develooper Front page | perl.perl5.porters | Postings from August 2012

Re: fixing smartmatch just hard enough (and when, too)

Thread Previous | Thread Next
Leon Timmermans
August 23, 2012 12:50
Re: fixing smartmatch just hard enough (and when, too)
Message ID:
On Thu, Aug 23, 2012 at 1:48 AM, Damian Conway <> wrote:
>> So maybe what we really want is to move that case down below the "Any ->
>> Code" case.
> I don't think so. If overloading is tested first, then the overloading
> itself can implement
> a fallback to the standard Any ~~ CodeRef case.
> If Any ~~ CodeRef is tested first, then new classes have no way to
> intercept and
> control that behaviour sensibly for their objects.
> In fact, I would argue that the overloading case should come *first*
> in the table,
> even before the Any ~~ undef case, so that classes overloading smartmatch can
> intercept and handle that case too.

I suspect in practice junctions may be the only use-case for it. I
can't think of any other use-case. I don't think it's a good idea to
special case, TBH.

> So what if I want *correct* junctive behaviour in that case: namely that
> each element of the junction is distributed over a separate call to the sub
> and the results are rejunctified.

Why do you expect that to work with smart-match when it doesn't with
subroutine calls, or a dozen other things?

> You're proposing that:
>     any(9,10,11) ~~ \&is_prime
> be equivalent to:
>    is_prime(any(9,10,11))
> but what if I want it to mean what it's supposed to mean:
>    any(is_prime(9), is_prime(10), is_prime(11))

If «is_prime(any(9,10,11))» is not equivalent to «any(is_prime(9),
is_prime(10), is_prime(11))», it's not a junction in the first place.
It's just an abstraction that fakes it half of the time and leaks like
a sieve the other half. Supporting them in a few circumstances and not
in others seems silly and wrong to me. IMO doing them half-arsed is
worse than not doing them at all.

> If overload testing comes first, I can implement the Junction class's
> overloading to do that. If CodeRef testing comes first, I'm screwed.

No you're not. If you solve the problem at the other end, it's
possible to write a solution that would actually work. That will
require an approach that is much less superficial than what you're
approaching here though, sadly.


Thread Previous | Thread Next Perl Programming lists via nntp and http.
Comments to Ask Bjørn Hansen at | Group listing | About