Haskell 类型类实例化

Question

我正在尝试将 (PolyA a) 和 (PolyB a) 设置为多项式类的实例，我想在其中实现 coeffs、fromCoeffs、coeffsB、fromCoeffsB。我不太确定我做错了什么，因为我收到一条错误消息，指出我的函数对多项式类不可见。有什么帮助吗？

class Polynomial p where

--default implementations
data PolyA a = Coeffs [a]
           deriving (Show)
data PolyB a = Const a | X (PolyB a) a
           deriving (Show)

--instances
instance Polynomial (PolyA a) where
    coeffs (Coeffs f)=f
    fromCoeffs f= Coeffs f

 instance Polynomial (PolyB a) where
 coeffsB (Const f)= [f]
 coeffsB (X f a)= coeffsB f ++ [a]
 fromCoeffsB [] = error "Wrong Input!"
 fromCoeffsB [f]= Const f
 fromCoeffsB lis@(_:t)= X (fromCoeffsB (init lis)) (last lis)

Answer 1

以下代码为我编译：

class Polynomial p where
  coeffs :: p a -> [a]
  fromCoeffs :: [a] -> p a

--default implementations
data PolyA a = Coeffs [a]
           deriving (Show)
data PolyB a = Const a | X (PolyB a) a
           deriving (Show)

--instances
instance Polynomial PolyA where
    coeffs (Coeffs f)=f
    fromCoeffs f= Coeffs f

instance Polynomial PolyB where
 coeffs (Const f)= [f]
 coeffs (X f a)= coeffs f ++ [a]
 fromCoeffs [] = error "Wrong Input!"
 fromCoeffs [f]= Const f
 fromCoeffs lis@(_:t)= X (fromCoeffs (init lis)) (last lis)

变更摘要：

• 将方法添加到Polynomial 类声明中。
• 删除实例声明中的类型参数。
• 将coeffsB 更改为coeffs 和fromCoeffsB 更改为fromCoeffs。
• 将PolyB 实例声明缩进一个空格。