[perl #134138] Eliminate modifiable variables in constants

On Sat, 25 May 2019 19:30:56 -0700, jkeenan wrote:
> On Sat, 25 May 2019 02:03:40 GMT, jkeenan@pobox.com wrote:
> > pod/perldeprecation.pod contains the following entry:
> > 
> > #####
> > Constants from lexical variables potentially modified
> > elsewhere
> > 
> > You wrote something like
> > 
> > my $var; $sub = sub () { $var };
> > 
> > but $var is referenced elsewhere and could be modified
> > after the "sub" expression is evaluated. Either it is
> > explicitly modified elsewhere ("$var = 3") or it is passed
> > to a subroutine or to an operator like "printf" or "map",
> > which may or may not modify the variable.
> > 
> > Traditionally, Perl has captured the value of the variable
> > at that point and turned the subroutine into a constant
> > eligible for inlining. In those cases where the variable
> > can be modified elsewhere, this breaks the behavior of
> > closures, in which the subroutine captures the variable
> > itself, rather than its value, so future changes to the
> > variable are reflected in the subroutine's return value.
> > 
> > If you intended for the subroutine to be eligible for
> > inlining, then make sure the variable is not referenced
> > elsewhere, possibly by copying it:
> > 
> > my $var2 = $var; $sub = sub () { $var2 };
> > 
> > If you do want this subroutine to be a closure that
> > reflects future changes to the variable that it closes
> > over, add an explicit "return":
> > 
> > my $var; $sub = sub () { return $var };
> > 
> > This usage has been deprecated, and will no longer be
> > allowed in Perl 5.32.
> > #####
> > 
> > The entry was made in this commit:
> > 
> > #####
> > commit 9840d1d66ee1648f6d7fb1554101450158cfee16
> > Author: Abigail <abigail@abigail.be>
> > Date:   Sat Jan 14 18:25:48 2017 +0100
> > 
> > Deprecating the modifyable variables in constants by 5.32.
> > 
> > It was already deprecated, but we're now adding a version number.
> > #####
> > 
> > Make it so.
> 
> Please review patch attached, which is smoking in this branch:
> 
> smoke-me/jkeenan/rt-134138-constants-in-variable
> 
> Thank you very much.

I'm not sure this is the desired behaviour, though maybe we do want it fatal for a release or two to ensure any code that depends on the old behaviour is updated.

As to your patch, you've left code in after the croak() that will no longer be executed (and since copied can no longer be true, some other code can be removed.)

So there's two issues:

a) should sub () { $x } return the current value of $x or croak at compile-time

b) if we croak at compile-time, the patch needs work

Tony

---
via perlbug:  queue: perl5 status: open
https://rt.perl.org/Ticket/Display.html?id=134138

• [perl #134138] Eliminate modifiable variables in constants by Tony Cook via RT
• [perl #134138] Eliminate modifiable variables in constants by James E Keenan via RT
• [perl #134138] Eliminate modifiable variables in constants by Tony Cook via RT