问题描述
Looking at a kind example from Learn You a Haskell:
data Frank a b = Frank {frankField :: b a} deriving (Show)
LYAH讨论:
在我想到一个示例时，这对我来说很有意义:Frank String Maybe = Frank {frankField :: Maybe String}.
This makes sense to me as I think of an example: Frank String Maybe = Frank {frankField :: Maybe String}.
但是我对如何考虑右侧的kind感到困惑
But I'm confused as to how to think about the kind of the right-hand side
... = Frank {frankField :: b a}.
由于返回类型为frankField :: b a，所以Frank的kind应该是:
Since the return type is frankField :: b a, how can the kind of Frank be:
* -> (* -> *) -> * -- what does the right-most * represent?
为什么右侧的kind等于*?
推荐答案
好吧，右侧的Frank是 value 构造函数，值没有种类，它们有类型.种类指的是左侧的Frank，它是类型构造函数.将值构造函数重命名为MkFrank:
Well, the Frank on the right-hand side is a value constructor and values don't have kinds, they have types. The kind refers to the Frank on the left-hand side, which is a type constructor. It might help to rename the value constructor to MkFrank:
data Frank a b = MkFrank {frankField :: b a} deriving (Show)
值构造函数的类型是
MkFrank :: b a -> Frank a b
类型构造器的类型是
Frank :: * -> (* -> *) -> *
这表示Frank接受一个(具体)类型(*)，而类型构造函数接受一个类型((* -> *))并为您提供类型(最后一个*).这类似于类型为A -> (B -> C) -> D的函数:它需要一个具体的"值和一个函数(一种获得给定另一个值的值的方法)，并且为您提供一个具体的"值，区别在于我们是处理类型而不是值.
This says that Frank takes one (concrete) type (*) and a type constructor that takes one type ((* -> *)) and gives you a type (the last *). This is analogous to a function with a type A -> (B -> C) -> D: it takes a "concrete" value and a function (a way to get a value given another value) and it gives you a "concrete" value, the difference is that we are dealing with types instead of values.
请注意，类型对类型类的作用略有不同(尽管类不包含在问题主体中).有一种叫做Constraint的特殊种类，代表签名中可以出现在=>左侧的东西.
As a side note, kinds work a bit differently for type classes (classes are not involved in the question body though). There's a special kind called Constraint which represents things that can go on the left hand side of => in a signature.
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