develooper Front page | perl.perl6.language | Postings from February 2002

Re: proposal: when-blocks, and binding $_

Thread Previous | Thread Next
Allison Randal
February 27, 2002 11:28
Re: proposal: when-blocks, and binding $_
Message ID:
On Wed, Feb 27, 2002 at 10:11:13AM -0800, Austin Hastings wrote:
> > C<when> is a conditional like C<if>, not a topicalizer.
> Right, it's a topicalizee, the victim of topicalization. And so it uses
> $_ or $x or $! or whatever the current topic is.

i.e. a "defaulting construct" or "topic sensitive keyword". I like the
latter, in this case, because it expresses what C<when> defaults to.

> But inside, you have to KNOW what the topic is again, you can't just
> use the default variable(s), so that differentiation of
> for @logged_exceptions -> $e {
>   when .survivable { if $e.isa('Exception::PEBKAC') { ... } }
> }
> versus
>   when .survivable { if $!.isa('Exception::PEBKAC') { ... } }
> }
> becomes necessary. 
> What I'm trying to suggest is that C<when>, because it already is doing
> some special things (huge list of conditionals to try, implied 'next'),
> is an orthogonal code element. It really IS going sideways, and unless
> you are coding sideways (using C<given> or C<CATCH>) the when-block
> represents a context-switch at the developer level.
> To that end C<when> should, when (urg!) used in a loop or other
> straight-ahead construct to check the current topic, force the current
> topic to become the default.

I absolutely agree with the problem you're trying to solve. The "when
..survivable" vs. "if $e.isa..." distinction bothers me as well. But I'm
greedy. I don't want to treat the symptom by adding to the list of ways
that C<when> is weird. I want a systematic solution for the whole
language. To me that means taking a step back from the familiar, but
limiting, concept of "$_ is default". Exactly how that will work out,
defaulting to topic everywhere, flagging the variable with an "assumed"
property, or something else entirely, I'm not sure. But however it's
done, I want it to be consistent in the bigger picture.


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