Hi, I'm trying to produce a list in the strict ST monad. The documentation of ST says that the monad is strict in the state, but not in the values. So I expect that, when returning a list, I get back only the Cons (with 2 unevaluated thunks). Now, when I need the first element (head), this will be evaluated (with whatever actions are necessary in the ST universe) and the tail is again a Cons with unevaluated parts. Internally my list is stored in a vector, and the elements are generated phasewise, each phase generating 0 or more elements in the vector, and a fuction splitMove is driving this process (see code below). I would expect that the first phase triggers, generates some moves, then (after these are consumed from the list) the next phase triggers generating the next few moves and so on. But when I trace the phases (Debug.Trace.trace) I get all the trace messages in front of the first move: Moves for fen: rnbqkbnr/pp3ppp/4p3/2pp4/3P4/2NQ4/PPP1PPPP/R1B1KBNR w After move generation... 0 >= 0 : next phase 3 >= 3 : next phase 3 >= 3 : next phase 42 >= 42 : next phase 44 >= 44 : next phase d4c5 g1f3 g1h3 c3b1 ... This seems not to be just an unhappy combination between trace and ST, as also the program without trace is beeing slower than the same implemented with plain lists, which is hard to believe (in many cases the move list is not consumed to the end). I wonder if my expectation is wrong, but I don't find a way to do this. Here is the (incomplete) code: produceList ... = runST $ do ml <- newMList ... listMoves ml -- Transforms a move list to a list of moves - lazy listMoves :: MList s -> ST s [Move] listMoves ml = do sm <- splitMove ml case sm of Just (m, ml') -> do rest <- listMoves ml' return $ m : rest Nothing -> return [] -- Split the first move from the move list and return it together with -- the new move list (without the first move). Return Nothing if there -- is no further move splitMove :: MList s -> ST s (Maybe (Move, MList s)) splitMove ml | mlToMove ml >= mlToGen ml = do mml <- trace trm $ nextPhase ml case mml of Nothing -> return Nothing Just ml' -> splitMove ml' | otherwise = do m <- U.unsafeRead (mlVec ml) (mlToMove ml) case mlCheck ml ml m of Ok -> return $ Just (m, ml1) Skip -> splitMove ml1 Delay -> splitMove ml1 { mlBads = m : mlBads ml } where ml1 = ml { mlToMove = mlToMove ml + 1 } trm = show (mlToMove ml) ++ " >= " ++ show (mlToGen ml) ++ " : next phase"