Add lead/follow combinators

Author: alexfmpe

src/Reflex/Workflow.hs

@@ -24,6 +24,8 @@ module Reflex.Workflow (
   , stop
   , replay
   , roundRobin
+  , lead
+  , follow
 
   , breadthFirst
   , depthFirst
@@ -102,6 +104,12 @@ braidFree = curry $ \case
   (Pure a, b) -> fmap (a,) b
   (a, Pure b) -> fmap (,b) a
 
+interleaveF :: Functor f => Bool -> f () -> F f a -> F f a
+interleaveF separatorAfter s = foldF $ \f ->
+  if separatorAfter
+  then liftF f <* liftF s
+  else liftF s *> liftF f
+
 append :: (Adjustable t m, MonadHold t m, PostBuild t m) => Event t (Step t m a) -> m (Event t a)
 append ev = do
   (h,t) <- headTailE ev
@@ -129,6 +137,9 @@ mkM = M . wrap . fmap pure . Compose
 braidM :: (Functor m, Reflex t) => M t m a -> M t m b -> M t m (a,b)
 braidM (M ma) (M mb) = M $ toF $ braidFree (fromF ma) (fromF mb)
 
+interleaveM :: (Functor m, Reflex t) => Bool -> m (Event t ()) -> M t m a -> M t m a
+interleaveM separatorAfter s = M . interleaveF separatorAfter (Compose s) . unM
+
 bottomUp
   :: forall t m a. PostBuild t m
   => (forall x. Step t m (Step t m x) -> Step t m x)
@@ -208,6 +219,10 @@ replay = callCC $ pure . fix
 roundRobin :: (Functor m, Reflex t) => Machine t m a -> Machine t m b -> Machine t m (a,b)
 roundRobin a b = Machine $ lift $ braidM (runMachine id a) (runMachine id b)
 
+lead, follow :: (Functor m, Reflex t) => m (Event t ()) -> Machine t m a -> Machine t m a
+lead   s m = Machine $ lift $ interleaveM False s $ runMachine id m
+follow s m = Machine $ lift $ interleaveM True  s $ runMachine id m
+
 --------------------------------------------------------------------------------
 -- Runners
 --------------------------------------------------------------------------------