On Mon, 2002-09-09 at 15:12, Luke Palmer wrote:
> > Going back to patterns, this gives us an added bonus. It not only
> > explains the behavior of hypotheticals, but also of subexpression
> > placeholders, which are created when the pattern returns:
[...]
> > I think this is a very clean and simple way to get everything that
> > patterns were supposed to do plus a lot of added benefit for things
> > like:
> This does bring up an interesting point. I think your solution is an
> interesting idea, but not really necessary. But consider this:
Before we consider your concern (which I will address below), why is
this "not really necessary"? As I see it, here are the needs addressed:
1. Creating lexically scoped variables in caller's namespace
2. Protecting existing lexicals from unexpected side-effects.
One of those is mandated by A5, and one of those is, IMHO, requisite for
maintainable programming.
We also achieve the following:
1. Limiting TCL-upvar-like manipulation of caller's stack
2. Allowing "pass-through" subroutines
This last bit is kind of crucial, IMHO. It makes a lot of sense to me
that this would work:
sub match_digits($str //= $_) { /(\d+)/ }
if match_digits { print $1 }
And if C<$0> contains a property that causes the lexicals to be created
upon return, then it would (because match_digits just returns C<$0> to
the caller).
> my $date;
> # lots of code
> sub foo {
> #lots more code
> sub bar {
> #lots more code
> m/ $date := <date> /;
> }
> }
If you used my suggestion, this would produce a warning or error
depending on strictness. That would have to be "my volatile $date" to
allow a thrown lexical to stomp it, in which case
> Oh. Duh. Why don't we have such a mechanism for matches?
My question exactly.
There is more that you can do once you can throw lexicals. For example,
you could provide a property for subroutines and rules which asserts the
lexicals which it can throw:
rule date is declaring($date) { # or is that declaring('date')?
$date := (<parse_date>)
}
# or
sub stat($filename//=$_) is declaring($mtime, $ctime, ...) {
# ...
return %statstruct but lexicals(%statstruct);
}
Now, the compiler can generate stomping warnings at compile-time instead
of just at run time.
--
Aaron Sherman <[EMAIL PROTECTED]>
http://www.ajs.com/~ajs
