Question

我正在为以下数据结构实现Foldable：

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

当我实现fold和foldMap时：

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，我会收到以下警告：

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

当我实现该函数时，我将得到它已经存在的错误，但是当我没有实现该函数时，我会收到一个警告，该函数丢失了？我做错了什么？

Answer 1

您只需缩进fold和foldMap声明，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

当你让它们缩进时，机器会看到一个空的instance声明和两个不相关的顶级函数fold和foldMap。空实例是＆＃34;没有明确实现的来源＆＃34;错误（为什么这是一个警告，而不是我不知道的错误）。＆＃34;模棱两可的事件＆＃34;错误是因为编译器无法判断对foldMap的递归调用是指引用隐式导入的Prelude.foldMap还是代码中的Main.foldMap。

不明确的'foldMap'

1 个答案: