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Postings from October 2015
On Wed, 7 Oct 2015 09:24:45 +0100
Dave Mitchell <davem@iabyn.com> wrote:
> 3. when { FOO } has condition FOO.
>
> For example:
>
> use constant ANS => 42;
> use constant ME => "davem";
>
> when (undef) # equivalent to when { !defined($_) }
> when (42) # equivalent to when { $_ == 42 }
> when (ANS) # equivalent to when { $_ == 42 }
> when (42.1) # equivalent to when { $_ == 42.1 }
> when ("42") # equivalent to when { $_ eq "42" }
> when ("davem") # equivalent to when { $_ eq "davem" }
> when (ME) # equivalent to when { $_ eq "davem" }
> when ($x) # equivalent to when { $_ eq $x }
Some trickier cornercases
3a) dualvar constants:
use Scalar::Util qw( dualvar );
use constant THREE => dualvar(3, "3");
when(THREE) # eq or == ?
3b) 'bigint' and friends:
use bigint;
when(5) # 5 is now a Math::BigInt instance surely?
> Rationale:
>
> I think 95% of what people actually *want* a smartmatch system for is
> so that they can write code of one of these three forms:
<snip>
Agree.
> Since perl is about making easy things easy, I think that achieving
> the above forms should be our basic goal. If we don't do that, we've
> failed. Of course perl likes to make hard things possible too, but I
> think that is an auxiliary consideration for now.
Agree.
> Why get rid of the smartmatch operator? I've never really seen the
> point of it, apart from being something that exposes the underlying
> mechanism of when(), c.f. readline() for <>. If we do keep it, then
> it should have exactly the same semantics as I've described above for
> when(). Then the condition for when(FOO) would be defined as always
> exactly equivalent to when { $_ ~~ expr }.
Agree doubly-so. I think a lot of people want a simple
dispatch-on-constantish things switch-alike, and the idea of the very
generic smartmatch came along accidentally from Perl 6. We don't
strictly need it to get a switch-alike out of this given/when syntax,
so it makes a lot of sense to consider it a separate operator, and work
on given/when as their own standalone thing.
> Finally, if the smartmatch operator were freed up, then maybe we could
> repurpose it as some sort of recursive grep operator that searches
> though nested arrays and hashes for something. That's just a vague
> thought though.
If we wanted a fancier recursive grep, surely that's more a candidate
for some kind of function in a module of somesort (List::Util?) rather
than really basic syntax using up an infix operator. Much easier to
backport, too.
> Finally finally, regardless of what semantics we finally choose,
> perhaps we should use a new feature name for all this, e.g.
>
> use feature 'switch2';
>
> with 'switch' becoming an error, so that existing code will noisily
> fail to compile rather than failing mysteriously on subtle changes to
> semantics.
A new name could be good, yes. I'm not sure I like the specific
"switch2" but now we're really bikeshedding...
Maybe new keywords while we're at it?
use feature 'dispatch';
dispatch($str) {
on("hello") { say "Hello, there"; }
on("bye") { say "Goodbye then"; }
}
--
Paul "LeoNerd" Evans
leonerd@leonerd.org.uk
http://www.leonerd.org.uk/ | https://metacpan.org/author/PEVANS
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