如何正确编写模式匹配？

Question

以下代码有效：

data MyList a = Atom a | Cons a (MyList a) deriving (Show)

getAtom (Atom a) = a

myFindMax :: (Ord a) => MyList a -> a 
myFindMax (Cons x xs) = let restMax = myFindMax xs in
                          if x > restMax then x else restMax
myFindMax x = getAtom x

但是当我写作

myFindMax (Atom x) = getAtom x

在其他模式之前，我收到错误

Couldn't match expected type ‘MyList a’ with actual type ‘a’
      ‘a’ is a rigid type variable bound by
          the type signature for myFindMax :: Ord a => MyList a -> a
          at myList.hs:5:14

为什么Atom案例与Cons案例不同？

Answer 1

没有什么不同，这是因为有错误：

myFindMax (Atom x) = getAtom x

将尝试将getAtom调用为非（通常）Atom值的值。该模式已经完成getAtom函数的工作。你应该写：

myFindMax (Atom x) = x