* Contributing Changes/Patches to xmobar

Want to contribute to xmobar? You've come to the right place! This
document will help guide you in this endeavour.

In case you want to make any non-trivial change to xmobar, it's always
best to talk with the community first. Currently the best way to do that
is to [[https://github.com/jaor/xmobar/issues/new][create an issue]] on GitHub. There, you can talk through the
problems you are having, as well as the proposed solution.

** Making the Change

It's best to create a separate branch in your clone of the [[https://github.com/jaor/xmobar/][xmobar repo]]
and then push this branch to your fork.

If your pull request undergoes several iterations and =master= has
changed in the meantime, you may be asked to rebase on top of it; in
this case please don't merge any branches, but actually rebase! This is
a very nifty feature that =git= offers in order to remain a clean
history.

Give your commits descriptive names!  Further, it's also highly recommended to
add a description of /why/ you made a certain change.  The syntax for this is
*commit message*, blank line, *more description*.  For example:

  #+begin_src shell
    Commit Message (max. 50 characters)

    Some more useful information of why this change was made; possibly
    how it connects with other commits in this same pr (wrapped at 72
    characters).
  #+end_src

** Opening a Pull Request

Once you have pushed the branch to your fork, making a pull request is
as easy as visiting that branch; GitHub will then even prompt you for
it!

Please include the following information in the description of your pull
request:

- Does your pull request close, or is related to, any existing issues?
- What does your pull request do?  If you add a new feature, an example
  of how you would use it would be most appreciated.
- A brief summary of how your commits fit together to achieve this (if
  necessary).

Please also remember to update the =changelog.md= file, as well as any
other documentation that your pull request touches upon.  For example,
if you add a new plugin you should update the [[./doc/plugins.org][plugins documentation]].

* 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

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.

** 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!