How to convert Integer in String and String in Integer in Haskell?

Question

I'm still learning Haskell and I'm really confused with this language... i have to implement two functions fromInteger :: Integer -> String and toInteger :: String -> Integer that translate between Haskell integers and numbers by strings of digits in reverse order, like: "25" -> "52". For the sake of functional decomposition i should first implement fromDigit :: Integer -> Char and toDigit :: Char -> Integer that translate between digits and characters. How should the function look like? Thanks a lot!

Answer 1

You can use read :: Read a => String -> a and show :: Show a => a -> String to convert from Integer and to a String:

Prelude> show 52
"52"
Prelude> read "52" :: Integer
52

But you do not need to convert values to a string to reverse the order of the digits. You can use recursion and quotRem :: Integral a => a -> a -> (a, a) to obtain the quotient and remainder.

You can also use recursing to convert an Integer to a String. Here we use as accumulator a String, and we each time calculate the last digit. This thus looks like:

fromInteger :: Integer -> String
fromInteger = go []
    where go ls x | x <= 9 = … : …
                  | otherwise = go (… : ls) …
                  where (q, r) = quotRem x 10

where you still need to fill in the … parts.