Add pausing functionality

Graphics/Command.hs

@@ -10,6 +10,8 @@ data Command
   | -- | A command to change the tick speed of the simulation,
     -- given by a function on the period between ticks.
     ChangeSpeed (Time -> Time)
+  | -- | A command to play/pause the simulation.
+    PlayPause
 
 getResize :: Command -> Maybe (Int, Int)
 getResize (Resize size) = Just size
@@ -19,5 +21,9 @@ getChangeSpeed :: Command -> Maybe (Time -> Time)
 getChangeSpeed (ChangeSpeed f) = Just f
 getChangeSpeed _ = Nothing
 
+isPlayPause :: Command -> Bool
+isPlayPause PlayPause = True
+isPlayPause _ = False
+
 -- | An event signalling that a command has been given by the user.
 type CommandEvent = Event Command

Graphics/Engine.hs

@@ -39,6 +39,7 @@ processEvent :: InputEvent -> CommandEvent
 processEvent = event noEvent $ \case
   EventKey (Char '-') Down _ _ -> Event $ ChangeSpeed (* 1.25)
   EventKey (Char '+') Down _ _ -> Event $ ChangeSpeed (* 0.8)
+  EventKey (SpecialKey KeySpace) Down _ _ -> Event PlayPause
   EventKey {} -> noEvent
   EventMotion {} -> noEvent
   EventResize size -> Event $ Resize size
@@ -49,9 +50,11 @@ run st =
    in proc inp -> do
         let cmdev = processEvent inp
 
+        playing <- accumHoldBy (const . not) True -< filterE isPlayPause cmdev
         time <- accum 0.2 -< mapFilterE getChangeSpeed cmdev
         windowSize <- hold (100, 100) -< mapFilterE getResize cmdev
 
-        space <- engine initSpace <<< tickSignal -< time
+        tick <- tickSignal -< time
+        space <- engine initSpace -< gate tick playing
 
         returnA -< drawGrid windowSize white (getSpace space)

Main.hs

@@ -10,4 +10,4 @@ space :: ToroidalSpace Bool
 space = tabulate (\(x,y) -> (x + y `mod` 5) * 10 + x - y > 30)
 
 main :: IO ()
-main = playYampa (InWindow "a" (2, 2) (2, 2)) black 30 (run space)
+main = playYampa (InWindow "a" (200, 200) (10, 10)) black 30 (run space)