I wrote some code as part of my effort to do backtracking search of musical structures. Most recently I wrote a function that "pruned" the search tree by looking for illegal combinations of notes. In the interest of efficiency I wanted this algorithm to terminate as soon as it encountered an illegal combination rather than hunt the entire structure for every illegality. This made me think of the laziness of "or": a function like g = x || y || z will not evaluate y and z if x turns out to be true. I was also writing this code within a stack of monad transformers: State, ErrorT, and WriterT. This was primarily for debugging reasons: I wanted to have ErrorT to give good error messages with context, WriterT to log events for later perusal, and State was there for convenience. I called this monad stack "Er." So the first thing I wrote was something like this: -- return True if an illegal combination is found prune :: Music -> Er Bool prune music = do flags <- forM [0..size music-1] (\ix -> checkOneNote music ix) return $ or flags checkOneNote :: Music -> Int -> Er Bool checkOneNote music ix = do tell $ "Checking note " ++ show ix ++ "\n" -- note: were thunks created by following line left unevaluated thanks to laziness of "or flags" above? doSomething music ix I already knew from past experience that mapM (and forM) evaluate every computation within the list so I wasn't really expecting this to be lazy, and it wasn't. Judging from what was "told" to my MonadWriter, I could see it was checking every note and not stopping early at the first illegal note. My first question is: by any chance was this an illusion through my use of "tell"? What I mean, is that was it leaving thunks unevaluated thanks to the "or flags" expressions lazy behavior, or would it have left them unevaluated if I hadn't had the "tell" there? I rewrote this using this function: pruneRecursive :: Music -> Int -> Int -> Er Bool pruneRecursive music maxIx ix | ix == maxIx = return False | do flag <- checkOneNote music maxIx ix if flag then return True else checkOneNote music maxIx (ix+1) According to the writer messages, this was not doing any unnecessary work. Just wondering if it was really necessary to rewrite this. What if I removed the tell messages later, or put them inside 'when' like this when (debugFlag) (tell ..) and then set debugFlag to False for time-critical program runs? Dennis