Question

我正在学习Haskell。

以下代码未编译

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

错误：无法将预期类型Nim与实际类型“a”匹配。 'a'是由player :: a - ＆gt;的输入签名约束的刚性类型变量。播放器。

任何帮助都将不胜感激。

Answer 1

Class和Instance声明需要包含其包含函数的间距：

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

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

无法匹配预期的类型

1 个答案: