[Haskell-beginners] (no subject)

[Daniel Seidel]

Ashley Banks wrote:
> I havent done much IO at all in haskell, only within the function itself.
> However I want to get the input from the interface for the function and
> havent done this before.
> 
> // 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]
> becomeFan _ _ [] = []
> becomeFan filmName fanName ((Film title director year fans):xs) 
> | filmName == title = (Film title director year fanName:fans) : xs
> | otherwise = (Film title director year fans) : becomeFan filmName fanName
> xs
> 
> filmsInGivenYear :: Int -> [Film] -> [String]
> filmsInGivenYear filmYear films = [ title | (Film title director year fans)
> <- films, year == filmYear]
> 
> // I want to ask the user what function they want to use, I have this so far
> 
> main :: IO()
> do putStr "Hi there! what is your name: "
> fanName = getLine
> do putStr "1 = Insert film, 2 = Become a Fan, 3 = The number of fans of a
> film, 4 = Film released in a year: "
> input = getLine
> read input :: Int
> (if input == 1 then main x = insertFilm [] else if input == 2 then main x =
> becomeFan [] else if input == 3 then main x = numberOfFans [])

that part is kind of confusing. My Suggestion: separate functions for 
the "submenus". And a main function calling them:

submenu "2" fanName =
   do putStr "What film you want to become a fan of? "
      filmName <- getLine
      putStr (show  (becomeFan filmName fanName testDatabase))
      return ()
submenu _ _ =
     do putStr "The function is not supported yet."
        return ()


main :: IO()
main =
   do putStr "Hi there! what is your name: "
      fanName <- getLine
      putStr "1 = Insert film, 2 = Become a Fan, 3 = The number of fans 
of a film, 4 = Film released in a year, 5 = Exit: "
      input   <- getLine
      (if input /= "5"
       then
        do submenu input fanName
           main
       else return ())

The example should work, but is not very useful, for just adding 
yourself to the fan list of a movie in testDatabase. You might need to 
pass around the database. It's not like manipulating a global object as 
you might do in imperative languages.

> 
> // I thought using the if statement would work, but now I cant think how to
> gather the needed input for the function they have chosen?
> 
> thanks
> 
> apple
> 
> 
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