在 Haskell 中，列表 ( '[Something] ) 前面的撇号是什么意思? [英] What does an apostrophe in front of a list ( '[Something] ) mean in Haskell?

问题描述

我正在阅读 Servant 文档 并发现了这个行:

I was reading the Servant documentation and came across this line:

type UserAPI = "users" :> QueryParam "sortby" SortBy :> Get '[JSON] [User]

' 对该列表做了什么?

推荐答案

这是DataKinds 在起作用，其中:

This is DataKinds in action, which:

• 提升类型级别的值，以及
• 将类型提升到种类级别

然而，这会导致类型级别的混淆.现在，在类型中，[X] 可能是 [X] :: *，X 列表类型，或者我们由于提升，可能有 [X] :: [T] -- 即值 [X](列表仅包含单个值 X)，带有 T 类型的 X，在类型级别提升.

This however causes confusion at the type level. Now, in types, [X] might either be [X] :: *, the list-of-X type, or instead we might have [X] :: [T] due to the lifting -- that is the value [X] (list containing only the single value X), with X of type T, lifted at the type level.

为了克服这种歧义，GHC 需要在提升的值构造函数前加上引号.所以，我们有 [X] :: * 和 '[X] :: [T].

To overcome this ambiguity, GHC requires a quote in front of lifted value constructors. So, we have [X] :: *, and '[X] :: [T].

具体来说，在您的情况下，Get '[JSON] [User] 涉及提升到类型级别的列表值 [JSON] 和列表类型 <代码>[用户].为了更好地理解差异，请注意没有 '[JSON] 类型的术语，因为这不是列表类型.我们甚至可以将 Get '[JSON,JSON,JSON] [User] 作为友好的表达式，甚至 Get '[] [User].相反，我们不能有 Get '[JSON] [User,User] 因为 [User,User] 不是类型.

Concretely, in your case, Get '[JSON] [User] involves both the list value [JSON] lifted to the type level, and the list type [User]. To better appreciate the difference, note that there are no terms of type '[JSON], since this is not a list type. We could even have Get '[JSON,JSON,JSON] [User] as a well-kinded expression, or even Get '[] [User]. Instead, we can't have Get '[JSON] [User,User] since [User,User] is not a type.

(类型 Get '[JSON,JSON,JSON] [User]，即使它是有效的，也不能被 Servant 库有意义地使用.我不知道这解除了什么列表用于Servant.)

(The type Get '[JSON,JSON,JSON] [User], even if it is valid, could not be meaningfully used by the Servant library. I have no idea of what that lifted list is used for in Servant.)

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