基于环境变量的Xmonad集合布局

Question

假设我有一个环境变量IS_REMOTE。我想有一套布局，如果它是"1"，另一套，如果它不是，或未定义。到目前为止，我已经

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)

这不能编译，因为(我认为) if的两个分支具有不同的类型。但这是我的Haskell知识的极限。执行此操作的正确方法是什么？

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

这里的核心问题来自于Haskell缺乏对存在类型或联合类型的一流支持。一个简单得多的例子是，即使if x then show 'a' else show "abc"是有效的，show $ if x then 'a' else "abc"也是无效的。无论如何，为了解决你的问题，这不是很好，但它是有效的：

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)

它使用the Layout existential type来使两个分支具有相同的类型。