Converting number bases using Haskell

BaseConverter.hs

import Data.List
import Data.Maybe

digits = ['0'.. '9'] ++ ['A'.. 'Z'] ++ ['a'.. 'z']

fromBase10 :: Int -> Int -> String
fromBase10 0 _ = ""
fromBase10 number base 
                | base > 0  = fromBase10 (number `div` base) base ++ [digits !! (number `mod` base)]
                | otherwise = error "Invalid base"

toBase10 :: String -> Int -> Maybe Int
toBase10 num base 
       | num == [] || base <=0 = Nothing
toBase10 (x:[]) base = x `elemIndex` digits
toBase10 (x:xs) base = do digit <- toBase10 [x] base
                          xsVal <- toBase10  xs base
                          return (digit * base ^ (length xs - 1) + xsVal)

main = do
    putStr "Input the base 10 number: "
    inputNumber <- readLn :: IO Int

    putStr "Input the target base: "
    targetBase <- readLn :: IO Int
    
    putStrLn $ fromBase10 inputNumber targetBase