Re^2: If you believe in Lists in Scalar Context, Clap your Hands

[gone2015]

OK. Lists cannot be lvalues, though you may have a list of lvalues...

Suppose I refine the points paying question: as an rvalue how does one describe the difference between a list and an array ?

[ikegami]

Lists cannot be lvalues, though you may have a list of lvalues...

Say again? An lvalue list returns a list of lvalues and enforces list context, just like an rvalue list returns a list of rvalues in list context.

In both of the following snippets, you'll see an list as an lvalue ("M" flag).

>perl -MO=Concise -e"($a,$b)=(4,5)"
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
9     <2> aassign[t3] vKS ->a
-        <1> ex-list lKP ->6           <-- RHS list
3           <0> pushmark s ->4
4           <$> const[IV 4] s ->5
5           <$> const[IV 5] s ->6
-        <1> ex-list lKPRM* ->9        <-- LHS list w/ "M"
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

>perl -MO=Concise -e"1 for ($a,$b)"
d  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
3     <;> nextstate(main 1 -e:1) v ->4
c     <2> leaveloop vK/2 ->d
8        <{> enteriter(next->9 last->c redo->9) lK/8 ->a
-           <0> ex-pushmark s ->4
-           <1> ex-list lKPM ->7       <-- lvalue list
4              <0> pushmark sM ->5
-              <1> ex-rv2sv sKM/1 ->6
5                 <#> gvsv[*a] s ->6
-              <1> ex-rv2sv sKM/1 ->-
6                 <#> gvsv[*b] s ->7
7           <#> gv[*_] s ->8
-        <1> null vK/1 ->c
b           <|> and(other->9) vK/1 ->c
a              <0> iter s ->b
-              <@> lineseq vK ->-
-                 <0> ex-const v ->9
9                 <0> unstack v ->a
-e syntax OK

[moritz]

An array is everything that could be used as an lvalue (perhaps with the exception of :lvalue subs, haven't tried those), independently of you actually use it as an lvalue.

That definition so far helped in all real-world cases that I've look at.

[ikegami]

perhaps with the exception of :lvalue subs, haven't tried those

lvalue subs are called in scalar context, so they can't return a list.

use strict;
use warnings;
my ($i,$j);
sub f :lvalue { ($i,$j) }
f() = (4,5);
print("$i,$j\n");

Useless use of a constant in void context at script.pl line 5.
Use of uninitialized value in concatenation (.) or string at script.pl
+ line 6.
,5

[ysth]

Wrong. Don't forget that plain = is the scalar assignment operator, unless there is a hash or array on the left. ()= is the list assignment operator:

$ perl
use strict;
use warnings;
my ($i,$j);
sub f :lvalue { ($i,$j) }
(f()) = (4,5);
print("$i,$j\n");
__END__
4,5

However, I would be very leery of using a relatively new feature like that that has probably not been used much in that way.