Perl-列表元素的评估顺序，在列表内赋值
根据 Perldoc（https://perldoc.perl.org/perlop#Comma-Operator）：
逗号运算符
...
在列表上下文中，它只是列表参数分隔符，并将其两个参数插入到列表中。这些参数也是从左到右计算的。
在下面的代码中，我认为$n应该是1但它变成了2. 我错过了什么？
my $v = 1;
my $n = ( $v, $v = 2 )[0];
# I thought left "$v" would be evaluated (and return 1) first, then right "$v = 2" would.
say "n = $n"; # but output was 2
say "v = $v"; # 2
或者
my $v = 1;
my @l = ( $v, $v = 2 );
say Dumper(@l); # [ 2, 2 ], not [ 1, 2 ]
即使我更改了顺序：
my $v = 1;
# my $n = ( $v, $v = 2 )[0];
my $n = ( $v = 2, $v )[1];
say "n = $n"; # still 2
say "v = $v"; # 2
回答
它确实从左到右进行评估。
$ perl -MO=Concise,-exec -e'my $v = 1; my $n = ( $v, $v = 2 )[0];'
1 <0> enter v
2 <;> nextstate(main 1 -e:1) v:{
3 <$> const[IV 1] s
4 <0> padsv[$v:1,3] sRM*/LVINTRO > my $v = 1;
5 <2> sassign vKS/2 /
6 <;> nextstate(main 2 -e:1) v:{
7 <0> pushmark s
8 <$> const[IV 0] s | |
9 <0> pushmark s | |
a <0> padsv[$v:1,3] s $v > ( ... )[0] |
b <$> const[IV 2] s | > my $n = ...
c <0> padsv[$v:1,3] sRM* > $v = 2 | |
d <2> sassign sKS/2 / | |
e <2> lslice sK/2 / |
f <0> padsv[$n:2,3] sRM*/LVINTRO |
g <2> sassign vKS/2 /
h <@> leave[1 ref] vKP/REFC
-e syntax OK
问题是与关联的标量$v放在堆栈上，而不是整数。你可以看到这里发生了什么：
use 5.014;
use warnings;
my @stack;
sub sassign {
my $rhs = pop(@stack);
my $lhs = pop(@stack);
$$rhs = $$lhs;
push @stack, $rhs;
}
# Stack $v $n
# ------- -- --
push @stack, 1; # 1
push @stack, my $v; # 1,$v !d
sassign(); # $v 1
@stack = (); # 1
push @stack, $v; # $v 1
push @stack, 2; # $v,2 1
push @stack, $v; # $v,2,$v 1
sassign(); # $v,$v 2
pop(@stack); # $v 2 # Net result of slice.
push @stack, my $n; # $v,$n 2 !d
sassign(); # $n 2 2
@stack = (); # 2 2
say $n; # 2
如果您更喜欢别名而不是引用：
use 5.014;
use warnings;
use experimental qw( refaliasing declared_refs );
my @stack;
sub sassign {
my $rhs = pop(@stack);
my $lhs = pop(@stack);
$rhs = $lhs;
$stack[@stack] = $rhs;
}
# Stack $v $n
# ------- -- --
$stack[@stack] = 1; # 1
$stack[@stack] = my $v; # 1,$v !d
sassign(); # $v 1
@stack = (); # 1
$stack[@stack] = $v; # $v 1
$stack[@stack] = 2; # $v,2 1
$stack[@stack] = $v; # $v,2,$v 1
sassign(); # $v,$v 2
pop(@stack); # $v 2 # Net result of slice.
$stack[@stack] = my $n; # $v,$n 2 !d
sassign(); # $n 2 2
@stack = (); # 2 2
say $n; # 2