From 20a64e67074460878a693c2d89842b1e1dad120c Mon Sep 17 00:00:00 2001
From: slotThe <soliditsallgood@mailbox.org>
Date: Fri, 15 Jan 2021 08:35:19 +0100
Subject: Extract writing your own plugin into its own file

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 doc/write-your-own-plugin.org | 74 +++++++++++++++++++++++++++++++++++++++++++
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 create mode 100644 doc/write-your-own-plugin.org

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+* Writing Your Own Plugin
+
+Writing a plugin for xmobar is very simple!
+
+First, you need to create a data type with at least one constructor.
+Next you must declare this data type an instance of the =Exec= class, by
+defining the one needed method (alternatively =start= or =run=) and 3
+optional ones (=alias=, =rate=, and =trigger=):
+
+#+begin_src haskell
+  start   :: e -> (String -> IO ()) -> IO ()
+  run     :: e -> IO String
+  rate    :: e -> Int
+  alias   :: e -> String
+  trigger :: e -> (Maybe SignalType -> IO ()) -> IO ()
+#+end_src
+
+=start= must receive a callback to be used to display the =String=
+produced by the plugin. This method can be used for plugins that need to
+perform asynchronous actions. See =src/Xmobar/Plugins/PipeReader.hs= for
+an example.
+
+=run= can be used for simpler plugins. If you define only =run= the
+plugin will be run every second. To overwrite this default you just need
+to implement =rate=, which must return the number of tenth of seconds
+between every successive runs. See [[https://github.com/jaor/xmobar/blob/master/examples/xmobar.hs][examples/xmobar.hs]] for an example of
+a plugin that runs just once, and [[https://github.com/jaor/xmobar/blob/master/src/Xmobar/Plugins/Date.hs][src/Xmobar/Plugins/Date.hs]] for one
+that implements =rate=.
+
+Notice that Date could be implemented as:
+
+#+begin_src haskell
+  instance Exec Date where
+      alias (Date _ a _) = a
+      start (Date f _ r) = date f r
+
+  date :: String -> Int -> (String -> IO ()) -> IO ()
+  date format r callback = do go
+      where go = do
+              t <- toCalendarTime =<< getClockTime
+              callback $ formatCalendarTime defaultTimeLocale format t
+              tenthSeconds r >> go
+#+end_src
+
+Modulo some technicalities like refreshing the time-zone in a clever
+way, this implementation is equivalent to the one you can read in
+=Plugins/Date.hs=.
+
+=alias= is the name to be used in the output template. Default alias
+will be the data type constructor.
+
+After that your type constructor can be used as an argument for the
+Runnable type constructor =Run= in the =commands= list of the
+configuration options.
+
+If your plugin only implements =alias= and =start=, then it is advisable
+to put it into the =Xmobar/Plugins/Monitors= directory and use one of
+the many =run*= functions in [[../src/Xmobar/Plugins/Monitors/Common/Run.hs][Xmobar.Plugins.Monitors.Run]] in order to
+define =start=. The =Exec= instance should then live in
+[[../src/Xmobar/Plugins/Monitors.hs][Xmobar.Plugins.Monitors]].
+
+** Using a Plugin
+
+To use your new plugin, you need to use a pure Haskell configuration for
+xmobar, and load your definitions there. You can see an example in
+[[../examples/xmobar.hs][examples/xmobar.hs]] showing you how to write a Haskell configuration that
+uses a new plugin, all in one file.
+
+When xmobar runs with the full path to that Haskell file as its argument
+(or if you put it in =~/.config/xmobar/xmobar.hs=), and with the xmobar
+library installed (e.g., with =cabal install --lib xmobar=), the Haskell
+code will be compiled as needed, and the new executable spawned for you.
+
+That's it!
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