List context and the comma operator in Perl

Question

Having some significant confusion about what it means to "evaluate in list context", specifically related to the comma operator. In the linked perlop doc, it says: In list context, it's just the list argument separator, and inserts both its arguments into the list. However, the code

@y = 9, 8, 7, 6;
say @y, ', ', scalar @y;

gives output 9, 1. This respects the fact that, when used as a binary operator on scalar values (?), the comma operator , has lower precedence that the assignment = operator. But since I'm assigning to the list @y, shouldn't the right hand side of the assignment be evaluated in list context and, by the above quote, treat the comma simply as a separator?

I suspect that I just don't truly understand what it "evaluate in list context" means, precisely...

Answer 1

这个问题与上下文无关。^[1]这是一个优先级问题。

赋值的优先级高于逗号，所以

my @y = 9, 8, 7, 6;

方法

( my @y = 9 ), 8, 7, 6;

但你想要

my @y = ( 9, 8, 7, 6 );

请注意，括号除了覆盖优先级之外什么也不做。

警告会发现这一点。始终使用use strict; use warnings;或等效！

我怀疑我只是没有真正理解“在列表上下文中评估”的含义，确切地说......

每个运算符根据评估的上下文决定要做什么和返回什么。

但是，它不会改变代码的解析方式。

1. my @y左边的足以导致使用列表分配，因此足以导致在列表上下文中评估 RHS。请参阅标量与列表赋值运算符。