Parse binary tree from file in Haskell - haskell

Question

I have task to write Quiz Animal game. I have to read Tree data from file (the structure of file is free). After end of game is possible to be added new data in tree(actually (Animal) to be replace by (Node (Animal) (Animal)).
My problem is that i can't parse content of "database.txt" in AnimalsTree
Example "database.txt" (this content is generated with writing before call 'New Game'):
Question "Is it fly?" (Animal "Bird") (Question "Is it swim?" (Animal "Fish") (Animal "Dog"))
EDIT
This is full code:
module Main where
import System.IO
data AnimalsTree = Animal String | Question String AnimalsTree AnimalsTree deriving (Read, Show)
-- Видовете отговори на потребителя
data Answer = Yes | No
main :: IO ()
main = do root <- "database.txt"
play (read root)
return ()
play :: AnimalsTree -> IO AnimalsTree
play root = do putStrLn "Think of an animal, I will try to guess what it is..."
newRoot <- play' root
writeFile "database.txt" (show newRoot)
playAgain <- ask "Do you want to play again?"
case playAgain of
Yes -> play newRoot
No -> do putStrLn "Bye!"
return newRoot
play' :: AnimalsTree -> IO AnimalsTree
play' question#(Question q l r) = do ans <- ask q
case ans of
Yes -> do y <- play' l
return $ Question q y r
No -> do n <- play' r
return $ Question q l n
play' animal#(Animal _) = do ans <- ask $ "Are you thinking of " ++ show' animal ++ "?"
case ans of
Yes -> do putStrLn "I win! :)"
return animal
No -> do putStrLn "I give up, you win!"
getNewAnimal animal
getNewAnimal :: AnimalsTree -> IO AnimalsTree
getNewAnimal animal = do putStrLn "Please help me!"
putStrLn "What is name of yout animal?"
name <- getLine
let newAnimal = Animal name
putStrLn $ "Now please enter a question that answers yes for " ++ show' newAnimal ++ " and no for " ++ show' animal
question <- getLine
return $ Question question newAnimal animal
ask :: String -> IO Answer
ask s = do putStrLn $ s ++ " (Yes/No)"
getAnswer
getAnswer :: IO Answer
getAnswer = do ans <- getLine
putStrLn ""
case ans of
"y" -> return Yes
"Y" -> return Yes
"yes" -> return Yes
"Yes" -> return Yes
"n" -> return No
"N" -> return No
"no" -> return No
"No" -> return No
_ -> putStrLn "This is incorect answer! Please try again with value in 'Yes' or 'No'!" >> getAnswer
show' (Animal name) = (if elem (head name) "AEIOUaeiou" then "an " else "a ") ++ name
show' (Question q _ _) = q
But I get following error:
test3.hs:10:19:
No instance for (Read (IO AnimalsTree))
arising from a use of ‘read’
In a stmt of a 'do' block: root <- read "database.txt"
In the expression:
do { root <- read "database.txt";
play (root);
return () }
In an equation for ‘main’:
main
= do { root <- read "database.txt";
play (root);
return () }

Answer 1

This looks like a great start! I only needed to add readFile to your main function to get it to compile:
main = do root <- readFile "database.txt"
It is possible that you may need to switch from readFile to explicitly opening the file for reading, doing the reading, and closing the file. This is because readFile does not guarantee when it will close the file, and writeFile is allowed to fail if it is given a path to a file that's already open.
You can read the documentation for openFile, hGetLine, and hClose for further details on how to do this (but beware hGetContents, as it has the same caveats readFile does).