promise - Promise 对象内外的分号

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

2 seconds are over!
result

;的作用是什么？内 then为什么如果我写

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

我会收到以下错误吗？

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

并不是 :

2 seconds are over!
result

像第一个例子？

Answer 1

'result'是块 { say "2 seconds are over!"; 'result' } 的最后一条语句.在 Perl 语言中，分号(不是换行符)决定了大多数语句的结尾。

在这段代码中:

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)

第二行可以改写为:

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!" .

在一个块之外，这条线

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

真的是两个陈述:

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!