如何从承诺中退出承诺？

Question

如何从承诺中退出承诺？ 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; }
    }
}

我不想永远处于承诺循环中。 break()和this.break()不承认，return不食言

Answer 1

使用last keyword退出循环。

start 块的保留值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`
}

如何从承诺中退出承诺？

How to exit a promise from within a promise?

promise

break

raku