Added additional definitions from Yampa to BearRiver as discussed in …

examples/bearriver/src/FRP/BearRiver.hs

@@ -143,6 +143,24 @@ hold a = feedback a $ arr $ \(e,a') ->
 loopPre :: Monad m => c -> SF m (a, c) (b, c) -> SF m a b
 loopPre = feedback
 
+-- | Event source that never occurs.
+never :: Monad m => SF m a (Event b)
+never = constant NoEvent
+
+-- | Event source with a single occurrence at time 0. The value of the event
+-- is given by the function argument.
+now :: Monad m => b -> SF m a (Event b)
+now b0 = Event b0 --> never
+
+-- | Suppress all but the first event.
+once :: Monad m => SF m (Event a) (Event a)
+once = takeEvents 1
+
+-- | Suppress all but the first n events.
+takeEvents :: Monad m => Int -> SF m (Event a) (Event a)
+takeEvents n | n <= 0 = never
+takeEvents n = dSwitch (arr dup) (const (NoEvent >-- takeEvents (n - 1)))
+
 after :: Monad m
       => Time -- ^ The time /q/ after which the event should be produced
       -> b    -- ^ Value to produce at that time
@@ -170,10 +188,24 @@ occasionally tAvg b
   timeDelta :: Monad m => SF m a DTime
   timeDelta = arrM_ ask
 
+-- | Initialization operator (cf. Lustre/Lucid Synchrone).
+--
+-- The output at time zero is the first argument, and from
+-- that point on it behaves like the signal function passed as
+-- second argument.
 (-->) :: Monad m => b -> SF m a b -> SF m a b
-b0 --> sf = MSF $ \a -> do
-  (_, ct) <- unMSF sf a
-  return (b0, ct)
+b0 --> sf = sf >>> replaceOnce b0
+
+-- | Input initialization operator.
+--
+-- The input at time zero is the first argument, and from
+-- that point on it behaves like the signal function passed as
+-- second argument.
+(>--) :: Monad m => a -> SF m a b -> SF m a b
+a0 >-- sf = replaceOnce a0 >>> sf
+
+replaceOnce :: Monad m => a -> SF m a a
+replaceOnce a = dSwitch (arr $ const (a, Event ())) (const $ arr id)
 
 accumHoldBy :: Monad m => (b -> a -> b) -> b -> SF m (Event a) b
 accumHoldBy f b = feedback b $ arr $ \(a, b') ->