Quickcheck card generator

Question

Ok, so I hacked this :

prop = forAll genCards $ \cards -> collect (go cards == True) $ isFlush cards == go cards

go cards = (length . nub .  map (\w -> last w)) cards == 1

genCard :: Gen String
genCard = elements[ "1C", "2C", "3C", "4C", "5C", "6C", "7C", "9C", "KC", "QC", "JC", "1H", "2H", "3H", "4H", "5H", "6H", "7H", "9H", "KH", "QH", "JH", "1S", "2S", "3S", "4S", "5S", "6S", "7S", "9S", "KS", "QS", "JS", "1D", "2D", "3D", "4D", "5D", "6D", "7D", "9D", "KD", "QD", "JD"] 

genCards :: Gen [String]
genCards = do
  replicateM 5 genCard

However there has to be a better way, but I could not figure out how to combine a generator for numbers + one for suits, and a way to ensure there are more flushes generated (it is ~0.4% by random chance according to collect).

Answer 1

To generate a card:

genVal = elements "123456789JQK"
genSuit = elements "CHSD"

genCard = do
    val <- genVal
    suit <- genSuit
    return [val, suit]

To generate a flush:

genFlush = do
    vals <- replicateM 5 genVal
    suit <- genSuit
    return [[val, suit] | val <- vals]

My genFlush , like your genCards , does not guarantee that the cards generated in this way are distinct. You can select a given probability for a flush vs. a normal hand with frequency ; eg for 60% flushes:

genHandThatIsProbablyAFlush = frequency
    [ (3, genFlush)
    , (2, genCards)
    ]