如何定义循环类型定义? [英] How to define a cyclic type definition?

问题描述

这不是有效的类型定义:

This is not a valid type definition:

scala>  type Addable = { def +(subject: Addable) }
<console>:4: error: illegal cyclic reference involving type Addable
        type Addable = { def +(subject: Addable) }

这可以用scala表达吗?

Can this be expressed in scala?

推荐答案

不，不能.

在 Scala 语言规范 2.7 版的第 40 页:

On page 40 of The Scala Language Specification Version 2.7:

然而，如果一个类型别名递归地指向定义类型构造函数本身.那即，类型别名中的类型 Ttype t[tps] = T 不得直接或间接提及名称t.

However, it is a static error if a type alias refers recursively to the defined type constructor itself. That is, the type T in a type alias type t[tps] = T may not refer directly or indirectly to the name t.

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