You can't derive Enum Player automatically, but you can program it. Here's one way how, using shorter example names. -- Dean data E1 = E1a | E1b | E1c deriving (Enum, Bounded, Show, Read) data E2 = E2a | E2b | E2c deriving (Enum, Bounded, Show, Read) data E = E1 E1 | E2 E2 deriving (Show, Read) instance Enum E where fromEnum (E1 e1) = fromEnum e1 fromEnum (E2 e2) = size_E1 + fromEnum e2 toEnum i | i < size_E1 = E1 (toEnum i) | otherwise = E2 (toEnum (i - size_E1)) instance Bounded E where minBound = toEnum 0 maxBound = toEnum (size_E1 + size_E2) size_E1 = fromEnum (maxBound :: E1) + 1 size_E2 = fromEnum (maxBound :: E2) + 1 Hal Daume III wrote:
succ Blue cannot return MrX unless Blue and MrX have the same type. What I meant was that you would say:
'succ (Detective Red)' ==> Detective Green 'succ (Detective Green)' ==> Detective Blue 'succ (Detective Blue)' ==> Fugitive MrX
-- Hal Daume III
"Computer science is no more about computers | hdaume@isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume
On Thu, 20 Feb 2003, Mike T. Machenry wrote:
I tried this. It doesn't work. succ Blue is an exception. Anybody else know how this should be done?
Thanks, -mike
On Thu, Feb 20, 2003 at 08:00:04AM -0800, Hal Daume III wrote:
Question 1: Is there an easier, more elegant way to write this code?
For the most part, no.
Question 2: Is there a way to express the following relationship? I want to have a set of symbols with ordering and another set that is part of that ordering but with a different parent. For exammple,
data Player = Detective | Fugitive deriving (Enum) data Detective = Red | Green | Blue deriving (Enum) data Fugitive = MrX deriving (Enum)
How about something like:
data Player = Detective Detective | Fugitive Fugitive deriving (Enum) data Detective = Red | Green | Blue deriving (Enum) data Fugitive = MrX deriving (Enum)
(I'm not sure if the deriving Enum on Player will be exactly what you want -- I think so though. I don't derive this class very often.)
Then you can test detectiveness by:
isDetective (Detective _) = True isDetective _ = False
HTH
- Hal