Reverse 'exact' to better match sipure and common use cases. Add sireplicate_ and sisequence_.

Control/Lens/SemiIso.hs

@@ -220,15 +220,15 @@ morphed = iso morph morph
 constant :: a -> SemiIso' () a
 constant x = semiIso (\_ -> Right x) (\_ -> Right ())
 
--- | \-> Always returns the argument.
+-- | \-> Filters out all values not equal to the argument.
 --
--- \<- Filters out all values not equal to the argument.
-exact :: Eq a => a -> SemiIso' () a
+-- \<- Always returns the argument.
+exact :: Eq a => a -> SemiIso' a ()
 exact x = semiIso f g
   where
-    f _ = Right x
-    g y | x == y    = Right ()
+    f y | x == y    = Right ()
         | otherwise = Left "exact: not equal"
+    g _ = Right x
 
 -- | Like 'filtered' but checks the predicate in both ways.
 bifiltered :: (a -> Bool) -> SemiIso' a a

Data/SemiIsoFunctor.hs

@@ -161,13 +161,19 @@ class SemiIsoMonad m => SemiIsoFix m where
     {-# MINIMAL sifix | (=//=) #-}
 
 -- | Equivalent of 'sequence'.
---
--- Note that it is not possible to write sequence_, because
--- you cannot void a SemiIsoFunctor.
 sisequence :: SemiIsoApply f => [f a] -> f [a]
 sisequence [] = sipure _Empty
 sisequence (x:xs) = _Cons /$/ x /*/ sisequence xs
 
+-- | Equivalent of 'sequence_', restricted to units.
+sisequence_ :: SemiIsoApply f => [f ()] -> f ()
+sisequence_ [] = sipure _Empty
+sisequence_ (x:xs) = unit /$/ x /*/ sisequence_ xs
+
 -- | Equivalent of 'replicateM'.
 sireplicate :: SemiIsoApply f => Int -> f a -> f [a]
 sireplicate n f = sisequence (replicate n f)
+
+-- | Equivalent of 'replicateM_', restricted to units.
+sireplicate_ :: SemiIsoApply f => Int -> f () -> f ()
+sireplicate_ n f = sisequence_ (replicate n f)