Hi,
On Sun, Dec 27, 2009 at 8:42 AM, legajid
Hello,
I have some trouble evaluating Enum class. Following is my source:
data Authors= Buzzati | Werber | Verne | Ray | Asimov | Voltaire deriving (Enum, Show)
auths=[Buzzati .. Verne] ++ enumFrom Ray pref=take 3 auths -- 1 disp_pref=mapM_ (putStrLn) pref
putStrLn wants a String as argument, here you are passing an Authors. Try this: disp_pref=mapM_ (putStrLn . show) pref
auth1 = Buzzati auth2 = succ auth1 -- Werber auth3 = pred Asimov -- Ray num_auth4=fromEnum Verne -- 2 -- 2 toEnum
You can use toEnum to get the nth constructor. For example: nthAuthor :: Int -> Authors nthAuthor i = toEnum i
main= display Buzzati display x = do putStrLn (show x) display (succ x) -- 3 end of enum
To use maxBound you need to make your type an instance of Bounded. You will also need to make it an instance of Eq in order to compare it. main= display Buzzati display x = do putStrLn (show x) if x == maxBound then return () else display (succ x) Patrick
1. could'nt match expected type [Char] against inferred type Authors I would like to display each data constructor name on one line , giving : Buzzati Werber Verne How can i translate data from one type to another (Authors to [Char])?
2. Like fromEnum gives data constructor index, is it possible with toEnum (or other way) to get the nth constructor of the given type ? eg : 2 applied to Authors (not in scope ?) would give Verne
3. tried to take 'succ' of last tag in enumeration How to detect the end of an enumeration ? Can i get the maxbound index ?
Thanks, Didier.
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-- ===================== Patrick LeBoutillier Rosemère, Québec, Canada