调试Haskell阅读功能

Question

我是Haskell的新手，我正在写一个简单的AI决策系统来玩20个问题的游戏风格。如果程序无法猜出正确的答案，它会要求提供一个问题来区分答案，并将该问题存储在树中。它在程序开始时从文件系统中读取树，并在最后将其写回。

我在Haskell中的序列化代码有很多问题。我收到错误“Prelude read：no parse”。这是怎么回事？这里是我的代码：

import Data.Char 
import System.IO 

-- Defines a type for a binary tree that holds the questions and answers 
data Tree a = 
     Answer String | 
     Question String (Tree a) (Tree a) 
     deriving (Read, Show) 

-- Starts the game running 
main = do 
     let filePath = "data.txt" 
     fileContents <- readFile filePath 
     animals <- return (read fileContents) 
     putStrLn "Think of an animal. Hit Enter when you are ready. " 
     _ <- getLine 
     ask animals 
     writeFile filePath (show animals) 

-- Walks through the animals tree and ask the question at each node 
ask :: Tree a -> IO() 
ask (Question q yes no) = do 
     putStrLn q 
     answer <- getLine 
     if answer == "yes" then ask yes 
          else ask no 
ask (Answer a) = do 
     putStrLn $ "I know! Is your animal a " ++ a ++ "?" 
     answer <- getLine 
     if answer == "yes" then computerWins 
          else playerWins 

computerWins = do putStrLn "See? Humans should work, computers should think!" 

playerWins = do putStrLn "TODO" 

而且这里是我使用的数据文件：

Question "Does it live in the water?" 
     ((Question "Does it hop?") (Answer "frog") (Answer "fish")) 
     (Answer "cow") 

Answer 1

read不打算可靠地处理之类的东西额外的括号。您的代码适用于以下数据文件：

Question "Does it live in the water?" 
     (Question "Does it hop?" (Answer "frog") (Answer "fish")) 
     (Answer "cow")