guard True = return () guard False = [] -- the textbook example f = do x <- [1..4] y <- [2..8] guard (x*y==8) return (x,y) -- the translation. f' = [1..4] >>= (\x -> [2..8] >>= (\y -> (if (x*y == 8) then return () else []) >> return (x,y))) -- another one. note that we can move the final -- return out of the parens. It would even work without it, but we'd -- get a list containing many empty lists: concat fixes that. f'' = [1..4] >>= (\x -> [2..8] >>= (\y -> if (x*y == 8) then [(x,y)] else [] >>= return )) -- bind desugared. Actually, it seems to be impossible to -- write this in monadic notation and I'm getting quite confused. f''' = concatMap (\x -> concatMap (\y -> if x*y == 8 then [(x,y)] else []) [2..8]) [1..4] -- That's the one I've been looking for. Remember that -- return e = [e] f'''' = concatMap (\x -> concatMap (\y -> concatMap (\_ -> [(x,y)]) (if x*y == 8 then [()] else [])) [2..8]) [1..4] Morale: Premature term elimination is the root of all misunderstanding. Haskell wins the wickedness of design contest by using [()] and [] as truth values. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.