in reply to Re^2: my (0?$a:$b): a koan
in thread my (0?$a:$b): a koan

> or it gets defeated by constant folding

thats the point, compare the optree of my (0?$a:$b)="X" with my ($b)="X".

Deparse has no chance to see any difference 

lanx:~$ perl -MO=Concise -e 'my (0?$a:$b)="X";print $a' 
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
7     <2> aassign[t3] vKS ->8
-        <1> ex-list lK ->5
3           <0> pushmark s ->4
4           <$> const[PV "X"] s ->5
-        <1> ex-list lK ->7
5           <0> pushmark s ->6
6           <0> padsv[$b:1,2] lPRM*/LVINTRO ->7
8     <;> nextstate(main 2 -e:1) v:{ ->9
b     <@> print vK ->c
9        <0> pushmark s ->a
a        <0> padsv[$a:1,2] l ->b
-e syntax OK

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lanx:~$ perl -MO=Concise -e 'my ($b)="X";print $a' 
c  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
7     <2> aassign[t2] vKS ->8
-        <1> ex-list lK ->5
3           <0> pushmark s ->4
4           <$> const[PV "X"] s ->5
-        <1> ex-list lK ->7
5           <0> pushmark s ->6
6           <0> padsv[$b:1,2] lPRM*/LVINTRO ->7
8     <;> nextstate(main 2 -e:1) v:{ ->9
b     <@> print vK ->c
9        <0> pushmark s ->a
-        <1> ex-rv2sv sK/1 ->b
a           <#> gvsv[*a] s ->b
-e syntax OK

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Note line a: In the first example $a is coded as entry "$a" from a lexical pad and in the second it's the scalar value of glob *a from a symbol table.

IMHO you found a case where the compiler should better warn about undefined behavior.

Cheers Rolf

UPDATE: we had a discussion about this just few weeks ago, but I can't find it. Take this instead: 'my' with 'if 0' retains old value

UPDATE: And here the warning I'm missing:

perl -wce 'my $a if 0; print $a' 
Deprecated use of my() in false conditional at -e line 1.
-e syntax OK

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UPDATE: Found it! see A curious case of of my() and Re^4: A curious case of of my() (What is GLOBAL?)

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Re^4: my (0?$a:$b): a koan
by educated_foo (Vicar) on May 04, 2011 at 21:13 UTC
    Nice -- it's a kind of clever inside-out way to say 
    my $a if 0;
my $b = 3;

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    EDIT: As seen with this: 
    $a=1;
$b=2;
sub x {
    my (0?$a:$b) = shift;
    print"a=$a\nb=$b\n";
    $a=shift
}
x(3,30);
x(4,40);
__END__
# OUTPUT:
a=
b=3
a=30
b=4

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      Ok, now that just confuses the fuck out me.

      Obviously this is a convoluted use of my and so I shouldn't worry about it too much. But these results don't match the explanation you gave earlier. If the theory is that both $a and $b get assigned a lexical at the beginning of the sub, than $a should be undef every sub call.

      $a=1;
$b=2;
sub x {
    my (0?$a:$b) = shift;
    print"a=$a\nb=$b\n";
    $a=shift
}
x(3,30);
x(4,40);
x(5,50);
__END__
#Output
a=
b=3
a=30
b=4
a=40
b=5

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In Section Meditations