Fix chapter 19

19-applying-structure/19-general-remarks.md

@@ -0,0 +1,55 @@
+# Chapter 19 - General Remarks
+
+## Concatenating key configurations
+
+In the example with **XMonad**, the following code is shown:
+
+```haskell
+main = do
+  xmonad def { keys =
+    \c -> fromList [
+      ((0, xK_F6),
+        lowerVolume 4 >> return ()),
+      ((0, xK_F7),
+        raiseVolume 4 >> return ())
+    ] `mappend` keys defaultConfig c
+  }
+```
+
+The monoid is quite clear from here, but I did not understand the
+`def { keys = ... }` part. After looking at the tutorial from XMonad for
+configuring: [XMonad Doc Configuring](https://bit.ly/2Il3kAO), I discovered
+that this was using **record update syntax**. This allows you to change fields
+of a record type, e.g.
+
+```shell
+Prelude> data User = User { name :: String, age :: Integer } deriving Show
+Prelude> let a = User { name = "John", age = 33 }
+Prelude> let b = b { name = "Jane" }
+Prelude> a
+User { name = "John", age = 33 }
+Prelude> b
+User { name = "Jane", age = 33 }
+```
+
+## 19.3 - lifting over web monad
+
+I don't quite understand how this works. I believe it should be a single fmap.
+I'm even more convinced after looking at the **snap** library:
+
+```haskell
+getRequest :: MonadSnap m => m Request
+getHeader :: HasHeaders a => CI ByteString -> a -> Maybe ByteString
+```
+
+`UserAgent` is probably defined as `type UserAgent = ByteString and with
+`OverloadedString`, we do have:
+
+```haskell
+userAgent' :: Request -> Maybe UserAgent
+userAgent' req = getHeader "user-agent" req
+```
+
+Which is correct. In order to apply the `userAgent'` function to the
+`getRequest`, we only to need lift over one structure, not two as `getRequest`
+only has one structure.