Haskell, returning String while using IO Integer to compute that string

You can't leave the IO monad once you're in it, so you'll need to return an IO String instead of a String e.g.

myPPCurrent :: WorkspaceId -> Integer -> String
myPPCurrent x = "Desktop: " ++ show x ++ show y

getWorkspaceName :: WorkspaceId -> IO String
getWorkspaceName id = do
    w <- getScreenWidth
    return $ myPPCurrent id w

I've never used XMonad: I'm basing this answer on general Haskell knowledge and what I've found on the internet in 2½minutes.

Anyway, I'm assuming somewhere you have a main that calls xmonad.

main = xmonad myConfig

Or maybe

main = xmonad XConfig {
          normalBorderColor = "#8080ff",
          -- etc

Or whatever. Let's look at the types.

main :: IO ()
xmonad :: (LayoutClass l Window, Read (l Window)) => XConfig l -> IO ()
myConfig :: XConfig l -- whatever type `l` is

Now suppose instead of myConfig, we have

makeMyConfig :: IO (XConfig l) -- I still don't know what `l` is

makeMyConfig isn't a configuration --- it's something that will make a configuration at runtime, potentially depending on files from disk, or the time, or the screen resolution.... You use it like so:

main = do
    config <- makeMyConfig
    xmonad config

And the point is that because the XConfig l we're using now is derived from an IO (XConfig l), it can incorporate Strings derived from IO Strings and Integers derived from IO Integers, etc. So that's your in for using getScreenWidth in your configuration.