Question

我试图将（PolyA a）和（PolyB a）设置为类Polynomial的实例，我想在其中实现coeffs，fromCoeffs，coeffsB，fromCoeffsB。我不太清楚我做错了什么，因为我收到一条错误消息，说我的函数对于Polynomial类是不可见的。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：

向Polynomial类声明添加方法。
删除实例声明中的类型参数。
将coeffsB更改为coeffs，将fromCoeffsB更改为fromCoeffs。
通过一个空格突出PolyB实例声明。

Haskell类型类实例化

1 个答案: