Adding Data.Lens.Lazy.Void module which duplicates Data.Lens.Lazy but…

… returns m () for setting operations.

This makes the setting operations easier to use with `when` and `unless`.

data-lens.cabal

@@ -41,6 +41,7 @@ library
   exposed-modules:
     Data.Lens.Common
     Data.Lens.Lazy
+    Data.Lens.Lazy.Void
     Data.Lens.Strict
     Data.Lens.Partial.Common
     Data.Lens.Partial.Lazy

src/Data/Lens/Lazy/Void.hs

@@ -0,0 +1,61 @@
+module Data.Lens.Lazy.Void
+  ( module Data.Lens.Common
+  -- * State API
+  , access         -- getter -- :: Monad m => Lens a b -> StateT a m b
+  , (~=), (!=)     -- setter -- :: Monad m => Lens a b -> b -> StateT a m b
+  , (%=), (!%=)    -- modify -- :: Monad m => Lens a b -> (b -> b) -> StateT a m b
+  , (%%=), (!%%=)  -- modify -- :: Monad m => Lens a b -> (b -> (c, b)) -> StateT a m c
+  , (+=), (!+=)    -- modify -- :: (Monad m, Num b) => Lens a b -> b -> StateT a m b
+  , (-=), (!-=)    -- modify -- :: (Monad m, Num b) => Lens a b -> b -> StateT a m b
+  , (*=), (!*=)    -- modify -- :: (Monad m, Num b) => Lens a b -> b -> StateT a m b
+  , (//=), (!/=)   -- modify -- :: (Monad m, Fractional b) => Lens a b -> b -> StateT a m b
+  , (&&=), (!&&=)  -- modify -- :: Monad m => Lens a Bool -> Bool -> StateT a m Bool
+  , (||=), (!||=)  -- modify -- :: Monad m => Lens a Bool -> Bool -> StateT a m Bool
+  , focus          -- modify -- :: Monad m => Lens a b -> StateT m b c -> StateT m a c
+  ) where
+
+import Data.Lens.Common
+import Control.Monad (void)
+import Control.Monad.Trans.State
+import Data.Lens.Lazy (access, (%%=), (!%%=), focus)
+import qualified Data.Lens.Lazy as L
+
+-- * State actions
+
+infixr 4 ~=, !=
+
+-- | set a value using a lens into state
+(~=), (!=) :: (Functor m, Monad m) => Lens a b -> b -> StateT a m ()
+l ~= b = void (l L.~= b)
+l != b = void (l L.!= b)
+
+infixr 4 %=, !%=
+
+-- | infix modification a value through a lens into state
+(%=), (!%=) :: (Functor m, Monad m) => Lens a b -> (b -> b) -> StateT a m ()
+l %= f = void (l L.%= f)
+l !%= f = void (l L.!%= f)
+
+infixr 4 +=, !+=, -=, !-=, *=, !*=
+
+(+=), (!+=), (-=), (!-=), (*=), (!*=) :: (Functor m, Monad m, Num b) => Lens a b -> b -> StateT a m ()
+f += b = f %= (+ b)
+f -= b = f %= subtract b
+f *= b = f %= (* b)
+f !+= b = f !%= (+ b)
+f !-= b = f !%= subtract b
+f !*= b = f !%= (* b)
+
+infixr 4 //=, !/=
+
+(//=), (!/=) :: (Functor m, Monad m, Fractional b) => Lens a b -> b -> StateT a m ()
+f //= b = f %= (/ b)
+f !/= b = f !%= (/ b)
+
+infixr 4 &&=, !&&=, ||=, !||=
+
+(&&=), (||=), (!&&=), (!||=) :: (Functor m, Monad m) => Lens a Bool -> Bool -> StateT a m ()
+f &&= b = f %= (&& b)
+f ||= b = f %= (|| b)
+f !&&= b = f !%= (&& b)
+f !||= b = f !%= (|| b)