[Haskell-cafe] Haskell IO problem

Tillmann Rendel rendel at cs.au.dk
Sun May 10 11:23:00 EDT 2009

Hi apple,

applebiz89 wrote:
> // Say I have this as my data type and list of films
> data Film = Film String String Int [String]
> -- List of films
> testDatabase :: [Film]
> testDatabase = [(Film "Casino Royale" "Martin Campbell" 2006 ["Garry",
> "Dave", "Zoe"]) ]
> // with functions such as:
> becomeFan :: String -> String -> [Film] -> [Film]
> filmsInGivenYear :: Int -> [Film] -> [String]
> // I want to ask the user what function they want to use

Once again, it is important to write small blocks of code first. Let's 
write an IO wrapper around one of the functions, e.g. around 

-- Given a films database, this operation asks the user for a year,
-- and then prints the names of all the films from that year.
doFilmsInGivenYear :: [Film] -> IO ()
doFilmsInGivenYear films = do
   -- an action
   putStrLn "which year?"

   -- an action which returns a result
   text <- getLine

   -- some pure computations
   let year = read text :: Int
   let answer = filmsInGivenYear year films

   -- an action
   print answer

For now, we want main to be just doFilmsInGivenYear using the testDatabase:

   main :: IO ()
   main = doFilmsInGivenYear films

If you compile and run that program, it should ask you for a year, and 
then print the films of that year. Then it exists, because there are no 
more actions to do.

If you want the program to go on asking you stuff, you can call main again:

   main :: IO ()
   main = do
     doFilmsInGivenYear films

If you compile this program and run it, it will run forever and keep 
asking you about years. (Strg-C should stop it again).

*After you have tried that out*, you could try to add code to main to 
ask the user whether she wants to stop.


PS. Please don't send questions only to me, I am not your private tutor 
:) Send to the list instead.

PPS. It is really important to compile your program often, and/or test 
it with ghci. It is much easier to do one small step after another

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