perl6 – 覆盖角色的属性

发布时间：2020-12-15 22:07:01 所属栏目：大数据来源：网络整理

导读：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 com

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

解决方法

由于R中的方法可能引用$！a,因此会引起含糊不清的属性.

使用子方法BUILD初始化inherited / mixedin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")??

（编辑：李大同）

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