[Haskell-beginners] Enumerated types

Brent Yorgey byorgey at seas.upenn.edu
Sun Dec 27 09:14:45 EST 2009

On Sun, Dec 27, 2009 at 02:42:48PM +0100, legajid wrote:
> 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

This is a type error since putStrLn expects a String and pref is a
list of Authors.  But the 'deriving Show' means that you can use the
'show' function to convert:

  disp_pref = mapM_ (putStrLn . show) pref

In fact, since  print = putStrLn . show, you can just say

  disp_pref = mapM_ print pref

> 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

Yes, with toEnum.  Just use e.g. 'toEnum 2'.  How does it know that
you want an Author and not some other Enum type?  Why, through the
magic of type inference!  As long as you use 'toEnum 2' in a context
where an Author value is expected it will evaluate to 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 ?

Yes, if you add 'Bounded' to the list of derived classes, then
'maxBound' gives you the last one.


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