Success story: novice haskeller moves console cursor with “netwire” FRP library
Hey, I’m definitely novice haskeller (just to point out: couple of weeks ago I’ve had understood State monad and monad transformers, before that I lived without state for almost 6 months), so I think it is just time to jump into FRP. Men say “FRP is cool” and “FRP is not slow” and “Conal Elliot, whah! beg beg beg”, so I, an novice hero have to tame that beast while the programming world starts to shift to Haskell!
So, what library for FRP sholud I use? Hm, lets examine Hackage…
Ouch. Large choice make me cry ‘( Need something else.
StackOverflow: What’s the status of current Functional Reactive Programming implementations?
netwire. Seems like installing
netwire will be faster, because typing
cabal install netwire is faster than
cabal install reactive-banana.
Me, remind myself, why do I need FRP? Oh, yes, I’ve recently done a dozen of variations of console tetris and have not forgot all the pain for dealing with timers, state, input, game loop and laziness. I am adequate and I don’t want to rewrite that fully with such a new thing for me as FRP. So let’s take a small subtask – move console cursor on screen with keyboard (discovered
System.Console.ANSI recently) – and do that right way!
Let’s start with reading documentation. Hm… Sure… Wow… Ehh… Aha, here it is, an Example:
module Main where import Control.Monad.Identity (Identity) import Control.Wire import Prelude hiding ((.), id) import Text.Printf testApp :: Wire () Identity a Time testApp = timeFrom 10 main :: IO () main = loop testApp clockSession where loop w' session' = do (mx, w, session) <- stepSessionP w' session' () case mx of Left ex -> putStrLn ("Inhibited: " ++ show ex) Right x -> putStrLn ("Produced: " ++ show x) loop w session
Runnig this we have:
Produced: 12.8301619000001 Produced: 12.8371623000001 Produced: 12.844162700000101 Produced: 12.849163000000102 Produced: 12.854163200000102 Produced: 12.860163600000101 Produced: 12.866163900000101 Produced: 12.871164200000102 Produced: 12.880164700000103 Produced: 12.885165000000104 Produced: 12.893165500000103 Produced: 12.900165900000104 Produced: 12.905166200000105 Produced: 12.912166600000106 Produced: 12.918166900000106 Produced: 12.924167300000105
Wow! But what changes should I do, to stop it printing after e.g. 15 seconds? Diving to hackage docs…
Effect… It is definitely hard to find something, for which you do not know the name… Maybe that –
when? Let’s try. Other examples combine wires with composition, so do I:
testApp :: Wire () Identity a Time testApp = when (< 15) . timeFrom 10
Produced: 14.961283800000212 Produced: 14.968284200000213 Produced: 14.974284500000213 Produced: 14.982285000000212 Produced: 14.987285300000213 Produced: 14.993285600000213 Inhibited: () Inhibited: () Inhibited: () Inhibited: () Inhibited: ()
Cool! Not what I wanted, but output changed after 15 seconds. Win.
After examining example a bit carefully, I see the global loop. Documentation says nothing about neediness of global loop in wire program, so I can only assume – it is necessary. To make things simpler, let’s abstract from it:
control whenInhibited whenProduced wire = loop wire clockSession where loop w' session' = do (mx, w, session) <- stepSessionP w' session' () case mx of Left ex -> whenInhibited ex Right x -> whenProduced x loop w session
Now we can run wires like this:
main = control return (putStrLn . show) $ when (< 15) . timeFrom 10
14.966284000000254 14.969284200000255 14.974284500000255 14.978284700000255 14.981284900000256 14.985285100000256 14.990285400000257 14.995285700000258 14.999285900000258 Interrupted. *Main>
Do you see it? The output has stopped printing after 15 seconds gone! Great discovery, this will help me later, no doubt.
Dealing with console keyboard is not a hard task for a man, who wrote console tetris. Let’s reuse our
keyPressed :: IO (Maybe Char)
foreign import ccall unsafe "conio.h getch" c_getch :: IO Char foreign import ccall unsafe "conio.h kbhit" c_kbhit :: IO Bool keyPressed = do isKey <- c_kbhit if isKey then Just <$> c_getch else return Nothing
Now we have to create wire from that function. How? Let’s read documentation. Hm.. Hm.. Hm-hm. As I see, the only wire creators are in
Control.Wire.Wire, and start with “
mk“. As I see, dealing with IO effects can only be done via
mkGen (but I’m not that sure)
mkGen :: (Time -> a -> m (Either e b, Wire e m a b)) -> Wire e m a bSource Construct an effectful wire from the given function.
Ok, maybe this way?
pressedKeyMaybe = mkGen isKey where isKey time () = do ky <- keyPressed return (Right ky, pressedKeyMaybe) -- strange, why do I use same -- function in the snd of the tuple?
Let’s test it:
main = control return (putStrLn . show) $ pressedKeyMaybe
Ha-ha! It doesn’t compile. Warum?
