promise - 如何从 promise 中退出 promise ？

Question

如何从 promise 中退出 promise ？ perl6 文档没有提供简单的方法。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 promise 循环中。 break()和 this.break()不被识别，和 return不违背 promise 。

Answer 1

使用 last keyword退出循环。
start 的保留值block 是其最后一条语句返回的值。

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`
}