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Postings from October 2016
* Dave Mitchell <davem@iabyn.com> [2016-10-19 10:24]:
> There's one further issue I've noticed: undef on the LHS:
>
> sub inc { $_++ for @_ }
>
> inc($a, undef ); # "Modification of a read-only value attempted"
> inc(($a, undef) = ($x, $y)); # increments $a and $y
I think that’s correct.
* David Nicol <davidnicol@gmail.com> [2016-10-19 11:00]:
> Undef appears to be special in that it gets replaced with an alias to
> the RHS element instead of throwing a compile-time error.
>
> $ perl -le '$x=$y=1; ($a,undef) = ($x,$y) ; print "a:$a x:$x y:$y"'
> a:1 x:1 y:1
> $ perl -le '$x=$y=1; ($a,2) = ($x,$y) ; print "a:$a x:$x y:$y"'
> Can't modify constant item in list assignment at -e line 1, near ") ;"
> Execution of -e aborted due to compilation errors.
Correct that it’s special, but wrong guess as to why.
sub x { join ":", \(@_) }
my $lhs = x my($l1,$l2);
my $rhs = x my($r2,$r2);
my $ret = x(($l1,$l2)=($r1,$r2));
say "lhs" if $lhs eq $ret;
say "rhs" if $rhs eq $ret'
This says “lhs”. It does not say “rhs” if you change the LHS of the
assignment to (undef,undef) – which would be what it should say if such
an assignment to undef were returning an alias to the RHS element.
Therefore,
* Dave Mitchell <davem@iabyn.com> [2016-10-19 10:24]:
> I think that second call to inc should pass ($a, undef) rather than
> ($a, $y) to inc, and should also die with "Modification of a read-only
> value attempted".
Apparently a literal `undef` on the LHS is special-cased to autovivify
an anonymous scalar.
Which is a scalar that gets assigned to, which is what list assignment
should return – so the current behaviour is correct.
Oddly, that seems to mean (@a,undef)=1 must return ($a[0],do{my$anon}).
Regards,
--
Aristotle Pagaltzis // <http://plasmasturm.org/>
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