Couldn't match expected type `Identity' with actual type `IO' Expected type: Wire () Identity () b0 Actual type: Wire () IO () (Maybe Char) In the second argument of `($)', namely `pressedKeyMaybe' In the expression: control return (putStrLn . show) $ pressedKeyMaybe Failed, modules loaded: none. Prelude>
I’m lucky and this problem went off with simple replace
Nothing Nothing Just 'j' Nothing Nothing Nothing Nothing Nothing Just 'k' Nothing Nothing Nothing Nothing Nothing Nothing Nothing
Just to make it less verbose:
main = control return (putStrLn . show) $ when (/= Nothing) . pressedKeyMaybe
*Main> main Just 'h' Just 'e' Just 'l' Just 'l' Just 'o' Just ' ' Just 'n' Just 'e' Just 't' Just 'w' Just 'i' Just 'r' Just 'e' Interrupted. *Main>
It’s great time I live in, excuse me for my bad English and egocentrism.
(BTW, I give thanks to netwire author for the “wackelkontakt” mention in the doc. Because it is in fact an effectfull wire, so it can be used as example implementation:
wackelkontaktM :: (MonadRandom m, Monoid e) => Double -- ^ Occurrence probability. -> Event e m a wackelkontaktM p = mkFixM $ \_ x -> do e <- getRandom return (if (e < p) then Right x else Left mempty)
So my key stroke wire looks like this now:
pressedKeyMaybe = mkFixM $ \_ _ -> do ky <- keyPressed return (Right ky)
Next small subtask in my great mission is to count keystrokes. I know already that FRP is about accumulating events or smth like that, so I want to use accum wire for this task. My plan is:
countingWire = accum (+) 0 . when (/= Nothing) . pressedKeyMayb
That doesn’t work, because
Maybes do not accumulate with
(+). We have to convert them to integers:
toInt :: Int -> Wire m e a Int toInt v = mkPure (\_ _ -> (Right v, toInt v)) countingWire = accum (+) 0 . toInt 1 . when (/= Nothing) . pressedKeyMaybe main = control return (putStrLn . show) countingWire
(…pressing some keys, while running wire…)
*Main> main 0 1 2 3 4 5 6 7 8 9 Interrupted. *Main>
Astonishing result! But we can do better, because of
<|> operator. As I’ve understood, it looks like “or”, so we can have power:
countingWire = accum (+) 0 . (toInt (-1) . when (== Just 'h') <|> toInt 1 . when (== Just 'l')) . pressedKeyMaybe
But when we have power, we want more power! Let’s control two values simulationously:
cursorWire = accum (\(a,b) (c,d) -> (a+c, b+d)) (0,0) . ( pure ((-1), 0 ) . when (== Just 'h') <|> pure ( 1, 0 ) . when (== Just 'l') <|> pure ( 0, (-1)) . when (== Just 'j') <|> pure ( 0, 1 ) . when (== Just 'k') ) . pressedKeyMaybe
(Have you noticed, how
toInt was replaced with
pure? That was an “Aha” moment when studying wires)
*Main> main (0,0) (1,0) (2,0) (3,0) (3,1) (3,0) (3,-1) (2,-1) (1,-1) (0,-1) Interrupted. *Main>
Now I am not that novice, that I was in the beginning, I am netwire-non-novice! Creating the cursor moving wire is now easy:
moveCursor = mkFixM $ \_ coords@(x,y) -> setCursorPosition y x >> return (Right coords)
And the result is:
main = control return (const (return ())) $ moveCursor . cursorWire
(Sorry, it's hard to show how I control the cursor with "hjkl" keys, simply believe me or try yourself)
I am pretty pleasant, that while we work with changing values, the whole system stays composable. I don’t know how to develop large systems with FRP architecure, but this small practical exercise gave me more than blind reading tons of reddit articles.
By the way, thanks for reading!
Have you ever noticed, that Windows console sends TWO keystrokes, when pressing arrow keys? Yeap, and if you worked with it, you may think it is headache with FRP. Suprisingly, no, with netwire it is simply a couple of compositions:
. edge ((/=) 224 . fromEnum . fromMaybe ' ') .
Putting this code before
pressedKeyMaybe in wire chain will filter only those intents, who had keycode
224 just a moment ago.
cursorWire = accum (\(a,b) (c,d) -> (a+c, b+d)) (0,0) . ( pure ((-1), 0 ) . when (== Just 'K') <|> pure ( 1, 0 ) . when (== Just 'M') <|> pure ( 0, (-1)) . when (== Just 'H') <|> pure ( 0, 1 ) . when (== Just 'P') ) . edge ((/=) 224 . fromEnum . fromMaybe ' ') . pressedKeyMaybe
Whole code – Github Gist: moving console cursor
One more thing I’ve left to readers. Limit the cursor movement to screen rectanlge. So that we will not exit the screen area.