Question

如何基于递归类型别名初始化值？

type alias ContentProvider =
    { profile : Profile
    , topics : List Topic
    , links : Links
    , subscribers : Subscribers
    }


type Subscribers
    = Subscribers (List ContentProvider)

我尝试初始化值失败：

contentProvider1 : ContentProvider
contentProvider1 =
    ContentProvider profile1 topics contentProvider1Links (Subscribers [ contentProvider2, contentProvider3 ])

请注意，最后一个参数导致错误：

(Subscribers [ contentProvider2, contentProvider3 ])

contentProvider1以偷偷摸摸的方式定义，导致无限循环。 - 以下定义直接取决于在彼此：
┌─────┐
│    contentProvider1
│     ↓
│    contentProvider2
│     ↓
│    contentProvider3
└─────┘

我引用了此documentation。但是，我不明白为什么我仍然遇到编译器错误。

附录

contentProvider1 : ContentProvider
contentProvider1 =
    ContentProvider profile1 topics contentProvider1Links (Subscribers [ contentProvider2 ])


contentProvider2 : ContentProvider
contentProvider2 =
    ContentProvider profile2 topics contentProvider2Links (Subscribers [ contentProvider1, contentProvider3 ])


contentProvider3 : ContentProvider
contentProvider3 =
    ContentProvider profile3 topics contentProvider3Links (Subscribers [ contentProvider1, contentProvider2 ])

Answer 1

你的代码在语法上是正确的，在Elm 0.18之前，它会被编译。但是当你运行程序时，你最终会陷入堆栈溢出，无限循环（因为尾部调用递归优化）或运行时异常because of a compiler bug that has since been fixed。

考虑运行时扩展contentProvider1所需的内容：

展开contentProvider1，其中引用了contentProvider2
展开contentProvider2，其中引用了contentProvider1
展开contentProvider1，其中引用了contentProvider2
依此类推，永恒

您所遇到的是Elm的超级有用的编译器，可以保存您无法使用的代码。仍然可以编写一个非暂停函数（and always will be），但是Elm的编译器现在会检查一些在运行时会爆炸的常见情况。

Here is some documentation that more thoroughly explains the reasoning behind this compiler error

Answer 2

contentProvider2和contentProvider3需要像Chad Gilbert那样定义，但这里的问题不是你的相互递归类型别名，而是circular value reference that is not possible。以下是编译的简化示例：

type alias ContentProvider =
    { profile : String
    , topics : List String
    , links : String
    , subscribers : Subscribers
    }

type Subscribers = 
    Subscribers (List ContentProvider)

contentProvider1 : ContentProvider
contentProvider1 =
    ContentProvider "profile" ["art", "science"] "test" (Subscribers [contentProvider2])

contentProvider2 : ContentProvider
contentProvider2 =
    ContentProvider "profile" ["art", "science"] "test" (Subscribers [])

如何根据递归类型别名初始化值？

2 个答案: