Question

我正在尝试以递归方式添加到Haskell中的二叉树。我正在关注Learn You A Haskell，只是做了一些修改，但我遇到了错误，我不明白：

data Male = Male { maleName :: String
                 , maleDOB :: Int
                 } deriving (Show, Read, Eq, Ord)

data Female = Female { femaleName :: String
                     , femaleDOB :: Int
                     } deriving (Show, Read, Eq, Ord)

data FamilyTree a = EmptyTree 
                  | Node a (FamilyTree Female) (FamilyTree Male)
                  deriving (Show, Read, Eq, Ord)

singleton :: a -> FamilyTree a
singleton x = Node x EmptyTree EmptyTree

treeInsert :: (Ord a) => a -> FamilyTree a -> FamilyTree a
treeInsert x EmptyTree = singleton x
treeInsert x (Node a left right)
    | x == a = Node x left right
    | x < a  = Node a (treeInsert x left) right
    | x > a  = Node a left (treeInsert x right)

以下是我收到的错误消息：

Couldn't match type `Female' with `Male'
Expected type: Male
  Actual type: a
In the first argument of `treeInsert', namely `x'
In the third argument of `Node', namely `(treeInsert x right)'
In the expression: Node a left (treeInsert x right)

我对Haskell很陌生，无法理解这里发生的事情。任何指向正确方向的指针都是受欢迎的！

Answer 1

写作时

treeInsert :: Ord a => a -> FamilyTree a -> FamilyTree a

类型系统确保第一个参数的类型等于第二个参数的索引。这意味着您只能在以Male开头的树中插入Male。我想，这不是你想要的。

然而，这是一个很好的问题，我会回答它。

中的问题

treeInsert :: Ord a => a -> FamilyTree a -> FamilyTree a

是a远非一般。什么会

treeInsert :: Int -> FamilyTree Int -> FamilyTree Int

意思？您需要将a限制为Female或Male。这是GADTs的工作：

{-# LANGUAGE GADTs #-}

data Person a where
    PFemale :: Female -> Person Female
    PMale   :: Male   -> Person Male

Person包含Female或Male，并在类型级别提供有关哪一个的信息。有了这个，我们可以定义

runPerson :: Person a -> a
runPerson (PFemale x) = x
runPerson (PMale   x) = x

treeInsert :: Person a -> FamilyTree a -> FamilyTree a
treeInsert p              EmptyTree          = singleton (runPerson p)
treeInsert p@(PFemale x) (Node a left right)
    | x == a    = Node x left right
    | otherwise = treeInsert p left
treeInsert p@(PMale   x) (Node a left right)
    | x == a    = Node x left right
    | otherwise = treeInsert p right

诀窍是，当您在Person a a上进行模式匹配时，会将其实例化为Female或Male，而不是其他内容。当a为Female时，您会继续插入＆＃34; Female＆＃34;子树，否则 - 进入＆＃34; Male＆＃34;。

递归添加到二叉树

1 个答案: