暧昧发生 'foldMap'

Question

我实现可折叠，数据结构如下：

data Tree a = Leaf a | Node (Tree a) (Tree a) deriving Show 

当我执行倍和foldMap：

instance Foldable Tree where 

--fold :: Monoid a => Tree a -> a 
fold (Leaf x) = x 
fold (Node l r) = fold l `mappend` fold r 

--foldMap :: Monoid b => (a -> b) -> Tree a -> b 
foldMap f (Leaf x) = f x 
foldMap f (Node l r) = foldMap f l `mappend` foldMap f r 

我会收到以下错误：

Ambiguous occurrence ‘foldMap’ 
    It could refer to either ‘Main.foldMap’, 
          defined at Chapterh14.hs:57:1 
          or ‘Prelude.foldMap’, 
          imported from ‘Prelude’ at Chapterh14.hs:1:1 
          (and originally defined in ‘Data.Foldable’) 

Chapterh14.hs:58:46: 
    Ambiguous occurrence ‘foldMap’ 
    It could refer to either ‘Main.foldMap’, 
          defined at Chapterh14.hs:57:1 
          or ‘Prelude.foldMap’, 
          imported from ‘Prelude’ at Chapterh14.hs:1:1 
          (and originally defined in ‘Data.Foldable’) 

Chapterh14.hs:71:13: 
    Ambiguous occurrence ‘foldMap’ 
    It could refer to either ‘Main.foldMap’, 
          defined at Chapterh14.hs:57:1 
          or ‘Prelude.foldMap’, 
          imported from ‘Prelude’ at Chapterh14.hs:1:1 
          (and originally defined in ‘Data.Foldable’) 

当我删除foldMap的定义时，我将得到以下警告：

No explicit implementation for 
     either ‘foldMap’ or ‘foldr’ 
    In the instance declaration for ‘Foldable Tree’ 
Ok, modules loaded: Main. 

当我实现函数时，我会得到它已经存在的错误，但是当我没有实现该函数时，我会得到一个警告，表明函数丢失了？我究竟做错了什么？

Answer 1

您只需要将您的fold和foldMap声明缩进，以便它们成为instance的一部分。

instance Foldable Tree where 
    --fold :: Monoid a => Tree a -> a 
    fold (Leaf x) = x 
    fold (Node l r) = fold l `mappend` fold r 

    --foldMap :: Monoid b => (a -> b) -> Tree a -> b 
    foldMap f (Leaf x) = f x 
    foldMap f (Node l r) = foldMap f l `mappend` foldMap f r 

当你让他们取消缩进，该机看到一个空instance声明，呼吁fold和foldMap两个不相干的顶级功能。空实例是“没有显式实现”错误的来源（为什么这是一个警告，而不是我不知道的错误）。 “模糊发生”错误是因为编译器无法分辨对foldMap的递归调用是指代码中隐式导入的Prelude.foldMap还是Main.foldMap。