G'day all. On Wed, Nov 20, 2002 at 08:44:36PM -0500, Mike T. Machenry wrote:

I am trying to construct an infinate list of pairs of random colors. I am hung up on getting a random color. I have:

data Color = Blue | Red | Green deriving (Eq, Ord, Show)

am I supposed to instantiate a Random class instance from color?

You could derive instances of Enum (and possibly also Bounded) and create random elements that way. Cheers, Andrew Bromage

"Mike T. Machenry" wrote:

Andrew and list,

I am a beginer. I really don't know what I would do if I derived Color from Enum. You say I could create elements that way. Is there some simple example someone could post to the list? Thank you for your help.

Here's one way to generate random values from an enumerable set: import Random randomR_Enum :: (Enum a, RandomGen g) => (a, a) -> g -> (a, g) randomR_Enum (lo, hi) gen = (toEnum val, gen') where (val, gen') = randomR (fromEnum lo, fromEnum hi) gen random_EnumBounded :: (Enum a, Bounded a, RandomGen g) => g -> (a, g) random_EnumBounded = randomR_Enum (minBound, maxBound) data Color = Blue | Red | Green deriving (Show, Enum, Bounded) instance Random Color where randomR = randomR_Enum random = random_EnumBounded main = do stdGen <- getStdGen print (take 10 (randoms stdGen :: [Color])) Dean

On Thu, 21 Nov 2002, Mike T. Machenry wrote:

This program yeilds: Fail: toEnum{Color}: tag (4) is outside of enumeration's range (0,3) in ghc... any ideas why?

-mike

Hmm. The program works fine for me, using GHC 5.04.1 or Hugs of Oct 2002. Dean

On Thu, Nov 21, 2002 at 10:07:12AM -0500, Dean Herington wrote:

...

"Mike T. Machenry" wrote:

...

Andrew and list,

I am a beginer. I really don't know what I would do if I derived Color from Enum. You say I could create elements that way. Is there some simple example someone could post to the list? Thank you for your help.

Here's one way to generate random values from an enumerable set:

import Random

randomR_Enum :: (Enum a, RandomGen g) => (a, a) -> g -> (a, g) randomR_Enum (lo, hi) gen = (toEnum val, gen') where (val, gen') = randomR (fromEnum lo, fromEnum hi) gen

random_EnumBounded :: (Enum a, Bounded a, RandomGen g) => g -> (a, g) random_EnumBounded = randomR_Enum (minBound, maxBound)

data Color = Blue | Red | Green deriving (Show, Enum, Bounded) instance Random Color where randomR = randomR_Enum random = random_EnumBounded

main = do stdGen <- getStdGen print (take 10 (randoms stdGen :: [Color]))

Dean