Question

I’m new to Haskell and I’m writing a simple AI decision system to play a 20 questions style of game. If the program is unable to guess the correct answer, it will ask for a question to use to distinguish the answer and it will store that question in a tree. It reads the tree in from the file system at the start of the program and writes it back out at the end.

I’m having a lot of problems with the serialization code in Haskell. I’m getting the error “Prelude read: no parse”. What’s going on? Here’s my code:

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"

And here’s the data file I’m using:

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

Answer 1

read is not intended to handle things like extra parentheses robustly. Your code works for me with the following data file:

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