A quick summary for those like me that didn't follow the change and were baffled to find out that "it's not a bug, it's a feature".
The change was asked for by Alain Frisch in 2006 (
http://caml.inria.fr/mantis/view.php?id=4052 ) and finally added in ocaml 3.11. The rationale is to make it easy to mechanically -- think camlp4 or another preprocessor -- generate pattern clauses to test for the head constructor of a data type, ignoring it's parameter.
Before that change, (K _) would work for all constructors K of arity greater than 1, but not for arity 0. After the change, (K _) work even for constant constructors. Generating a match clause that says "looks if it's the constructor K, I don't care about the arguments" is much easier as you don't have to carry arity information around.
The downside of this behaviour is that the universal pattern _ has an different meaning in this setting. It does not only matches any value (as the manual says :
http://caml.inria.fr/pub/docs/manual-ocaml/patterns.html ), but also "matches any number of arguments, possibly 0". The nice compositional interpretation of patterns -- K (p1, .., pN) matches a value with constructor K and whose N arguments match p1..pN -- is lost.
Note that this was already the case before the change suggested by Alain Frisch : _ would work for two-arguments constructors as well, while a named variable wouldn't -- this is well-known subtle difference between (Foo of a * b) and (Foo of (a * b)). The pattern _ ignored any non-zero number of arguments.
Note that since ocaml 3.12, there is a warning available for this very error.
$ ocaml -warn-help
[...]
28 Wildcard pattern given as argument to a constant constructor.
[...]
type ty = A | B
let test = function
| A _ -> ()
| B -> ()
File "
test.ml", line 4, characters 4-5:
Warning 28: wildcard pattern given as argument to a constant constructor
I think than, in the end, it's all a matter of compromise.
Thanks to Julia and Mehdi for casting light on the dark corners of the ocaml syntax!
PS : I haven't found that behaviour documented anywhere. Maybe it would be good to describe that special behaviour of _ on constructors in the manual?
On Fri, Nov 26, 2010 at 11:02 PM, Julia Lawall
<julia@diku.dk> wrote:
On Fri, 26 Nov 2010, Mehdi Dogguy wrote:
> On 11/26/2010 10:46 PM, Julia Lawall wrote:
> > The following code compiles in 3.12.0 but doesn't compile in 3.10.2.
> > Is it a bug or a feature?
> >
>
> It's a feature that was implemented in 3.11.0 (iirc).
>
> See:
http://caml.inria.fr/mantis/view.php?id=4675 (and other related
> bugreports).
julia