Read a rose tree separated by new lines and tabs and transform it to a Data.Tree

HierachicalParser.hs

import Data.List
import Data.Tree
import Control.Arrow (first)
import Data.Function (on)

toHierarchical :: String -> [(Int, [String])]
toHierarchical =  map simplify . groupBy ((==) `on` fst) . map (first length . span (== '\t')) . lines
    where 
        simplify = first head . unzip
        
toTree :: Ord t => [(t, [a])] -> [Tree a]
toTree [] = []        
toTree ((pos, nodes):xs) =  mapOnlyLast nodes ++ toTree xs'
    where 
        mapOnlyLast [] = []
        mapOnlyLast [x] = [Node x (toTree childs)]
        mapOnlyLast (x:xs) = Node x [] : mapOnlyLast xs
        (childs, xs') = span (\(i,_)-> i > pos) xs
        
main = (drawForest . toTree . toHierarchical) <$> readFile "testTab.txt" >>= putStrLn