Re^3: ${^POSTMATCH} problem

in reply to Re^2: ${^POSTMATCH} problem
in thread ${^POSTMATCH} problem

Yeah, The behaviour of ($foo = 1) . ($foo = 2) . ($foo = 3) used to be roughly equivalent to

my $x = "";
$x .= ( $foo = 1 );
$x .= ( $foo = 2 );
$x .= ( $foo = 3 );
[download]

But now, it's roughly equivalent to

my $x = join("",
   ( $foo = 1 ),
   ( $foo = 2 ),
   ( $foo = 3 ),
);
[download]

In both cases, $foo itself is being put on the stack. The difference is when it's used (before it has a chance to be changed by later assignments, or after).

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Re^4: ${^POSTMATCH} problem by LanX (Saint) on Jun 14, 2020 at 16:01 UTC
I'm not happy about the behaviour of join here, because there is no way how CORE::join() will ever assign to the lvalue. So it should act like handling copies. I'm aware that we can override built-ins like join, so accessing the alias should still be possible. Theoretically that is. Update On second thought, a general new `Warning` in case of side effects of altered repeated variables as arguments may be more effective. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply]
Re^5: ${^POSTMATCH} problem by ikegami (Patriarch) on Jun 15, 2020 at 04:22 UTC
Short of completely rewriting Perl, it would have to make an actual copy to act like it was handling copies.	[reply]
Re^6: ${^POSTMATCH} problem by ikegami (Patriarch) on Jun 15, 2020 at 04:26 UTC
No, it could also rewrite the call to `join` in terms of pre-multiconcat concatenation. I thought at first this would cause problems for magical vars, but that's not the case. But the point stands. To change this would impose a performance hit. Rather than slowing down `join` to provide more consistent behaviour, they've opted to speed up concatenation while providing less consistent behaviour. And I agree with this. In general, a programmer should never read and change a variable in the same statement. It introduces readability issues if nothing else, but it's common for it to invoke undefined behaviour.	[reply] [d/l] [select]

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