cat.hs

-- ページ
data Page = Page [String]

-- 文字列をページに変換
toPage :: String -> Page
toPage = Page . lines

main = getContents >>= print . toPage

instance Show Page where
    show (Page ls) = unlines $ map (padding 6) (zip [1..] ls)

-- 行 line に幅 width で行番号 n をふる
padding width (n,line) = spaces ++ num ++ "  " ++ line
    where
      spaces = replicate (width - length num) ' '
      num    = show n