Convert Maybe Int to Int in Haskell

Question

I am working on the following code and wanted to find the index of the number in the box string. So i used findIndex but it returns the Maybe Int value whereas i want only Int value.

How can i convert Maybe Int to Int value or is there any way in which i can extract Int from Maybe Int. The code should print an error message if Maybe Int is nothing

box:: String
box = unlines $ ["0 | 1 | 2",
                 "---------",
                 "3 | 4 | 5",
                 "---------",
                 "6 | 7 | 8"]

moves = do
        putStrLn " Enter the number"
        number <- readLn :: IO Int
        print number
        findpostion number box

findposition number box = findIndex (==number) box

Answer 1

You can easily do this using pattern matching in your do statement:

case findposition number box of
  Just n  -> -- do whatever with n
  Nothing -> putStrLn "Invalid number!" -- you can handle the error however you want.

A good option would be to create a separate IO action to get the number:

getNumber = do putStrLn "Enter the number:"
               number <- readLn
               case findposition number box of
                 Just n  -> -- Do whatever
                 Nothing -> putStrLn "Please try again." >> getNumber

This way if the user enters an invalid number, it just asks again.

Also, as written now, your code won't work. You should have some other way of storing the numbers in box as actual numbers; right now, they're in Strings.

Answer 2

Obviously, it's not possible in general: when the search doesn't succeed there is no canonical integer return value, so you get a Nothing without any such value then.

If you don't really care about the Nothing case (e.g. because you will always make sure there is one such element) you can use the fromJust function out of Data.Maybe, which you can also quickly implement yourself:

findposition number = (\(Just i)->i) . findIndex (==number)

However that's not really recommendable because you will need to make sure this doesn't break, and doing this is much easier by means of a proper pattern matching.