什么是实物签名约束 [英] What is Constraint in kind signature

问题描述

如果我检查Maybe的kind，我会得到:

If I inspect the kind of Maybe I get this:

λ> :k Maybe
Maybe :: * -> *

现在，如果我检查Monad的种类，我会得到:

Now, if I inspect the kind of Monad I get this:

λ> :k Monad
Monad :: (* -> *) -> Constraint

那里的Constraint是什么，为什么需要它?为什么不只是这个* -> *?

What is Constraint there and why it is needed ? Why not just this * -> * ?

推荐答案

与Maybe不同，Monad不是 type ；这是一个 typeclass .

Unlike Maybe, Monad is not a type; it is a typeclass.

其他类型类也是如此:

Num :: * -> Constraint
Functor :: (* -> *) -> Constraint
Bifunctor :: (* -> * -> *) -> Constraint

其中*代表具体类型(例如Bool或Int)，->代表种类更高的类型(例如Maybe)，而Constraint代表类型约束的概念.这就是为什么:

Where * represents concrete types (such as Bool or Int), -> represent higher-kinded types (such as Maybe), and Constraint represents the idea of a type constraint. This is why:

我们知道我们不能像这样签名:

As we know we can't make a signature like this:

return :: a -> Monad a -- This is nonsense!

因为Monad应该用作约束，也就是说，这必须是一个单子才能工作":

Because Monad should be used as a constraint, to say that, 'this must be a monad to work':

return :: (Monad m) => a -> m a

之所以这样做，是因为我们知道return不能在任何旧类型m上运行，因此我们以名称Monad为不同类型定义了return的行为.换句话说，没有一个可以被称为Monad的事物，只有一个可以被称为Monadic的行为.

We do this because we know that return can't work on any old type m, so we define the behaviour of return for different types under the name Monad. In other words, there is no single thing that can be called a Monad, but only behaviour that can be called Monadic.

由于这个原因，我们创建了这个类型约束，说我们必须预先定义一个Monad才能使用此功能.这就是为什么Monad是(* -> *) -> Constraint的原因-它本身不是类型！

For this reason, we have created this type constraint, saying that we must have pre-defined something as a Monad to use this function. This is why the kind of Monad is (* -> *) -> Constraint - it itself is not a type!

Maybe是Monad的实例.这意味着有人在某处写过:

Maybe is an instance of Monad. This means that somewhere, someone has written:

instance Monad Maybe where
  (>>=) = ... -- etc

...并定义Maybe应如何表现为Monad.这就是为什么我们可以将Maybe与具有前缀约束Monad m => ...的函数或类型一起使用的原因.本质上，这是定义Monad施加的约束的地方.

...and defined how Maybe should behave as a Monad. This is why we can use Maybe with functions or types that have the prefix constraint Monad m => .... This is essentially where one defines the constraint applied by Monad.

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