Re^5: Accessing Arguments inside Subroutines via @

Hello LanX,

Thanks for that, it’s good to know. Perl continues to surprise! I looked in the Camel Book, but the section on “List Assignment”¹ doesn’t cover this behaviour. Do you know where it’s documented?

I’m intrigued to know how Perl accomplishes this. I wrote the following, but it left me none the wiser:

#! perl
use strict;
use warnings;
use feature qw(say state);

#say 'f = ', f(), ', g = ', g();

( f(), g() ) = ( g(), f() );

#say 'f = ', f(), ', g = ', g();

sub f : lvalue
{
    state  $f = 'first';
    say 'f(', $f, ')';
    return $f;
}

sub g : lvalue
{
    state  $g = 'second';
    say 'g(', $g, ')';
    return $g;
}
[download]

Output:

12:58 >perl 1193_SoPW.pl
g(second)
f(first)
f(first)
g(second)

12:58 >
[download]

It looks as though the virtual machine:

accesses the right-hand variables
accesses the left-hand variables
performs the assignments

So it must create temporary copies of the right-hand variables “under the hood”?

¹Chapter 2, pages 82–83 in the 4^th Edition.

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^6: Accessing Arguments inside Subroutines via @_
by LanX (Saint) on Mar 21, 2015 at 12:09 UTC

Athanasius

No idea about the camel book, I have to admit I never read it.

my understanding is that the values from RHS are temporarily pushed on a stack like structure and from there assigned to the LHS.

B::Concise shows

~$ perl -MO=Concise -e '($a,$b)=($b,$a)'
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v:{ ->3
9     <2> aassign[t5] vKS/COMMON ->a
-        <1> ex-list lKP ->6
3           <0> pushmark s ->4
-           <1> ex-rv2sv sK/1 ->5
4              <#> gvsv[*b] s ->5
-           <1> ex-rv2sv sK/1 ->-
5              <#> gvsv[*a] s ->6
-        <1> ex-list lKPRM* ->9
6           <0> pushmark sRM* ->7
-           <1> ex-rv2sv sKRM*/1 ->8
7              <#> gvsv[*a] s ->8
-           <1> ex-rv2sv sKRM*/1 ->-
8              <#> gvsv[*b] s ->9
-e syntax OK
[download]

so you might wanna look into ex-list (where both sides are stored) and aassign ("array-assign" which is a misnomer).

Unfortunately I couldn't find a good compilation explaining all these ops.

From the Llama (Learning Perl, 3rd Edition)