the built-in Failure exception. I guess this is because it
> is so much more convenient when it comes to making C bindings.
> Am I right that such scenario will always trigger this warning because
> the warning attribute will never be removed from the OCaml upstream
> definition of Failure?
>
> PS: my only comment about the warning is maybe to state that all(?)
> exceptions from the stdlib have the corresponding attribute set.
>
> --
> Adrien Nader
>

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