关于“FlexibleInstances”的类型类问题

给定类型类

class Dictionary w where
  insert :: String -> String -> w -> w
  remove :: String -> w -> w
  lookUp :: String -> w ->  String

我写不了

instance Dictionary [(String,String)] where
  insert key value dic = (key,value) : remove key dic
  remove key dic = filter (\entry -> (fst entry) /= key) dic
  lookUp key [] = "not found"
  lookUp key ((k,v):xs) | k == key = v
                        | otherwise = lookUp key xs 

因为

Illegal instance declaration for `Dictionary[(String, String)]'
    (All instance types must be of the form (T a1 ... an)
     where a1 ... an are type *variables*,
     and each type variable appears at most once in the instance head.
     Use -XFlexibleInstances if you want to disable this.)
In the instance declaration for `Dictionary[(String, String)]'

……我不太明白。像这样的东西有效：

newtype Dic = Dic [(String,String)]

instance Dictionary Dic where
  insert key value (Dic dic) = Dic $ (key,value) : filter (\entry -> (fst entry) /= key) dic
  remove key (Dic dic) = Dic $ filter (\entry -> (fst entry) /= key) dic
  lookUp key (Dic []) = "not found"
  lookUp key (Dic ((k,v):xs)) | k == key = v
                              | otherwise = lookUp key (Dic xs)  

有没有更好的办法？或者我应该使用建议的编译器指令？

原因很简单。 Haskell 98 只允许“不饱和”类型的实例，这是在其类型变量中不固定的类型。仔细阅读它给出的错误消息，它准确地描述了编译器想要的内容。

为了做你想做的事，基本上有两种你已经尝试过的方法：

• 打开灵活实例。这是最常见的方式，因为此扩展是最常用的扩展之一。
• 将其包裹成一个newtype。提供兼容性，但很丑。

选一个 ;）