Haskell IO Recursion (with binary tree)

Question

I'm new to Haskell and I'm loving it so far, but I'm struggling with some aspects of IO. I'm making a sort of simple "Akinator" application and I'm having trouble understanding how to modify a binary tree using IO recursion. I have my own data type:

data QA = P String | Q QA String QA   -- P representing person, Q representing a question
 deriving (Show, Read)

We go down the tree by answering a question with yes or no. Yes takes you to the left and no takes you to the right. But when I get to the end of the tree while we run the program (we reach a person), I want to be able to modify the tree by adding another QA. (We ask the user for input in case the person was not found)

The function i use is:

play :: QA -> IO QA

How do you convert the QA data type to a IO QA during the recursion and return the same QA (in IO format) but with an added Leaf/Branch?

Thank you very much!

-------Some of my code--------

play :: QA -> IO QA
play (P s) = do
   a <- yesNoQuestion ("Is ( " ++s ++" ) your person?")
   case a of
      True -> do
          putStrLn "I win omg so ez!"
          exitSuccess -- (to exit the application without modifying
                      -- dunno if this is a good solution)
      False -> do
        p <- question "Just curious: Who was your famous person?"
        n <- question "Give me a question for which the answer for this person is yes"
        return q -- (Here I want to return a new QA with an added question -- 
                 --and with a leaf representing a person)


play (Q qaY s qaN) = do
    a <- yesNoQuestion s
    case a of
        True -> do
            pY <- play qaY
            return pY -- (I want this to say (return Q pY s qaN) But this 
                      -- does not work 
          --  since all types aren't IO)
        False -> do
            pN <- play qaN
            return pN -- (Same as previous)

Li-yao Xia · Accepted Answer · 2020-10-20 04:35:59Z

2

You can write

            return (Q pY s qaN)

play (Q qaY s qaN) = do
    a <- yesNoQuestion s
    case a of
        True -> do
            pY <- play qaY
            return (Q pY s qaN)
        False -> do
            pN <- play qaN
            return (Q qaY s pN)

answered Oct 20, 2020 at 4:35

Li-yao Xia

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