Add execWriter

Author: garyb

src/Control/Monad/Writer.purs

@@ -11,5 +11,8 @@ type Writer w a = WriterT w Identity a
 runWriter :: forall w a. Writer w a -> Tuple a w
 runWriter = runIdentity <<< runWriterT
 
+execWriter :: forall w a. Writer w a -> w
+execWriter m = snd (runWriter m)
+
 mapWriter :: forall w1 w2 a b. (Tuple a w1 -> Tuple b w2) -> Writer w1 a -> Writer w2 b
 mapWriter f = mapWriterT (Identity <<< f <<< runIdentity)