如何从承诺中退出承诺？ perl6文档不提供简单的方法.例如：
my $x = start {
loop { # loop forever until "quit" is seen
my $y = prompt("Say something: ");
if $y ~~ / quit / {
# I want to exit the promise from here;
# "break" and "this.break" are not defined;
# "return" does not break the promise;
# I do NOT want an error exception when exiting a promise;
# I want to return a value as the result of this promise;
}
else { say $y; }
}
}
我不想永远处于承诺循环中. break()和this.break()无法识别,返回不会破坏承诺.
解决方法
使用
last keyword退出循环.
保持的起始块值是其最后一个语句返回的值.
所以：
my $x = start {
loop { # loop forever until "quit" is seen
my $y = prompt("Say something: ");
if $y ~~ / quit / {
last
}
else { say $y; }
}
42 # <-- The promise will be kept with value `42`
}