“实例显示状态"不编译 [英] "instance Show State where" doesn't compile

问题描述

这是我要弄清楚的State Monad代码

This is the State Monad code I am trying to figure out

data State a = State (Int -> (a, Int)) 
instance Monad State where
    return x = State (\c -> (x, c))     
    State m >>= f = State (\c ->
        case m c of { (a, acount) ->
            case f a of State b -> b acount})

getState = State (\c -> (c, c))
putState count = State (\_ -> ((), count))

instance Show State where  -- doesn't work
    show (State a) = show a   -- doesn't work

我试图将State设为Show的实例，以便在ghci提示符下看到getState和putState count的动作.

I am trying to make State as instance of Show so that I can see the action of getState and putState count on the ghci prompt.

任何教程或指向State Monad资料的链接也将很好.

Any tutorial or link to State Monad material would be nice too.

推荐答案

在Haskell中，类型类仅对相同种类的类型进行分类. Monad对类型为* -> *的类型进行分类，而Show对类型为*的类型进行分类.您的State类型的类型为* -> *，这就是为什么您的Monad实例没有问题，但您的Show实例却有问题 的原因. State之所以称为类型构造函数"，是因为它消耗一个类型以产生另一个类型.可以认为它在类型级别上类似于函数应用程序.因此，您可以应用特定的类型并为其创建实例:

In Haskell, typeclasses classify only types of the same kind. Monad classifies types of kind * -> *, while Show classifies types of kind *. Your State type has kind * -> * which is why there isn't a problem with your Monad instance, but there is a problem with your Show instance. State is called a "type constructor" because it consumes a type in order to produce another type. Think of it sort of like function application at the type level. You can therefore apply a specific type and make instances of that:

instance Show (State Char) where
    show _ = "<< Some State >>"

现在，这不是一个非常有用的实例，请尝试使用Sjoerd的建议来获得更有意义的Show实例.请注意，他的版本使用带有约束的通用类型.

Now, this isn't a very useful instance, try something like Sjoerd's suggestion to get a more meaningful Show instance. Notice that his version uses a generic type with a constraint.

instance (Show a) => Show (State a) where
    -- blah blah

通用类型为a，约束为(Show a) =>，换句话说，a本身必须是Show的实例.

The generic type is a, and the constraint is (Show a) =>, in other words, a itself must be an instance of Show.

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