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这是一棵树的代码。

import Control.Exception
import Control.Monad
import Control.DeepSeq

import qualified Data.List as List

import Test.HUnit


data Tree a  =  Empty
         |  Node a (Tree a) (Tree a)
         deriving (Show, Eq)



insertTree :: ( Ord a, Show a ) => Tree a -> a -> Tree a
insertTree Empty x  =  Node x Empty Empty
insertTree ( Node v tLeft tRight ) x
     | x == v = Node v tLeft tRight
     | x < v = Node v (insertTree tLeft x) tRight
     | x > v = Node v tLeft (insertTree tRight x)


createTree :: ( Ord a, Show a ) => [ a ] -> Tree a
createTree = foldl insertTree Empty

member:: ( Ord a, Show a ) => Tree a -> a -> Bool
member Empty x = False
member (Node v tLeft tRight ) x
| x == v = True
| x < v = member tLeft x
| x > v = member tRight x



intTree = createTree [ 9, 7, 2, 8, 6, 0, 5, 3, 1 ]

listTree = createTree ( List.permutations [ 0 .. 3 ] )

strTree = createTree [ "hello"
                     , "world"
                     , "lorem"
                     , "ipsum"
                     , "dolor"
                     , "sit"
                     , "amet"
                    ]

我的代码基本上是查看一个元素是否包含在树内。当我运行我的代码时例如：ghci.> member 5 intTree 我得到一个错误（Ord（Tree a0））没有实例。我想知道如何解决这个问题。任何帮助是极大的赞赏。

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1 回答 1

5

no instance for ...错误几乎总是因为你的论点放错了地方。不幸的是，Possible fix: add an instance for...建议通常是错误的做法。

在这种情况下，因为member:: ( Ord a, Show a ) => Tree a -> a -> Bool它首先想要它的树，然后是元素，所以你需要尝试member intTree 5而不是member 5 intTree.

于 2013-05-06T09:08:21.200 回答