diff --git a/reactive-banana.cabal b/reactive-banana.cabal
--- a/reactive-banana.cabal
+++ b/reactive-banana.cabal
@@ -1,5 +1,5 @@
 Name:                reactive-banana
-Version:             0.4.3.0
+Version:             0.4.3.1
 Synopsis:            Small but solid library for
                      functional reactive programming (FRP).
 Description:         
diff --git a/src/Reactive/Banana/Model.hs b/src/Reactive/Banana/Model.hs
--- a/src/Reactive/Banana/Model.hs
+++ b/src/Reactive/Banana/Model.hs
@@ -14,8 +14,9 @@
     FRP(..),
     Event, Behavior,
     -- $classes
+    
     -- * Derived Combinators
-    whenE, mapAccum, Apply(..),
+    whenE, filterJust, mapAccum, Apply(..),
     
     -- * Model implementation
     Model,
@@ -24,6 +25,7 @@
 
 import Control.Applicative
 import qualified Data.List
+import Data.Maybe
 import Prelude hiding (filter)
 import Data.Monoid
 
@@ -179,6 +181,11 @@
 whenE :: FRP f => Behavior f Bool -> Event f a -> Event f a
 whenE bf = filterApply (const <$> bf)
 
+-- | Variant of 'filterE'. Keep only the 'Just' values.
+filterJust :: FRP f => Event f (Maybe a) -> Event f a
+filterJust = fmap (maybe err id) . filterE isJust
+    where err = error "Reactive.Banana.Model.filterJust: Internal error. :("
+
 -- | Efficient combination of 'accumE' and 'accumB'.
 mapAccum :: FRP f => acc -> Event f (acc -> (x,acc)) -> (Event f x, Behavior f acc)
 mapAccum acc ef = (fst <$> e, stepper acc (snd <$> e))
@@ -239,8 +246,9 @@
         accumE' acc []     = []
         accumE' acc (e:es) = e' : accumE' acc' es
             where
-            e'   = tail $ scanl' (flip ($)) acc e
-            acc' = last e'
+            vals = scanl' (flip ($)) acc e
+            e'   = tail $ vals
+            acc' = last vals
 
 -- strict version of scanl
 scanl' :: (a -> b -> a) -> a -> [b] -> [a]