[ Perl 6 ] 定义类型的子集

[ Perl 6 ] 定义类型的子集

There is more than one way to do it.

• 有些时候，我们希望定义的变量或传递的参数的类型应当是有限制的基本类型。为了实现这一需求，Perl 6提供了subset关键字，它用于定义一个类型的子集

subset Non-neg-int of Int where { $_ >= 0 }

# factorial as example
sub factorial(Non-neg-int $n) {
  return 1 if $n == 0;
  return $n * factorial $n-1;
}

> fact(1.5)
Type check failed in binding to parameter ‘$n‘; expected Int but got Rat (1.5)
  in sub fact at <unknown file> line 1
  in block <unit> at <unknown file> line 1

> fact(-1)
Constraint type check failed in binding to parameter ‘$n‘; expected Non-neg-int but got Int (-1)
  in sub fact at <unknown file> line 1
  in block <unit> at <unknown file> line 1