So... I must say I am rather pleased with the following code. It allows you to use any value of type Bounded and Enum as a member of Random, or Arbitrary, which means you can quickCheck properties on it as well. For quickchecking, the code below "cheats" by not defining the coarbitrary funciton, which I confess I don't really understand and never use. Even so, I can see myself using this in a number of places... Does it seem reasonable to add a ticket to get this added to http://hackage.haskell.org/packages/archive/QuickCheck/2.1.0.1/doc/html/Test... perhaps modulo the definition of an appropriate coarbitrary function? thomas. thartman@patchwiki:~/haskell-learning/testing>cat BoundedEnum.hs {-# LANGUAGE FlexibleInstances, UndecidableInstances, ScopedTypeVariables, OverlappingInstances #-} module Main where import Test.QuickCheck import System.Random class (Bounded a, Enum a) => BoundedEnum a instance (Bounded a, Enum a) => BoundedEnum a instance BoundedEnum a => Random a where random g = let min = fromEnum (minBound :: a) max = fromEnum (maxBound :: a) (i,g') = randomR (min,max) $ g in (toEnum i,g') randomR (low,high) g = let min = fromEnum low max = fromEnum high (i,g') = randomR (min,max) $ g in (toEnum i,g') instance BoundedEnum a => Arbitrary a where arbitrary = do let min = fromEnum (minBound :: a) max = fromEnum (maxBound :: a) i <- arbitrary return . toEnum $ min + (i `mod` (max-min)) coarbitrary = undefined data DayOfWeek = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday deriving (Show, Read, Eq, Enum, Ord, Bounded) t :: IO DayOfWeek t = randomIO t2 :: IO Int t2 = randomIO pDayEqualsItself :: DayOfWeek -> Bool pDayEqualsItself day = day == day -- a trivial property, just so we can show t3 = quickCheck pDayEqualsItself -- show what days are being tested t4 = verboseCheck pDayEqualsItself 2009/5/1 michael rice <nowgate@yahoo.com>:
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
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[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>
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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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