beginner – Is it wrong to split this function into two?

I wrote this module to find the Hamming distance between two strings. (It’s a problem from exercism.io‘s Haskell track.)

As I saw it, the problem has two distinct parts: check if the length of the two strings are equal (if not return Nothing), and recursive pattern matching on equal-length strings.

Since my score is a monad (Maybe), I didn’t know how to implement the recursive adding without dealing with the Nothing case, so I broke it off into a separate function using a simple Int type.

GHC raises warning: (-Wincomplete-patterns) since my pattern matching in the helper function is incomplete. So:

  1. Is it bad practice to ignore this warning?
  2. Would it be better to implement the recursive part on the Maybe?
  3. Is there a better way to solve this problem?

I’m very new to Haskell, all help appreciated.

module Hamming (distance) where

distance :: String -> String -> Maybe Int
distance a b
  | length a /= length b = Nothing
  | otherwise = Just $ getDistance a b

getDistance :: String -> String -> Int
getDistance () () = 0
getDistance (x) (y) = if x == y then 0 else 1
getDistance (x : xs) (y : ys) = getDistance (x) (y) + getDistance xs ys