为什么 GHCi 无法解析 [[]] 类型？
为什么 Haskell 无法解析 [[]]（列表的列表）类型？
为什么不简单地 * -> *，因为我可以给它一个像 Int 这样的类型，并得到 [[Int]]，这是一种 *.
Why can't Haskell resolve the kind of [[]] (A list of lists)?
Why isn't it simply * -> *, as I can give it a type like Int, and get [[Int]], which is of kind *.
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我认为这与
Maybe Maybe相同，尽管在后一种情况下，原因可能更清楚：“外部”类型构造函数期望传递一种*类型，但看到类型构造函数* -> *（“内部”也许/[]）并抱怨。如果我是正确的，这实际上并不是 GHCi 的:kind功能的问题，而是找到正确的语法来表达更高级类型构造函数的组合的问题。作为一种解决方法，
可以使用类似的东西（我认为打开适当的语言扩展 -
ExistentialQuantification- 以启用forall语法）。I think it's the same as with
Maybe Maybe, although in the latter case the reason is perhaps clearer: the "outer" type constructor expects to be passed a type of kind*, but sees a type constructor of type* -> *(the "inner"Maybe/[]) and complains. If I'm correct, this is not really a problem with the:kindfunctionality of GHCi, but rather with finding the correct syntax to express the composition of higher-kinded type constructors.As a workaround, something like
can be used (with the appropriate language extension turned on --
ExistentialQuantification, I think -- to enable theforallsyntax).如果我们将
[[]]脱糖为[] []，那么很明显它的种类很差，因为[] :: * -> *。如果您实际上想要一个“列表的列表”，您需要编写两个
* -> 类型的构造函数。 *。如果没有一些样板文件，你就无法做到这一点，因为 Haskell 没有类型级 lambda。不过，您可以这样做：现在您可以编写：
并使用它编写函数：
像这样的函子组合在多个领域都有应用，尤其是 Swierstra 的 "数据类型点菜"
If we desugar
[[]]as[] []then it's obvious that it is poorly kinded because[] :: * -> *.If you actually wanted a "list of lists" you need to compose two type constructors of kind
* -> *. You can't do that without a little boilerplate because Haskell doesn't have a type-level lambda. You can do this though:Now you can write:
And write functions using it:
Functor composition like this has applications in several areas, notably Swierstra's "Data types a la carte"