equi in Haskell

equi.hs

equi :: [Int] -> Maybe Int
equi [] = Nothing
equi ns = go 0 0 ns
  where
    fullSum = sum ns
    go _ _ [] = Nothing
    go n leftSum (x:xs) =
      let rightSum = fullSum - leftSum - x
      in if rightSum == leftSum
            then Just n
            else go (n + 1) (leftSum + x) xs