问题描述
我尝试使用 scalaz 的 Tagged Type 来加强类型安全.
I try to use Tagged Type from scalaz to strengthen type safety.
我遇到了一个我不明白的警告和错误.
I encountered a warning and an error that I don't understand.
你能解释一下吗?
这是控制台的输出:
scala> sealed trait PostId
defined trait PostId
scala> def PostId(l: Long) : Long @@ PostId = Tag[Long, PostId](l)
PostId: (l: Long)scalaz.@@[Long,PostId]
warning: previously defined trait PostId is not a companion to method PostId.
Companions must be defined together; you may wish to use :paste mode for this.
scala> case class Post(id: PostId)
defined class Post
scala> Post(PostId(2l))
<console>:26: error: type mismatch;
found : scalaz.@@[Long,PostId]
(which expands to) Long with AnyRef{type Tag = PostId}
required: PostId
Post(PostId(2l))
推荐答案
在您的示例中,PostId
只是一种标记类型.实际的标记ed 类型(您应该操作的类型)是Long @@ PostId
.
In your example, PostId
is just a tag type.The actual tagg-ed type (the type that you should manipulate) is Long @@ PostId
.
错误是您定义了 Post
以获取 PostId
的实例,而您确实想让它获取 Long @@ PostId,因此类型不匹配.
The error is that you have defined Post
to take an instance of PostId
when you really meant to have it take an instance of Long @@ PostId
, hence the type mismatch.
我建议将 PostId
重命名为 PostIdTag
并将 PostId
定义为 Long @@ PostId
的别名:
I suggest renaming PostId
to PostIdTag
and defining PostId
as an alias to Long @@ PostId
:
sealed trait PostIdTag
type PostId = Long @@ PostIdTag
def PostId(l: Long) : PostId = Tag[Long, PostIdTag](l)
然后你可以保持你的 Post
定义不变.
Then you can keep your Post
definition as is.
UPDATE:事实证明,scalaz 标记类型似乎只适用于 AnyRef
类型,即不能从 创建标记类型AnyVal
子类型.
UPDATE: As it turns out, scalaz tagged types seem to only work with types <: AnyRef
, ie one cannot create a tagged type from an AnyVal
sub-type.
解决方案是将 Long
替换为 java.lang.Long
(这是透明的,因为 Scala 会自动转换 java.lang.Long
> 值到 Long
):
The solution then is to replace Long
with java.lang.Long
(which works transparently because scala will automatically convert java.lang.Long
values to Long
):
sealed trait PostIdTag
type PostId = java.lang.Long @@ PostIdTag
def PostId(l: java.lang.Long) : PostId = Tag[java.lang.Long, PostIdTag](l)
case class Post(id: PostId)
Post(PostId(2l))
这篇关于标记类型:类型不匹配的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!