Question

I try to implement a function (Tree a) -> (Tree a) -> (Tree a). The function should sum the node values and return a tree with the sums. Unfortunatley i got the following error message:

Aufgabe10.hs:4:11: Unexpected type ‘Tree a’ In the class declaration for ‘+’ A class declaration should have form class + a b c where ...

This is my code:

data Tree a = Node a (Tree a) (Tree a) 
    |Empty

class (+) (Tree a) where
    (+) :: (Tree a) (Tree a) -> (Tree a)

instance (Num a) => (+) (Tree a) (Tree a) where
    (+) (Node a1 b1 c1)  (Node a2 b2 c2) = (Node (a1+a2) ((+) b1 b2)  ((+) c1 c2)) 
    (+) Empty (Node a b c) = (Node a b c) 
    (+) (Node a b c) Empty = (Node a b c)

Ok i changed now the class to Plus and named the function plus, cause i don't want to implement all the numb functions. This is the new code:

data Tree a = Node a (Tree a) (Tree a) 
    |Empty

class Plus a where
    plus:: (Tree a)  -> (Tree a)  -> (Tree a) 

instance (Num a) => Plus (Tree a) where
    Plus (Node a1 b1 c1)  (Node a2 b2 c2) = (Node (a1+a2) (Plus b1 b2)  (Plus c1 c2)) 
    Plus Empty (Node a b c) = (Node a b c) 
    Plus (Node a b c) Empty = (Node a b c)

I get the following errors:

Aufgabe10.hs:8:9: Pattern bindings (except simple variables) not allowed in instance declarations Plus (Node a1 b1 c1) (Node a2 b2 c2) = (Node (a1 + a2) (Plus b1 b2) (Plus c1 c2))

Aufgabe10.hs:9:9: Pattern bindings (except simple variables) not allowed in instance declarations Plus Empty (Node a b c) = (Node a b c)

Aufgabe10.hs:10:9: Pattern bindings (except simple variables) not allowed in instance declarations Plus (Node a b c) Empty = (Node a b c)

Answer 1

看看What are typeclasses?：你会注意到类型类声明需要：

类型名称;
一个类型变量。

因此，您的声明应该以

class (+) a where

不

class (+) (Tree a) where

虽然我更喜欢

class Plus a where

这解决了你的第一个问题。：）

回答您的更新版本......

您是否看到Plus和plus之间的区别？是的，一个是资本化而另一个不是。你知道Haskell的含义吗？

Plus，就像Haskell中的每个大写单词一样，指的是类型（在这种情况下，类型类称为Plus）。

另一方面，函数必须始终为小写 - 这就是您以小写形式定义plus :: (Tree a) -> (Tree a) -> (Tree a)的原因。

你已经解决了问题吗？

而不是

instance (Num a) => Plus (Tree a) where
    Plus (Node a1 b1 c1)  (Node a2 b2 c2) = (Node (a1+a2) (Plus b1 b2)  (Plus c1 c2)) 
    Plus Empty (Node a b c) = (Node a b c) 
    Plus (Node a b c) Empty = (Node a b c)

你应该写的

instance (Num a) => Plus (Tree a) where
    plus (Node a1 b1 c1)  (Node a2 b2 c2) = (Node (a1+a2) (Plus b1 b2)  (Plus c1 c2)) 
    plus Empty (Node a b c) = (Node a b c) 
    plus (Node a b c) Empty = (Node a b c)

因为当plus是Tree的实例时，您正在定义函数Plus的行为。得到它了？：）

Haskell unexpected type in class declaration

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