基于环境变量的 Xmonad 设置布局

Question

Suppose I have an env variable IS_REMOTE . I would like to have a set of layouts if it is "1", and another set if it is not, or undefined. So far I have

import XMonad
import XMonad.Config.Desktop
import XMonad.Hooks.ManageDocks
import XMonad.Layout.LayoutModifier
import XMonad.Layout.MultiColumns
import XMonad.Layout.PerWorkspace
import XMonad.Layout.ThreeColumns
import Data.Maybe
import System.Environment

main = do
  isRemoteEnv <- lookupEnv "IS_REMOTE"
  xmonad $ desktopConfig
    {
      layoutHook = myLayout (fromMaybe "0" isRemoteEnv)
    }

myLayout remote = if remote == "1"
  then onWorkspace "web" (avoidStruts $ (multiCol [1] 1 0.02 (-0.5)) ||| Full) $
       (avoidStruts $ (ThreeColMid 1 0.02 (1/2)) ||| Full)
  else onWorkspace "web" (avoidStruts $ Mirror (multiCol [1] 1 0.02 (-0.5)) ||| Full) $
       (avoidStruts $ Mirror (ThreeColMid 1 0.02 (1/2)) ||| Full)

This does not compile, because (I think) the two branches of if have different types. But this is at the limit of my Haskell knowledge. What is the correct way to do this?

xmonad.hs:21:8: error:
    * Couldn't match type `Mirror MultiCol' with `MultiCol'
      Expected type: PerWorkspace
                       (ModifiedLayout AvoidStruts (Choose MultiCol Full))
                       (ModifiedLayout AvoidStruts (Choose ThreeCol Full))
                       a
        Actual type: PerWorkspace
                       (ModifiedLayout AvoidStruts (Choose (Mirror MultiCol) Full))
                       (ModifiedLayout AvoidStruts (Choose (Mirror ThreeCol) Full))
                       a
    * In the expression:
        onWorkspace
          "web" (avoidStruts $ Mirror (multiCol [1] 1 0.02 (- 0.5)) ||| Full)
          $ (avoidStruts $ Mirror (ThreeColMid 1 0.02 (1 / 2)) ||| Full)
      In the expression:
        if remote == "1" then
            onWorkspace
              "web" (avoidStruts $ (multiCol [1] 1 0.02 (- 0.5)) ||| Full)
              $ (avoidStruts $ (ThreeColMid 1 0.02 (1 / 2)) ||| Full)
        else
            onWorkspace
              "web" (avoidStruts $ Mirror (multiCol [1] 1 0.02 (- 0.5)) ||| Full)
              $ (avoidStruts $ Mirror (ThreeColMid 1 0.02 (1 / 2)) ||| Full)
      In an equation for `myLayout':
          myLayout remote
            = if remote == "1" then
                  onWorkspace
                    "web" (avoidStruts $ (multiCol [1] 1 0.02 (- 0.5)) ||| Full)
                    $ (avoidStruts $ (ThreeColMid 1 0.02 (1 / 2)) ||| Full)
              else
                  onWorkspace
                    "web" (avoidStruts $ Mirror (multiCol [1] 1 0.02 (- 0.5)) ||| Full)
                    $ (avoidStruts $ Mirror (ThreeColMid 1 0.02 (1 / 2)) ||| Full)
   |
21 |   else onWorkspace "web" (avoidStruts $ Mirror (multiCol [1] 1 0.02 (-0.5)) ||| Full) $
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

Answer 1

The core problem here comes from Haskell's lack of first-class support for existential or union types. A much simpler example is that show $ if x then 'a' else "abc" is invalid, even though if x then show 'a' else show "abc" is valid. Anyway, to solve your problem, this isn't very pretty, but it works:

import XMonad
import XMonad.Config.Desktop
import XMonad.Hooks.ManageDocks
import XMonad.Layout.LayoutModifier
import XMonad.Layout.MultiColumns
import XMonad.Layout.PerWorkspace
import XMonad.Layout.ThreeColumns
import Data.Kind
import Data.Maybe
import System.Environment

main = do
  isRemoteEnv <- lookupEnv "IS_REMOTE"
  case myLayout (fromMaybe "0" isRemoteEnv) of Layout c -> xmonad $ desktopConfig {
      layoutHook = c
    }

myLayout remote = if remote == "1"
  then Layout $ onWorkspace "web" (avoidStruts $ (multiCol [1] 1 0.02 (-0.5)) ||| Full) $
       (avoidStruts $ (ThreeColMid 1 0.02 (1/2)) ||| Full)
  else Layout $ onWorkspace "web" (avoidStruts $ Mirror (multiCol [1] 1 0.02 (-0.5)) ||| Full) $
       (avoidStruts $ Mirror (ThreeColMid 1 0.02 (1/2)) ||| Full)

It uses the Layout existential type to make both branches be the same type.