Re: [Caml-list] zero-arity constructor

[<mark@proof-technologies.com>]

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.dylc@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 <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).
>>
>> OK, thanks.  I agree wth those that don't like the change...
>>
>> julia
>>
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>
>
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