[more updates to IxSetDataLens
Jeremy Shaw <jeremy@seereason.com>**20120122014851
 Ignore-this: 5aa7ef9b59abc1f2c3be0f0da34650db
] hunk ./IxSetDataLens.markdown.lhs 1
-
hunk ./IxSetDataLens.markdown.lhs 13
+It can take a bit of practice to get used to lenses. But, fortunately, using them is completely optional -- so if they are not your thing, you don't have to use them. In this tutorial we will start with a general tutorial on using lenses, and then finish up with showing how to use them with `IxSet` and `acid-state`.
+
hunk ./IxSetDataLens.markdown.lhs 21
+> import Control.Category   ((.), (>>>))
hunk ./IxSetDataLens.markdown.lhs 30
->
+> import Prelude            hiding ((.))
hunk ./IxSetDataLens.markdown.lhs 114
-#ifdef HsColour
-> stepcutFirstName' :: Text
-> stepcutFirstName' = _firstName $ _name $ stepcut
-#endif
+
+> stepcutFirstName2 :: Text
+> stepcutFirstName2 = _firstName $ _name $ stepcut
+
hunk ./IxSetDataLens.markdown.lhs 134
-> stepcutFirstName'' :: Text
-> stepcutFirstName'' = (stepcut ^. name) ^. firstName
+> stepcutFirstName3 :: Text
+> stepcutFirstName3 = (stepcut ^. name) ^. firstName
hunk ./IxSetDataLens.markdown.lhs 147
+<h3><a name="ixset_lens_category"><kbd>Lens</kbd> is an instance of <code>Category</code></a></h3>
+
+`Lens` is an instance of `Category`. That means we can use the `.` operator from `Category` to compose lenses.
+
+The normal `.` looks like this:
+
+<div class="code">
+#ifdef HsColour
+-- | as defined in 'Prelude'
+(.) :: (b -> c) -> (a -> b) -> a -> c
+#endif
+</div>
+
+But `.` can be generalized to work for any `Category` like this:
+
+<div class="code">
+#ifdef HsColour
+-- | as defined in 'Control.Category'
+(.)  :: (Category cat) => cat b c -> cat a b -> cat a c
+#endif
+</div>
+
+If you look closely at the imports at the top, you will notice that we hide `.` from the `Prelude` and imported the version from `Control.Category` instead. Now we can write this:
+
+<div class="code">
+
+> stepcutFirstName4 :: Text
+> stepcutFirstName4 = firstName . name ^$ stepcut
+
+</div>
+
+Which looks very similar to the non-lens version:
+
+<div class="code">
+
+> stepcutFirstName5 :: Text
+> stepcutFirstName5 = _firstName . _name $ stepcut
+
+</div>
+
hunk ./IxSetDataLens.markdown.lhs 197
-Once again we have a lazier version `^=` and a stricter vesrion `^!=`.
+Once again we have a lazier version `^=` and a stricter version `^!=`.
hunk ./IxSetDataLens.markdown.lhs 251
+Another option would be to leverage the `Category` instance and use the `.` operator:
+
+<div class="code">
+
+> setNick2 :: Text -> (User -> User)
+> setNick2 newNick = (nickName . name) ^= newNick
+
+</div>
+
+However, I find that a bit confusing to read, because the field names are listed right-to-left, but the overall flow of that line is left-to-right. If we want a consistent left-to-right feel we can use the `>>>` operator:
+
+<div class="code">
+#ifdef HsColour
+> (>>>) :: Control.Category.Category cat => cat a b -> cat b c -> cat a c
+#endif
+</div>
+
+<div class="code">
+
+> setNick3 :: Text -> (User -> User)
+> setNick3 newNick = (name >>> nickName) ^= newNick
+
+</div>
hunk ./IxSetDataLens.markdown.lhs 362
+We can also try to use `>>>`  instead of all those `^%=`, but the `fmap` is a bit troublesome:
+
+<div class="code">
+
+> changeNick2 :: UserState
+> changeNick2 = ((users >>> user (UserId 0)) ^%= fmap ((name >>> nickName) ^= "stepkut")) userState
+
+</div>
+
+Additionally, it seems like `^%=` binds to tightly and so we need some extra `( )` to keep things happy.
+
hunk ./IxSetDataLens.markdown.lhs 407
+
+`data-lens-fd` provides a few other functions that you can use to get, set, and modify the state in an `Update` or `Query` event. Check out the <a href='http://hackage.haskell.org/package/data-lens-fd'> haddock documentation for data-lens-fd</a>.
+
+