Adjust keybinding depending on number of screens in Xmonad

Question

I'm trying to adjust my key bindings in Xmonad depending on the number of screen (physical screens/monitors) I am currently using. Somehow I am failing with this task, probably due to my lack of Haskell knowledge.

Somewhere in my xmonad.hs file I define my keybindings:

myKeys =
    [ ...
    , ...
    , ("M-x", showSet1)
    , ...
    ]

So far I defined showSet1 elsewhere in the file (abc is a placeholder for the actual definition:

showSet1 = abc

Then I used myKeys like this:

main :: IO ()
main = xmonad
     . ewmhFullscreen
     ...
     $ myConfig

myConfig = def
    { modMask = myModMask
    , ...
    } `additionalKeysP` myKeys

This worked perfectly so far, but now I want to adjust showSet1 depending on the number of screens. I found this thread and from there use the following to get the number of screens:

numScreens :: X Int
numScreens = withDisplay (io.fmap length.getScreenInfo)

My plan was to use the following to define showSet1:

showSet1 = if numScreens == 4
    then abc
    else xyz

When compiling xmonad, I get the following error:

    • No instance for (Eq (X Int)) arising from a use of ‘==’
    • In the expression: numScreens == 4
      In the expression:
        if numScreens == 4 then
            abc
        else
            xyz

If I understand that correctly, numScreens is of type X Int and I cannot compare that to 4. Any idea how I can do that? I have tried a lot of things, including playing with the function screenCount, but nothing worked so far.

Answer 1

Since you're getting the number of screens from X, it isn't a pure computation. Hence, it is a X Int rather than plain Int. Depending on what asd and xyz are, this might help:

showSet1 = do
  ns <- numScreens
  if ns == 4
    then abc
    else xyz