如何从承诺中退出承诺？

Question

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()并且this.break()不被承认,并且return没有违背承诺.

Answer 1

使用last关键字退出循环.

start块的保留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`
}