[Haskell-beginners] Enumerated types

legajid legajid at free.fr
Sun Dec 27 08:42:48 EST 2009


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

auth1 = Buzzati
auth2 = succ auth1       -- Werber
auth3 = pred Asimov      -- Ray
num_auth4=fromEnum Verne -- 2
-- 2 toEnum

main= display Buzzati
display x = do
        putStrLn (show x)
        display (succ x)
-- 3 end of enum

1. could'nt match expected type [Char] against inferred type Authors
I would like to display each data constructor name on one line , giving :
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 ?


More information about the Beginners mailing list