When you derive Enum, you get fromEnum and toEnum for free. You don't need your dow2Int stuff or int2Dow. Replace those with fromEnum and toEnum respectively.

/jve

On Fri, May 1, 2009 at 12:26 PM, michael rice <nowgate@yahoo.com> wrote:

I'm using the code below to generate random days of the week [Monday..Sunday].

Is there a better/shorter way to do this?

Michael

==============

[michael@localhost ~]$ ghci dow
GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( dow.hs, interpreted )
Ok, modules loaded: Main.
*Main> random (mkStdGen 100) :: (DayOfWeek, StdGen)
Loading package old-locale-1.0.0.1 ... linking ... done.
Loading package old-time-1.0.0.1 ... linking ... done.
Loading package random-1.0.0.1 ... linking ... done.
(Friday,4041414 40692)
*Main> random (mkStdGen 123) :: (DayOfWeek, StdGen)
(Tuesday,4961736 40692)
*Main>

==============

import System.Random

data DayOfWeek
    = Monday
    | Tuesday
    | Wednesday
    | Thursday
    | Friday
    | Saturday
    | Sunday
    deriving (Show, Read, Eq, Enum, Ord, Bounded)

instance Random DayOfWeek where
randomR (a,b) g =
      case (randomIvalInteger (toInteger (dow2Int a), toInteger (dow2Int b)) g) of
        (x, g) -> (int2Dow x, g)
       where
         dow2Int Monday    = 0
         dow2Int Tuesday   = 1
         dow2Int Wednesday = 2
         dow2Int Thursday = 3
         dow2Int Friday    = 4
         dow2Int Saturday = 5
         dow2Int Sunday    = 6

   int2Dow 0   = Monday
   int2Dow 1   = Tuesday
   int2Dow 2   = Wednesday
   int2Dow 3   = Thursday
   int2Dow 4   = Friday
   int2Dow 5   = Saturday
   int2Dow 6   = Sunday

random g      = randomR (minBound,maxBound) g

randomIvalInteger :: (RandomGen g, Num a) => (Integer, Integer) -> g -> (a, g)
randomIvalInteger (l,h) rng
| l > h     = randomIvalInteger (h,l) rng
| otherwise = case (f n 1 rng) of (v, rng') -> (fromInteger (l + v `mod` k), rng')
     where
       k = h - l + 1
       b = 2147483561
       n = iLogBase b k

       f 0 acc g = (acc, g)
       f n acc g =
          let
       (x,g')   = next g
      in
      f (n-1) (fromIntegral x + acc * b) g'

iLogBase :: Integer -> Integer -> Integer
iLogBase b i = if i < b then 1 else 1 + iLogBase b (i `div` b)

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--
/jve