raku - 覆盖角色的属性

Question

是否可以覆盖角色的属性以提供默认值？

role A {
     has $.a;
}
class B does A {
    has $.a = "default";
}
my $b = B.new;

这会导致编译错误:

===SORRY!=== Error while compiling:
Attribute '$!a' already exists in the class 'B', but a role also wishes to compose it

Answer 1

由于 R 中的方法可引用$!a所指的属性会有歧义。

使用子方法 BUILD 初始化继承/混合属性。

role R { has $.a };
class C does R {
    submethod BUILD { $!a = "default" }
};
my $c = C.new;
dd $c;
# OUTPUT«C $c = C.new(a => "default")␤»

根据您的用例，您最好通过角色参数设置默认值。

role R[$d] { has $.a = $d };
class C does R["default"] { };
my $c = C.new;
dd $c;
# OUTPUT«C $c = C.new(a => "default")␤»