OSD to show STDOUT from a command in XMonad

Recently I’ve been looking a bit into Haskell. This has proven specially useful since I’ve been using XMonad as my primary window manager for some time now. This newly acquired understanding of Haskell allows me therefore to go a step further in the configuration of my working environment.

A few days ago, I’ve written some configuration to map a keybinding to an action where a command is ran, and then the output of that command is captured, and shown in a OSD (On Screen Display).

The first problem I had to deal with, as a newcomer to Haskell, was how to pass the output of the CLI command, which is an IO action, and therefore not pure, into the Dzen function that triggers the OSD. The function, dzenConfig, will show a OSD with some text, using a specified configuration, and it has the following signature:

dzenConfig :: DzenConfig -> String -> X ()

So the type of the first parameter is a DzenConfig. This should be quite straightforward, as, not surprisingly, most of the Dzen configuration functions return a DzenConfig. I wanted the OSD to show up in the current screen, centered, with a size of 800x30, so that I can show a long…ish single line of text, and to use a font like terminus. To sort out this first parameter here’s what I’ve done:

import qualified XMonad.Util.Dzen as Dzen

terminus = "-*-terminus-*-*-*-*-24-*-*-*-*-*-*-*"

Dzen.onCurr (Dzen.center 800 30) Dzen.>=> Dzen.font terminus

This one is pretty straightforward. I’ve imported the Dzen module using a qualified import because font and >=> clashed with functions similarly named in other namespaces. If you’re not too familiarised with Haskell, this might look a bit weird, but it actually makes sense in the end. I’m going to try to explain it here. The function center has the following signature:

center :: Int -> Int -> ScreenId -> DzenConfig

This means that it will take 2 Int, one ScreenId and will return a DzenConfigm but, you’ll notice that we’re calling it with only the 2 Int parameters. Because in Haskell all functions are curried by default, when we do this, we actually get back a function with the signature

ScreenId ->
DzenConfig

. The reason why we get something of this type back, is because we’ve applied the two Int parameters to the function, so we get back a function that requires only the two last ones, and will always use 800 and 30 as the two first parameters.

onCurr :: (ScreenId -> DzenConfig) -> DzenConfig

You’ll see that it takes exactly a function from ScreenId to DzenConfig, and returns a DzenConfig. So, by partially applying the two Int parameters to center, we get exactly what we need to pass to onCurr, and we get out of it a DzenConfig.

Then, we compose the DzenConfig returned by that expression with the DzenConfig we get out of our call to font, and we get a composition of our desired configuration.

Now that that’s sorted, let’s look at the second parameter. The second parameter, is of type String. This is the string that will be displayed in the OSD. To get this string, we need to run our command and somehow capture its output. The way I was able to do this, was by using the runProcessWithInput function, from the package XMonad.Util.Run.

runProcessWithInput :: MonadIO m => FilePath -> [String] -> String -> m String

Now this is… expected, but it kind of causes us a problem. The return value of our function call is a MonadIO of String, but our OSD expects a String. For someone with more Haskell experience this would have been straightforward, but me coming from a non-functional background, my first instinct was to get the output of the command and try to somehow unpack it so that I could pass the string into my dzen call. As it turns out, I can’t.

My IO action has to be contained, and I can’t get my string out of it to use it in pure functions, so, my call to dzen, had to be moved into my IO function. So I ended up with this:

import qualified XMonad.Util.Dzen as Dzen

import XMonad.Util.Run (runProcessWithInput)

terminus = "-*-terminus-*-*-*-*-24-*-*-*-*-*-*-*"

externalCommandInPopUp :: String -> [String] -> X ()
externalCommandInPopUp c p = do
    s <- runProcessWithInput c p ""
    Dzen.dzenConfig (Dzen.onCurr (Dzen.center 800 30) Dzen.>=> Dzen.font terminus) s

To use it, I bind a call to externalCommandInPopUp to a key combination, and specify there which command to run. So, for example to have WinKey+Shift+m showing me the song currently playing on mpd (music player daemon) using mpc (music player client), I do the following:

, ((modMask .|. shiftMask, xK_m),
  (externalCommandInPopUp "mpc" ["current"]))

Important to note that the preceding comma in this example is there only to indicate that this keybinding is one amongst others in a list of keybindings. Had this be the first one on the list, it wouldn’t have the comma. For more details on how to define Xmonad keybindings, refer to the Xmonad documentation.

The full XMonad configuration where this example is in use, can be found here. This will show what I’m talking about in this post, in context.