如何将二叉树的字符串表示形式读入树中？

Question

我有以下代码，它接受输入作为ex中给出的示例，并创建一个具有给定类型的树：

data QA = Leaf String | Question QA String QA

ex :: QA
ex = (Question 
       (Question (Leaf "Marie Curie") "Is she a scientist?" (Leaf "Queen Elisabeth II"))
       "Is she from Europe?"
       (Question (Leaf "Marilyn Monroe") "Is she an actress?" (Leaf "Hilary Clinton")))

showQa :: QA -> String 
showQa (Leaf x) = "(" ++ x ++ ")"
showQa (Question left x right) = "(" ++ x ++ showQa left ++ showQa right ++ ")"

instance Show QA where
  show = showQa

然后，我使用showQa函数将树转换为字符串，以便能够将其存储在文件中，输出如下字符串：

"(Is she from Europe?(Is she a scientist?(Marie Curie)(Queen Elisabeth II))(Is she an actress?(Marilyn Monroe)(Hilary Clinton)))"

问题是，当我再次从文件中读取字符串时，如何将这个字符串转换为原始类型的树。

Answer 1

使QA成为显示和读取的实例

data QA = Leaf String | Question QA String QA deriving (Show, Read)

您可以通过这种方式摆脱showQa，只需调用QA类型的显示即可。读书也是如此。

当你写到文件时你可以

writeFile "[FileName].qa" (show ex)

" ex“应该是QA类型的名称，在您的例子中称为ex

从文件中读取，只需使用readFile并读取结果：

fileReader = do
    content <- tryIOError (readFile "[FileName].qa")
    case content of
        Left e  -> do
            return (ex) --In case of reading file goes wrong
        Right r -> do 
            return (read r)