Your message dated Sun, 8 Jan 2012 19:03:23 +0000
with message-id <[email protected]>
and subject line Re: Bug#655063: syntax error not caught ($h+=9)=(3)
has caused the Debian Bug report #655063,
regarding syntax error not caught ($h+=9)=(3)
to be marked as done.
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655063: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=655063
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--- Begin Message ---
Package: perl
Version: 5.14.2-6
Severity: wishlist
The first syntax error is caught, but not the next, even with use strict.
$ perl -lwe '($h{a}+ 9,$h{f})=(3,2); print for values %h'
Can't modify addition (+) in list assignment at -e line 1, near ");"
Execution of -e aborted due to compilation errors.
$ perl -lwe '($h{a}+=9,$h{f})=(3,2); print for values %h'
3
2
$ perl -lwe 'use strict;my %h;($h{a}+=9,$h{f})=(3,2); print for values %h'
3
2
$ perl -lwe 'use strict;my $h;($h+=9,undef)=(3,2); print $h'
3
$ perl -lwe 'use strict;my $h;($h+=9)=(3); print $h'
3
If it is not a syntax error then it should be.
--- End Message ---
--- Begin Message ---
On Sun, Jan 08, 2012 at 10:14:38AM -0800, Russ Allbery wrote:
> [email protected] writes:
>
> > $ perl -lwe 'use strict;my $h;($h+=9)=(3); print $h'
> > 3
>
> > If it is not a syntax error then it should be.
>
> If Perl were a different language with different semantics, it could be a
> syntax error, but it's not. I don't see how that could be a syntax error
> without changing the way Perl works in ways that wouldn't be backward
> compatible.
>
> The expression "$h += 9" adds 9 to $h and then returns $h as an lvalue
> (meaning that it can be assigned to again). This is because "$h += 9" is
> entirely equivalent to "$h = $h + 9" and assignment returns an lvalue, as
> documented in perlop:
>
> Unlike in C, the scalar assignment operator produces a valid lvalue.
> Modifying an assignment is equivalent to doing the assignment and then
> modifying the variable that was assigned to. This is useful for
> modifying a copy of something, like this:
>
> ($tmp = $global) =~ tr [0-9] [a-j];
>
> Likewise,
>
> ($a += 2) *= 3;
>
> is equivalent to
>
> $a += 2;
> $a *= 3;
>
> The expression "($h += 9)" is therefore a list of lvalues containing one
> lvalue, and you can assign a list of values to a list of lvalues in Perl.
> (If you couldn't, the expression "my ($a, $b) = (1, 2)" wouldn't work,
> which would break a ton of Perl code.) Also, this expression is very
> similar to the "($a += 2) *= 3" expression that's explicitly documented as
> working in perlop.
>
> So while this is confusing, this is the language working as it is
> documented to work.
Hi Russ,
Thanks for taking the time to explain this. I'm now resolving this bug
report.
Cheers,
Dominic.
--
Dominic Hargreaves | http://www.larted.org.uk/~dom/
PGP key 5178E2A5 from the.earth.li (keyserver,web,email)
--- End Message ---
