Hi Thomas This is "iffy versus miffy", a standard applicative problem. When you use the result of one computation to choose the next computation (e.g., to decide whether you want to keep doing-and-taking), that's when you need yer actual monad. It's the join of a monad that lets you compute computations. The applicative interface does not allow any interference between the value and computation layers. It's enough for effects which facilitate but do not determine the flow of computation (e.g. threading an environment, counting how often something happens, etc...). So, you ask a sensible... On 4 May 2009, at 22:15, Thomas Hartman wrote:
{-# LANGUAGE NoMonomorphismRestriction #-} import Data.List import Control.Monad import Control.Applicative
-- Can the function below be tweaked to quit on blank input, provisioned in the applicative style? -- which function(s) needs to be rewritten to make it so? -- Can you tell/guess which function(s) is the problem just by looking at the code below? -- If so, can you explain what the strategy for doing so is?
...nostril question.
notQuiteRight = takeWhile (not . blank) <$> ( sequence . repeat $ echo )
^^^ Here, we're doing all the computations, then post-processing the values with a pure function. There's no way the pure function can tell the computation to stop bothering.
echo = do l <- getLine putStrLn l return l
-- this seems to work... is there a way to make it work Applicatively, with lifted takeWhile? seemsToWork = sequenceWhile_ (not . blank) (repeat echo)
sequenceWhile_ p [] = return () sequenceWhile_ p (mx:mxs) = do x <- mx if p x ^^^ Here, you're exactly using the result of a computation to choose which computations come next. That's a genuinely monadic thing to do: miffy not iffy.
then do sequenceWhile_ p mxs else return ()
If you're wondering what I'm talking about, let me remind/inform you of the definitions. iffy :: Applicative a => a Bool -> a t -> a t -> a t iffy test yes no = cond <$> test <*> yes <*> no where cond b y n = if b then y else n miffy :: Monad m => m Bool -> m t -> m t -> m t miffy test yes no = do b <- test if b then yes else no Apologies for slang/pop-culture references, but "iffy" means dubious, questionable, untrustworthy "miffy" is a cute Dutch cartoon character drawn by Dick Bruna The effect of iffy askPresident launchMissiles seekUNResolution is to ask the President, then launch the missiles, then lobby the UN, then decide that the result of seeking a UN resolution is preferable. Remember folks: Missiles need miffy! Cheers Conor