Did you also already need Random instances of enumerations? Wouldn't it be
nice to have some helper functions in System.Random:
enumRandomR :: (Enum a, RandomGen g) => (a,a) -> g -> (a,g)
enumRandomR (l,r) =
mapFst toEnum . randomR (fromEnum l, fromEnum r)
boundedEnumRandom :: (Enum a, Bounded a, RandomGen g) => g -> (a,g)
boundedEnumRandom = enumRandomR (minBound, maxBound)
mapFst :: (a -> c) -> (a,b) -> (c,b)
mapFst f ~(x,y) = (f x, y)
Example application:
instance Random Day where
random = boundedEnumRandom
randomR = enumRandomR
