haskell - 无法理解编码逻辑/原理/约定 - 为什么需要 'Show a'？为什么 'Show Car"或 "Show String"不起作用？

Question

下面给出的代码可以编译，ok。

data Car p q r = Car {company :: p  
                     , model :: q  
                     , year ::r  
                     } deriving (Show)


tellCar :: (Show a) => Car String String a -> String

什么基本原则/约定/逻辑可以提醒我只需要在“tellCar”中选择“Show a”，而不是任何其他选项？我在哪里可以找到资源来学习这些原则/约定/逻辑？

如果我在tellCar中错误地采用了“Show Car”，则在编译时会收到以下错误消息:

*Main> :load "/home/optimight/baby.hs"  
[1 of 1] Compiling Main             ( /home/optimight/baby.hs, interpreted )  

/home/optimight/baby.hs:96:18:  
    Expecting three more arguments to `Car'  
    In the type signature for `tellCar':  
      tellCar :: Show Car => Car String String a -> String  
Failed, modules loaded: none.  

如果我在tellCar中错误地使用了“Show z”，编译时会收到以下错误消息:

*Main> :load "/home/optimight/baby.hs"  
[1 of 1] Compiling Main               ( /home/optimight/baby.hs, interpreted )

/home/optimight/baby.hs:96:1:  
    Ambiguous constraint `Show z'  
      At least one of the forall'd type variables mentioned by the constraint  
      must be reachable from the type after the '=>'  
    In the type signature for `tellCar':  
      tellCar :: Show z => Car String String a -> String  
Failed, modules loaded: none.   

如果我在tellCar中错误地采用了“显示字符串”，则在编译时会收到以下错误消息:

Prelude> :load "/home/optimight/baby.hs"  
[1 of 1] Compiling Main             ( /home/optimight/baby.hs, interpreted )  

/home/optimight/baby.hs:96:1:  
    Non type-variable argument in the constraint: Show String  
    (Use -XFlexibleContexts to permit this)  
    In the type signature for `tellCar':  
      tellCar :: Show String => Car String String a -> String  
Failed, modules loaded: none.  

Answer 1

tellCar :: (Show a) => Car String String a -> String

请记住这里Show a的含义是:

In the type specification that follows the =>, a should have an instance for Show.

这意味着 Car 数据构造函数的第三个参数必须是具有 Show 实例的某种类型。

指定(Show Car) => …或(Show String) => …是没有意义的；这些具体类型可能有也可能没有 Show 实例(在本例中它们都有)，但它并没有阐明实际类型 Car String String a -> String 任意。

指定(Show z) => … 表示:

In the type specification that follows the =>, z should have an instance for Show.

请注意，后面的类型是 Car String String a -> String，我们可以得出结论，a) z 从未被提及，所以我们可以忽略 (Show z) => …，并且 b) a 是多态的，没有约束! (即您不能依赖它拥有任何特定的类型类实例，包括 Show)