我尝试为森林的每个节点分配一个数字，这样就不会有 2 个节点具有相同的数字。我尝试使用 2 个相互调用递归的函数，但我得到了一些编译错误。
这是代码:

numberTree :: Int -> Tree a -> (Tree Int,Int)

numberTree n (Node c []) = (Node n [], n+1)
numberTree n (Node _ (x:xs)) =  (Node n  fst.numberForest  (n+1) xs, snd.numberForest   (n+1) xs)


numberForest :: Int -> [Tree a] -> ([Tree Int],Int)

numberForest n (x:xs) = ((fst(numberTree n x )):(fst(numberForest (n+1) xs)), snd(numberForest (n+1) xs)+1)

numberForest n (x:xs) = ((fst(numberTree n x )):(fst(numberForest (n+1) xs)), snd(numberForest (n+1) xs)+1)

.hs:27:34: Couldn't match expected type b0 -> c0' with actual typeTree Int Possible cause: Node' is applied to too many arguments In the first argument of(.)', namely `Node n fst' In the expression: Node n fst . numberForest (n + 1) xs

.hs:27:34:
Couldn't match expected type `Tree Int' with actual type `a0 -> c0
In the expression: Node n fst . numberForest (n + 1) xs
In the expression:
  (Node n fst . numberForest (n + 1) xs,
   snd . numberForest (n + 1) xs)
In an equation for `numberTree':
    numberTree n (Node _ (x : xs))
      = (Node n fst . numberForest (n + 1) xs,
         snd . numberForest (n + 1) xs)

 .hs:27:42:
Couldn't match type `(a1, b1) -> a1' with `[Tree Int]'
Expected type: Forest Int
  Actual type: (a1, b1) -> a1
Probable cause: `fst' is applied to too few arguments
In the second argument of `Node', namely `fst'
In the first argument of `(.)', namely `Node n fst'

 .hs:27:46:
Couldn't match expected type `a0 -> b0'
            with actual type `([Tree Int], Int)'
Possible cause: `numberForest' is applied to too many arguments
In the second argument of `(.)', namely `numberForest (n + 1) xs'
In the expression: Node n fst . numberForest (n + 1) xs

.hs:27:70:
Couldn't match expected type `Int' with actual type `a2 -> c1'
In the expression: snd . numberForest (n + 1) xs
In the expression:
  (Node n fst . numberForest (n + 1) xs,
   snd . numberForest (n + 1) xs)
In an equation for `numberTree':
    numberTree n (Node _ (x : xs))
      = (Node n fst . numberForest (n + 1) xs,
         snd . numberForest (n + 1) xs)

.hs:27:74:
Couldn't match expected type `a2 -> (a3, c1)'
            with actual type `([Tree Int], Int)'
Possible cause: `numberForest' is applied to too many arguments
In the second argument of `(.)', namely `numberForest (n + 1) xs'
In the expression: snd . numberForest (n + 1) xs

最佳答案

这条线

numberTree n (Node _ (x:xs)) =  (Node n  fst.numberForest  (n+1) xs, snd.numberForest   (n+1) xs)

numberTree n (Node _ (x:xs)) =  ( (Node n fst) . (numberForest (n+1) xs)
                                , snd . (numberForest (n+1) xs))

numberTree n (Node _ (x:xs)) =  ( Node n  (fst (numberForest  (n+1) xs))
                                , snd (numberForest (n+1) xs))

numberTree n (Node _ (x:xs)) =  let result = numberForest (n+1) xs
                                in (Node n (fst result), snd result)

numberTree n (Node _ (x:xs)) =  let (forest, n') = numberForest (n+1) xs
                                in (Node n forest, n')

https://stackoverflow.com/questions/27425200/