diff --git a/src/Control/Varying/Spline.hs b/src/Control/Varying/Spline.hs
--- a/src/Control/Varying/Spline.hs
+++ b/src/Control/Varying/Spline.hs
@@ -29,6 +29,7 @@
     runSplineT,
     evalSplineT,
     execSplineT,
+    output,
     -- * Special operations.
     untilEvent,
     race,
@@ -249,8 +250,15 @@
                   ec' = (mb',) <$> ec
               return (Step fb ec', runSplineT $ capture' mb' v'')
 
+-- | Produce the argument as an output value exactly once, then return ().
+output :: (Applicative m, Monad m, Monoid (f b), Applicative f)
+       => b -> SplineT f a b m ()
+output b = SplineT $ Var $ \_ ->
+    return (Step (pure b) NoEvent, pure $ Step (pure b) $ Event ())
+
 -- | Map the output value of a spline.
 mapOutput :: (Functor f, Monoid (f t), Applicative m, Monad m)
-          => Var m a (f b -> f t) -> SplineT f a b m c -> SplineT f a t m c
+          => Var m a (b -> t) -> SplineT f a b m c -> SplineT f a t m c
 mapOutput _ (SplineTConst c) = SplineTConst c
-mapOutput vf (SplineT vx) = SplineT $ toIter <$> vf <*> vx
+mapOutput vf (SplineT vx) = SplineT $ toIter <$> vg <*> vx
+    where vg = (<$>) <$> vf
diff --git a/src/Control/Varying/Tween.hs b/src/Control/Varying/Tween.hs
--- a/src/Control/Varying/Tween.hs
+++ b/src/Control/Varying/Tween.hs
@@ -151,7 +151,7 @@
     where g t = let c = end - start
                     b = start
                     x = f c t b
-                in if t <= 1.0 then Event x else NoEvent
+                in if t >= 1.0 then NoEvent else Event x
 
 -- | Creates a tween that performs no interpolation over the duration.
 constant :: (Applicative m, Monad m, Num t, Ord t)
@@ -161,7 +161,7 @@
 -- | Varies 0.0 to 1.0 linearly for duration `t` and 1.0 after `t`.
 timeAsPercentageOf :: (Applicative m, Monad m, Ord t, Num t, Fractional t)
                    => t -> Var m t t
-timeAsPercentageOf t = (\t' -> min 1 (t' / t)) <$> accumulate (+) 0
+timeAsPercentageOf t = ((\t' -> min 1 (t' / t)) <$> accumulate (+) 0)
 
 --------------------------------------------------------------------------------
 -- $writing
diff --git a/varying.cabal b/varying.cabal
--- a/varying.cabal
+++ b/varying.cabal
@@ -10,7 +10,7 @@
 -- PVP summary:      +-+------- breaking API changes
 --                   | | +----- non-breaking API additions
 --                   | | | +--- code changes with no API change
-version:             0.2.0.0
+version:             0.3.0.0
 
 -- A short (one-line) description of the package.
 synopsis:            FRP through value streams and monadic splines.