Main.hs

{-# LANGUAGE OverloadedStrings, TupleSections #-}

import Control.Monad
import Control.Lens
import Data.Function
import Data.List
import Data.Ord
import qualified Data.Text.Lazy.IO as TL
import qualified Text.Taggy.Lens as T

main :: IO ()
main = do
    allElements <- TL.getContents <&> toListOf (T.html . T.elements . T.allNamed (only "body") . to universe . traverse)
    let result = sortBy (comparing $ Down . length . snd) [ (e, xs) | e <- allElements, xs <- groupByElement (children e) ]
    forM_ result $ \(e, xs) -> print (T.eltName e, map T.eltName xs)
  where groupByElement = groupBy ((==) `on` view T.name) . sortBy (comparing $ view T.name)