Haskell version of diamond kata, complement to https://gist.github.com/ddellacosta/ba7e03951ba1bafd3ec9

gistfile1.hs

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

-- Generates a single diamond line with the alphabet char in the right position.
dline :: Int -> Int -> Int -> [Char] -> [Char] -> [Char]
dline len idx idx2 as ls
  | idx2 == (succ len) = ls ++ (reverse . init) ls
  | idx2 == idx = (dline len idx (succ idx2) as (ls ++ [(as !! (len - idx2))]))
  | otherwise = (dline len idx (succ idx2) as (ls ++ "-"))

-- Helper function to generate all the lines based on initial args.
diamond' :: Int -> Int -> [Char] -> [[Char]] -> [[Char]]
diamond' len idx as ds
  | idx == (succ len) = ((init . reverse) ds) ++ ds
  | otherwise = diamond' len (succ idx) as (ds ++ [dline len idx 1 as ""])

-- takes alphabet index n and alphabet as, returns list 
-- of diamond strings in the proper order. 
diamond :: Int -> [Char] -> [[Char]]
diamond n as = diamond' n 1 as []

-- can dump out a diamond like:
-- *Main λ > dump $ diamond 8 alphabet 
-- "-------A-------"
-- etc....
dump :: [[Char]] -> IO ()  
dump [] = return ()  
dump (x:xs) = do  
    print x  
    dump xs