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