无法匹配预期类型

Question

我正在学习Haskell。

下面的代码不编译

data Player = Max | Min 
    deriving (Show,Eq) 

class Position a where 
score :: a -> Int 
player :: a -> Player 


data Nim = Nim { turn :: Player, count :: Int} 

instance Position Nim where 
score a = count a 
player a = turn a 

error: Could not match expected type Nim with actual type 'a'. 'a' is a rigid type variable bound by the input signature for player :: a -> Player.

任何帮助，将不胜感激。

Answer 1

的类和实例声明需要间距为他们的职能包括：

class Position a where 
    score :: a -> Int 
    player :: a -> Player 

instance Position Nim where 
    score a = count a 
    player a = turn a