haskell - 为什么:k [False] result in an error in GHCI?

Question

我对以下 session 结束时收到的错误感到困惑:

$ ghci
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
Ok, modules loaded: Main.
*Main> :set -XDataKinds

*Main> :t [False, True]
[False, True] :: [Bool]

*Main> :t [False]
[False] :: [Bool]

*Main> :k [False, True]
[False, True] :: [Bool]

*Main> :k [False]

<interactive>:1:2:
    Expected kind ‘*’, but ‘False’ has kind ‘Bool’
    In a type in a GHCi command: [False]

为什么会出错？

future 的实验表明:

*Main> :k [Int]
[Int] :: *

*Main> :k [Int, Int]
[Int, Int] :: [*]

[Int]可以有居住值(value)，所以它是一种 * , 但它也是有道理的 [*] .

更多数据点:

*Main> :k []
[] :: * -> *

*Main> :k [Bool]
[Bool] :: *

Answer 1

如果您的类型级别列表只有一个元素，GHC 认为它不是提升列表，而是应用于某种类型的常规列表类型构造函数 * .

您应该在列表前加上撇号以明确选择提升的列表:

> :k '[False]
'[False] :: [Bool]

与空列表类似:

> :k '[]
'[] :: [k]