Surely it's preferable to use a syntactically distinct mechanism for this subtly different concept. Given that we're talking about patterns and not general expressions here, surely there's plenty of space in the syntax. Perhaps something like '*' to mean 0 or more. Or is it already too late because '_' has already been incorporated and backwards compatibility dictates that this cannot be changed?
type ty = A | B let test = function | A * -> () | B -> () Mark Adams on 26/11/10 10:35 PM, bluestorm <bluestorm.d...@gmail.com> wrote: > 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. > [...] > > $ cat test.ml > type ty = A | B > > let test = function > | A _ -> () > | B -> () > > $ ocaml -w +28 test.ml > 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 <ju...@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). >> >> OK, thanks. I agree wth those that don't like the change... >> >> julia >> >> _______________________________________________ >> Caml-list mailing list. Subscription management: >> http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list >> Archives: http://caml.inria.fr >> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners >> Bug reports: http://caml.inria.fr/bin/caml-bugs >> > > > > ---------------------------------------- > _______________________________________________ > Caml-list mailing list. Subscription management: > http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list > Archives: http://caml.inria.fr > Beginner's list: http://groups.yahoo.com/group/ocaml_beginners > Bug reports: http://caml.inria.fr/bin/caml-bugs > > > _______________________________________________ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
