props.hs

module Main where

import Control.Monad

data S = C | C1 | C2 | C3 | D | O Integer deriving (Eq, Show)
data A = A | Is deriving (Eq, Show)

props = [(C3, A, O 3), (C2, A, O 2), (C1, Is, C3), (C2, Is, C3), (C, Is, C1), (C, Is, C2), (D, A, O 0)]

find f s lst = [x | x@(s', _, _) <- lst, s == s', f x] `mplus` concat [find f p lst | (s', Is, p) <- lst, s == s']

main = print $ map trd $ find f C props where
	f (_, a, _) = a == A
	trd (_, _, x) = x