Loading/Modifying/Saving Changes to Text File - haskell

Question

Allow me to start by sharing what I have so far:
main :: IO ()
main = do contents <- readFile "filmList.txt"
let database = (read contents :: [Film])
putStr "Please enter your username: "
userName <- getLine
menu database
where menu newDb = do putStrLn "\nPlease select an option:"
putStrLn "1: Display all films currently in the database"
putStrLn "2: Add a new film to the database (and display all films)"
putStrLn "3: "
putStrLn "4: Save Database"
putStrLn "5: Exit"
putStr "\nSelected option: "
option <- getLine
case option of "1" -> putStrLn(displayFilms newDb)
"2" -> do putStr "Name of film: "
title <- getLine
putStr "Name of director: "
director <- getLine
putStr "Year of release: "
year <- getLine
putStrLn(displayFilms (addNewFilm title director (read year) newDb))
menu newDb
`
I have managed to load a txt file with the database of Film types but I cannot figure out how to go about actually making changes to the data. When I try to run option 2, I get a list of all the films with the newly added one as well but if I then run option 1 to list all films, it doesn't include the newly added film. Should I be saving back to the txt file each time a new Film instance is added? Any help is greatly appreciated, thank you!

Answer 1

Keep in mind that everything in Haskell is immutable. When you write addNewFilm (addNewFilm title director (read year) newDb, that doesn't change the database represented by newDb to include the new information; it merely returns a new database that has all the old information plus the new row. It's your responsibility to choose where and when to use the old database (named, ironically, newDb) or the new one returned by the function.
For example, you might write this case instead:
case option of "1" -> putStrLn (displayFilms newDb) >> menu newDb
"2" -> do ...
let evenMoreReallyNewDb = addNewFilm title director (read year) newDb
putStrLn (displayFilms evenMoreReallyNewDb)
menu evenMoreReallyNewDb
You might notice that both cases end with a very similar pattern (a putStrLn . displayFilms and then a menu). Unifying them would be a good idea; see if you can see how to do that yourself.
If you want to write the modified database back to the file, you will of course need to do that explicitly using writeFile or a similar function.