Re: What's happening in this expression?

Can you monks please explain step-by-step what computations will happen here?

Under strict, none, because it will not compile unless $x, $y, and $z have been previously declared. Otherwise, B::Deparse helps:

$ perl -MO=Deparse,-p -e 'my $a, $x, $y, $z = foo()'
(my($a), $x, $y, ($z = foo()));
[download]

In other words, it's declaring a lexical $a, then $x and $y are simply sitting there, and then $z is assigned the return value of the function call foo() - although if that's not defined, that call will fail. See also Comma Operator. Turning on warnings gives some relatively helpful hints:

$ perl -w -e 'my $a, $x, $y, $z = foo()'
Parentheses missing around "my" list at -e line 1.
Useless use of a variable in void context at -e line 1.
Useless use of a variable in void context at -e line 1.
Name "main::x" used only once: possible typo at -e line 1.
Name "main::z" used only once: possible typo at -e line 1.
Name "main::y" used only once: possible typo at -e line 1.
Undefined subroutine &main::foo called at -e line 1.
[download]

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Re^2: What's happening in this expression? by zapdos (Sexton) on Oct 11, 2020 at 09:58 UTC
Thank you so much. What are the two warnings "Useless use of a variable in void context at -e line 1." referring to?	[reply]
Re^3: What's happening in this expression? by haukex (Archbishop) on Oct 11, 2020 at 10:08 UTC
What are the two warnings "Useless use of a variable in void context at -e line 1." referring to? They're referring to `$x` and `$y`, because the statement as written above is in void context, so these variables are in void context too. I was intentionally a little vauge with my "simply sitting there" because it's possible for them to not be in void context, for example if this statement was the last thing in a `sub` (the context that the `sub` is called in is passed through): `use warnings; use strict; my ($x,$y,$z); sub foo { qw/a b c d/ } sub bar { my $a, $x, $y, $z = foo() } my @x = bar(); my $r = bar();` [download] This only produces the warning "`Parentheses missing around "my" list"`. `@x` will contain the values `(undef, undef, undef, "d")`, because `$a`, `$x`, and `$y` are `undef` (nothing is assigned to them), while `$z` is assigned the return value of `foo()`, which means it is assigned `"d"` because my example `sub foo` is returning a list, and a list in scalar context evaluates to its last value. `$r` will contain `"d"` for the same reason.	[reply] [d/l] [select]
Re^4: What's happening in this expression? by zapdos (Sexton) on Oct 11, 2020 at 14:02 UTC
Please, why the statement `my $a, $x, $y, $z = foo()` is in void context? I don't get it.	[reply] [d/l]
Re^5: What's happening in this expression? by haukex (Archbishop) on Oct 11, 2020 at 14:26 UTC
Re^3: What's happening in this expression? by perlfan (Vicar) on Oct 11, 2020 at 10:09 UTC
>then $x and $y are simply sitting there	[reply]


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