Steve Grazzini wrote:
> On Mon, Jul 21, 2003 at 02:18:48PM -0400, Bob Showalter wrote:
> > Steve Grazzini wrote:
> > > On Mon, Jul 21, 2003 at 10:34:59AM -0400, Jeff 'japhy'
> Pinyan wrote:
> > > > The documentation states that the $DIGIT variables are set after
> > > > each SUCCESSFUL pattern match.
> > >
> > > The docs also claim that the digit variables will be "dynamically
> > > scoped to the current BLOCK", which isn't quite accurate, and is
> > > presumably what confused Ed.
> >
> > The docs are correct. $1 is dynamically scoped to the block. Ed's
> > code didn't leave the block, so this dynamic scoping didn't apply.
>
> [ snip well-behaved example ]
>
> > Thus, $1 is saved before entry to the block and restored afterward.
>
> You can't prove that there isn't a bug by showing well-behaved
> examples! :-)
>
> First: the digit variables are supposed to be dynamically scoped,
> but really they're not scoped at all. Compare these two examples,
> which, according to perlvar, ought to be equivalent.
Where in the world does perldoc perlvar say these should be equivalent? I
disagree that these are or should be equivalent.
The dynamic scoping behavior of the $1..n variables works the same way that
$_ and foreach iterator variables works.
>
> # local()ized variable -----------------
>
> our $x = 0;
>
> sub F {
> local $x = $x + 1;
> print $x;
> F() if $x<2;
> print $x;
> }
>
> F(); # prints "1221"
>
>
> # digit variable ------------------------
>
> "0" =~ /(\d)/;
>
> sub F {
> ($1 + 1) =~ /(\d)/;
> print $1;
> F() if $1<2;
> print $1;
> }
>
> F(); # prints "1222"
>
> Now, you can almost explain this by saying that Perl's notion of the
> "last match" is dynamically scoped, and that the digit variables just
> refer to captured substrings from the last match.
>
> [ Which leaves out a crucial part of the explanation, but at least
> it's almost correct. ]
>
> But then there's a second problem: the notional "last match" isn't
> scoped to the body of the loop either.
>
> #-- local()ized variable ------------------
>
> our $x = 1;
> print $x;
>
> for (1..3) {
> local $x = $x + 1;
> print $x;
> }
>
> print $x; # output is "12221"
>
> #-- digit variable ------------------------
>
> "1" =~ /(\d)/;
> print $1;
>
> for (1..3) {
> ($1 + 1) =~ /(\d)/;
> print $1;
> }
>
> print $1; # output is "12341"
>
> The actual behavior is more like what you said: the "last match"
> is saved before loop entry and restored loop exit. But that's a bit
> of a cheat, since everything else that we call "dynamically scoped"
> would get saved and restored with each iteration.
Uh-uh. $_ and foreach iterators don't behave that way, nor should they. Look
at what $_ is doing in that example. $1 is doing the same thing.
>
> --
> Steve
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