Diamond Kata in Haskell - First Attempt

diamond.hs

import Data.Traversable

main =
  traverse print (genAll 'J')

-- [_,_,A,_,_]
-- [_,B,_,B,_]
-- [C,_,_,_,C]
-- [_,B,_,B,_]
-- [_,_,A,_,_]

genAll :: Char -> [[Char]]
genAll z = top z ++ bottom z
  where
    top = genTop
    bottom = tailElse . reverse . top

-- [_,_,A,_,_]
-- [_,B,_,B,_]
-- [C,_,_,_,C]

genTop :: Char -> [[Char]]
genTop z = revList (processChars ['A'..z] [])

-- [_,_,A]
-- [_,B,_]
-- [C,_,_]

tailElse :: [a] -> [a]
tailElse [] = []
tailElse (_ : []) = []
tailElse (_ : xs) = xs

revList :: [[Char]] -> [[Char]]
revList l =
  fmap (\r -> (reverse r) ++ tailElse r) l

-- [A,_,_]
-- [_,B,_]
-- [_,_,C]

-- [A,_]
-- [_,B]

dup :: Char -> Int -> [Char] -> [Char]
dup _ 0 l = l
dup val i l = dup val (i - 1) (l ++ [val])

addRow :: Char ->  [[Char]] -> [[Char]]
addRow z lol =
  rows ++ [newRow]
  where
    rows = fmap (\l -> l ++ [' ']) lol
    newRow = (dup ' ' (length lol) []) ++ [z]

processChars :: [Char] -> [[Char]] -> [[Char]]
processChars [] arr = arr
processChars (c : cs) arr = 
  processChars cs (addRow c arr)

-- [A]