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…

Hackage: FRP

Ouch. Large choice make me cry ‘( Need something else.

StackOverflow: What’s the status of current Functional Reactive Programming implementations?

Okay, so reactive-banana or netwire . Seems like installing netwire will be faster, because typing cabal install netwire is faster than cabal install reactive-banana .

Installed

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… wackelkontakt wtf?!.. Accum … Analyze … 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.

Reality

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.

Keyboard

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 stepSessionP to stepSession in control function.

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)

)

Counting keypresses

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 Maybe s 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>

Result

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!

Post Scriptum

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

Hometask

One more thing I’ve left to readers. Limit the cursor movement to screen rectanlge. So that we will not exit the screen area.