I was writing some haskell code for fun to solve some "knights and knaves" problems, and I have troubles with http://en.wikipedia.org/wiki/Knights_and_knaves#Question_3 So knights always tell the truth and knaves always lie. John and Bill are two persons, but you don't know what they are, and you have to find out. Question 3 You: Are you both knights? John: answers either Yes or No, but you don't have enough information to solve the problem. You: Are you both knaves? John: answers either Yes or No, and you can now solve the problem. My haskell code gave the following result: --(what is john, what is bill, first answer from john, second answer from john) ("John is a knight","Bill is a knight","Yes","No ") ("John is a knight","Bill is a knave ","No ","No ") ("John is a knave ","Bill is a knight","Yes","Yes") ("John is a knave ","Bill is a knave ","Yes","No ") Anyone has an idea what I missed here? Thanks, Peter PS: The (quick and dirty) haskell code I wrote for this is: infixl 1 <=> infixl 1 ==> -- Equivalence x <=> y = (x == y) -- Implication True ==> False = False _ ==> _ = True -- Knights tell the truth name `isa` True = name ++ " is a knight" -- Knaves always lie name `isa` False = name ++ " is a knave " answer True = "Yes" answer False = "No " --Logician: Are you both knights? --John answers either Yes or No, but the Logician does not have enough information to solve the problem. --Logician: Are you both knaves? --John answers either Yes or No, and the Logician can now solve the problem. condition j b answer1 answer2 = (j <=> ((b && j) == answer1)) && (j <=> ((not b && not j) == answer2)) solution = [("John" `isa` j, "Bill" `isa` b, answer answer1, answer answer2) | j <- [True,False], b <- [True, False], answer1 <- [True, False], answer2 <- [True, False], condition j b answer1 answer2 ] main = mapM_ putStrLn (map show solution)