Need Help With Haskell Conversion

[Hejime]

hi everyone

i want to ask a help regarding my haskell homework

here's the scenario

i want to take whatever which exists within a .txt file and then create index where all the words which have the length of 3 and above will be saved into another output file in list form

i'm having problems with the conversion Doc -> [([Int], Word)] and vise versa back to String or Doc so i can write into output file