Question

当我运行以下代码时：

my $timer = Promise.in(2);
my $after = $timer.then({ say "2 seconds are over!"; 'result' });
say $after.result;  # 2 seconds are over
                    # result

我得到了

2 seconds are over!
result

;中then的作用是什么？如果我写

，为什么呢？

say "2 seconds are over!"; 'result';

我是否收到以下错误？

WARNINGS:
Useless use of constant string "result" in sink context (line 1)
2 seconds are over!

而不是：

2 seconds are over!
result

和第一个例子一样？

Answer 1

'result'是块{ say "2 seconds are over!"; 'result' }的最后一个语句。在Perl语言中，分号（不是换行符）决定了大多数语句的结尾。

在此代码中：

my $timer = Promise.in(2);
my $after = $timer.then({ say "2 seconds are over!"; 'result' }); # 'result' is what the block returns
say $after.result;  # 2 seconds are over (printed by the say statement)
                    # result ('result' of the block that is stored in $after)

第二行可以改写：

my $after = $timer.then( {
                          say "2 seconds are over!";
                          'result'
                         }
); # 'result' is what the block returns

该分号只是结束语句say "2 seconds are over!"。

在一个区块之外，这一行

say "2 seconds are over!"; 'result';

实际上是两个陈述：

say "2 seconds are over!";  # normal statement
'result';                   # useless statement in this context (hence the warning)

将多个语句放在一行中很少会改变它们的行为方式：

my $timer = Promise.in(2); my $after = $timer.then({ say "2 seconds are over!"; 'result' }); say $after.result; # Still behaves the same as the original code. ... Do not edit. This is intentionally crammed into one line!

Promise对象内外的分号

1 个答案: