Commits on Dec 28, 2020

1. XMonad: configured recompile (#107696) …

   * nixos/xmonad: xmonad config w/ghc+xmessage
   
   When the "config" option isn't set, we use xmonad-with-packages to
   provide xmonad with runtime access to an isolated ghc, ensuring it can
   recompile and exec a user's local config (e.g. $HOME/.xmonad/xmonad.hs)
   regardless of which ghc (if any) is on PATH.
   
   When the "config" option is set, however, we compile a configured xmonad
   executable upfront (during nixos-rebuild), and prior to this commit, it
   was not provided with runtime access to an isolated ghc.
   
   As a result, with the "config" option set, it was not possible
   to recompile and exec a user's local config unless there was a
   compatible version of ghc on PATH with the necessary packages (xmonad,
   xmonad-contrib, etc.) in its package database. Adding such a ghc to
   environment.systemPackages, e.g.
   
     (haskellPackages.ghcWithPackages (ps: with ps; [xmonad xmonad-contrib]))
   
   is problematic because it adds both ghc and an unconfigured xmonad to
   PATH, e.g.
   
     $ ls -l $(which xmonad ghc)
     lrwxrwxrwx ... /run/current-system/sw/bin/ghc -> /nix/store/...-ghc-8.10.2-with-packages/bin/ghc
     lrwxrwxrwx ... /run/current-system/sw/bin/xmonad -> /nix/store/...-ghc-8.10.2-with-packages/bin/xmonad
   
   Having the unconfigured xmonad on PATH is particularly bad because
   restarting xmonad will dump the user into the unconfigured version, and
   if no local config exists (e.g. in $HOME/.xmonad/xmonad.hs), they'll be
   left in this unconfigured state.
   
   In this commmit, we give the configured xmonad runtime access to ghc
   like xmonad-with-packages does for the unconfigured version. The aim
   is to allow the user to switch between the nixos module's config and a
   local config (e.g. $HOME/.xmonad/xmonad.hs) at will, so they can try out
   config changes without performing a nixos-rebuild.
   
   Since the xmonad on PATH is the configured executable, there's no
   danger a user could unwittingly restart into the unconfigured version,
   and because xmonad will refuse to recompile when no local config
   exists, there's no danger a user could unwittingly recompile into an
   unconfigured version.
   
   Given that a local config exists, the recompile/restart behavior depends
   on two factors:
   - which entry point is used
     * 'XMonad.xmonad' (default)
     * 'XMonad.launch' (recommended in "config" option description)
   - what operation is triggered (i.e. via mod+q)
     * `spawn "xmonad --recompile && xmonad --restart"` (default)
     * `restart "xmonad" True`
     * custom function
   
   If the default 'XMonad.xmonad' entrypoint and default mod+q operation
   are used, hitting mod+q will compile and exec the local config, which
   will remain in use until next time the display manager is restarted.
   
   If the entrypoint is changed to 'XMonad.launch' but mod+q left with its
   default operation, hitting mod+q will have no visible effect. The logs
   (as seen by running `journalctl --identifier xmonad --follow`) will show
   an error,
     X Error of failed request:  BadAccess (attempt to access private resource denied)
   which indicates that the shell was unable to start xmonad because
   another window manager is already running (namely, the nixos-configured
   xmonad).
   https://wiki.haskell.org/Xmonad/Frequently_asked_questions#X_Error_of_failed_request:_BadAccess_.28attempt_to_access_private_resource_denied.29
   
   Changing the mod+q operation to `restart "xmonad" True` (as recommended
   in the "config" option's description) will allow a restart of the
   nixos-configured xmonad to be triggeredy by hitting mod+q.
   
   Finally, if the entrypoint is 'XMonad.launch', mod+q has been
   bound to `restart "xmonad" True` and another key bound to a custom
   recompile/restart function (e.g. `compileRestart` as shown in the
   "config" option example), the user can switch between the nixos module's
   config and their local config, with the custom key switching to the
   local config and mod+q switching back.
   
   * nixos/xmonad: refactor let binding
   
   * nixos/xmonad: refactor (eliminate duplicate code)
   
   * nixos/xmonad: install man pages
   
   Prior to this commit, man pages were not installed if the "config"
   option was set.
   
   * nixos/xmonad: comment grammar fixups
   
   * nixos/xmonad: writeStateToFile in example config
   
   Calling writeStateToFile prior to recompiling and restarting allows
   state (workspaces, etc.) to be preserved across the restart.
   
   * nixos/xmonad: add ivanbrennan to maintainers
   
   * nixos/xmonad: adjust compileRestart example
   
   * nixos/xmonad: add missing import to example config

   

   ivanbrennan committed Dec 28, 2020
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