diff --git a/Control/Lens/SemiIso.hs b/Control/Lens/SemiIso.hs
--- a/Control/Lens/SemiIso.hs
+++ b/Control/Lens/SemiIso.hs
@@ -52,7 +52,8 @@
     unit,
     swapped,
     associated,
-    constant
+    constant,
+    exact
     ) where
 
 import Control.Lens.Internal.SemiIso
@@ -114,10 +115,22 @@
 
 -- | \-> Always returns the argument.
 --
+-- \<- Maps everything to a @()@.
+--
+-- Note that this isn't an @Iso'@ because
+--
+-- > unapply (constant x) >=> apply (constant x) /= id
+--
+-- But SemiIso laws do hold.
+constant :: a -> SemiIso' () a
+constant x = semiIso (\_ -> Right x) (\_ -> Right ())
+
+-- | \-> Always returns the argument.
+--
 -- \<- Filters out all values not equal to the argument.
-constant :: Eq a => a -> SemiIso' () a
-constant x = semiIso f g
+exact :: Eq a => a -> SemiIso' () a
+exact x = semiIso f g
   where
     f _ = Right x
     g y | x == y    = Right ()
-        | otherwise = Left "constant: not equal"
+        | otherwise = Left "exact: not equal"
diff --git a/semi-iso.cabal b/semi-iso.cabal
--- a/semi-iso.cabal
+++ b/semi-iso.cabal
@@ -1,5 +1,5 @@
 name:                semi-iso
-version:             0.1.0.0
+version:             0.2.0.0
 synopsis:            Weakened partial isomorphisms that work with lenses.
 description:         Semi-isomorphisms are partial isomorphisms with weakened iso laws.
                      And they work with Iso and Prism from @lens@!
@@ -15,9 +15,13 @@
 build-type:          Simple
 cabal-version:       >=1.10
 
+source-repository head
+  type:     git
+  location: git@github.com:Pawel834/semi-iso.git
+
 library
   exposed-modules:     Control.Lens.SemiIso
                        Control.Lens.Internal.SemiIso
                        Data.SemiIsoFunctor
-  build-depends:       base >=4.7 && <4.8, profunctors, transformers, lens
+  build-depends:       base >= 4 && < 5, profunctors, transformers, lens
   default-language:    Haskell2010