Fix example of overriding Logger (digitallyinduced#1944)

* Fix example of overriding Logger

* Start add guide

* Log also anon users

* tabs

* Complete example

* Apply suggestions from code review

* Improve code readability

* Code review fixes

* Apply suggestions from code review

Guide/logging.markdown

@@ -198,5 +198,56 @@ Would log a timestamp as:
 
 > Sunday, 2020-1-31 22:10:21
 
+### Decorating the Logs with the User ID
 
+You can override the default logger and have it decorated with additional information. A typical use case is adding the current user's ID or name to the log messages.
 
+
+```haskell
+-- Web/FrontController.hs
+
+-- Add imports
+import IHP.Log.Types as Log
+import IHP.Controller.Context
+
+instance InitControllerContext WebApplication where
+    initContext = do
+        initAuthentication @User
+        -- ... your other initContext code
+
+        putContext userIdLogger
+
+userIdLogger :: (?context :: ControllerContext) => Logger
+userIdLogger =
+    defaultLogger { Log.formatter = userIdFormatter defaultLogger.formatter }
+    where
+        defaultLogger = ?context.frameworkConfig.logger
+
+
+userIdFormatter :: (?context :: ControllerContext) => Log.LogFormatter -> Log.LogFormatter
+userIdFormatter existingFormatter time level string =
+    existingFormatter time level (prependUserId string)
+
+prependUserId :: (?context :: ControllerContext) => LogStr -> LogStr
+prependUserId string =
+    toLogStr $ userInfo <> show string
+    where
+        userInfo =
+            case currentUserOrNothing of
+                Just currentUser -> "Authenticated user ID: " <> show currentUser.id <> " "
+                Nothing -> "Anonymous user: "
+```
+
+From your controller you can now add a log message
+
+```haskell
+    action PostsAction = do
+        Log.debug ("This log message should have user info" :: Text)
+        -- Rest of the action code.
+```
+
+In your log output, you will see the user info prepended to the log message.
+
+```
+[30-Mar-2024 18:28:29] Authenticated user ID: 5f32a9e3-da09-48d8-9712-34c935a72c7a "This log message should have user info"
+```

IHP/Controller/Context.hs

@@ -143,21 +143,36 @@ instance HasField "frameworkConfig" ControllerContext FrameworkConfig where
 --
 -- This can be useful to customize the log formatter for all actions of an app:
 --
--- > import IHP.Log.Types
+-- > -- Web/FrontController.hs
+-- >
+-- > import IHP.Log.Types as Log
 -- > import IHP.Controller.Context
--- > 
+-- >
 -- > instance InitControllerContext WebApplication where
 -- >     initContext = do
--- >         let defaultLogger :: Logger = ?context.frameworkConfig.logger
--- >         let withUserIdLogger :: Logger = { Log.formatter = userIdFormatter defaultLogger.formatter }
--- >         putContext withUserIdLogger
--- > 
--- > userIdFormatter :: (?context :: Context) => Log.LogFormatter -> Log.LogFormatter
+-- >     -- ... your other initContext code
+-- >
+-- >     putContext userIdLogger
+-- >
+-- > userIdLogger :: (?context :: ControllerContext) => Logger
+-- > userIdLogger =
+-- >     defaultLogger { Log.formatter = userIdFormatter defaultLogger.formatter }
+-- >     where
+-- >         defaultLogger = ?context.frameworkConfig.logger
+-- >
+-- >
+-- > userIdFormatter :: (?context :: ControllerContext) => Log.LogFormatter -> Log.LogFormatter
 -- > userIdFormatter existingFormatter time level string =
--- >         existingFormatter time level (prependUserId string)
--- > 
--- > preprendUserId :: (?context :: Context) => Text -> Text
--- > preprendUserId string = "userId: " <> show currentUserId <> " " <> string
+-- >     existingFormatter time level (prependUserId string)
+-- >
+-- > prependUserId :: (?context :: ControllerContext) => LogStr -> LogStr
+-- > prependUserId string =
+-- >     toLogStr $ userInfo <> show string
+-- >     where
+-- >         userInfo =
+-- >             case currentUserOrNothing of
+-- >                 Just currentUser -> "Authenticated user ID: " <> show currentUser.id <> " "
+-- >                 Nothing -> "Anonymous user: "
 --
 -- This design mistake should be fixed in IHP v2
 instance HasField "logger" ControllerContext Logger where