fill.hs

-- Seq {aa :: String, gaps :: [(Int, Int)]}
fill' :: Int -> Seq -> String
fill' n Seq {aa = aa, gaps = gaps} = go (sortOn fst gaps) (zip [1 ..] aa) n
  where
    go :: [(Int, Int)] -> [(Int, Char)] -> Int -> String
    go _ [] l = replicate l '-'
    go [] str l = map snd str ++ replicate l '-'
    go gps@((start, leng):gs) ((i, c):xs) l
      | start <= i = replicateAppend leng '-' (c : go gs xs (l - leng - 1))
      | otherwise = c : go gps xs (l - 1)
      
replicateAppend :: Int -> Char -> String -> String
replicateAppend n c str = foldr (\_ b -> c : b) str [1 .. n]