When and Where

When 可以用在主题化($_)的语句中 #

Raku 里面有个特殊的变量叫 $_, 即主题化变量, the variable in question.

for ('Swift', 'PHP', 'Python', 'Perl') -> $item  { 
    say $item when $item ~~ /^P/;
} 

输出:

PHP
Python
Perl

for (12, 24, 56, 42) {
    .say when *>40 
}

输出:

56
42

而 where 用于对类型进行约束:

for ('Swift', 'PHP', 'Python', 'Perl', 42) -> $item where $item ~~ Str {
    say $item;
}

输出:

Swift                                                                               
PHP                                                                                 
Python                                                                              
Perl                                                                                    
Constraint type check failed for parameter '$item'