ENH clean up docs in shell

lib/XMonad/Internal/Shell.hs

@@ -1,55 +1,4 @@
 -- | Functions for formatting and spawning shell commands
---
--- TLDR: spawning a "command" in xmonad is complicated for weird reasons, and
--- this solution is the most sane (for me) given the constraints of the xmonad
--- codebase.
---
--- A few facts about xmonad (and window managers in general):
--- 1) It is single-threaded (since X is single threaded)
--- 2) Because of (1), it ignores SIGCHLD, which means any subprocess started
---    by xmonad will instantly be reaped after spawning. This guarantees the
---    main thread running the WM will never be blocked.
---
--- In general, this means that 'System.Process.waitForProcess' (and similar)
--- will not work since these call wait() on the child process, which will fail
--- because the child has already been cleared and thus there is nothing on which
--- to wait. By extension this also means we don't have access to a child's exit
--- code.
---
--- XMonad and contrib use their own method of spawning subprocesses using the
--- extremely low-level 'System.Process.Posix' API. See the code for
--- 'XMonad.Core.spawn' or 'XMonad.Util.Run.safeSpawn'. Specifically, the
--- sequence is (in terms of the low level Linux API):
--- 1) call fork()
--- 2) uninstall signal handlers
--- 3) call setsid()
--- 4) start new thing with exec()
---
--- In practice, I'm guessing the main reason for 2 and 3 is so that child
--- processes don't inherit the weird SIGCHLD behavior of xmonad itself. The
--- setsid thing is one way to guarantee that killing the child thread will also
--- kill its children (if any). Note that this obviously will not block since
--- we are calling fork() without wait() (which would throw an error anyways).
---
--- What if I actually want the exit code?
---
--- The best solution (I can come up with), is to use bracket to uninstall
--- handlers, run process (with wait), and then reinstall handlers. I can use
--- this with a much higher-level interface which will make things easier. This
--- obviously means that if the process is running in the main thread, it needs
--- to be almost instantaneous (since it actually will be blocking). NOTE: I
--- shouldn't use this to replace the existing functions in xmonad since
--- 'spawning' a new process in a non-blocking manner with a higher-level API
--- will produce lots of Haskell objects that need to be cleaned, and it will be
--- hard (perhaps impossible) to keep track and deal with these after spawning.
---
--- This works, albeit with the cost of using almost every process API in Haskell.
---
--- Briefly:
--- 1) 'System.Process.Posix' (where xmonad lives)
--- 2) 'System.Process' (wraps 1)
--- 2) 'System.Process.Typed' (wraps 2, which I prefer for getting exit codes)
--- 3) 'RIO.Process' (wraps 3, which I prefer at the app level)
 
 {-# LANGUAGE OverloadedStrings #-}
 
@@ -78,70 +27,108 @@ import qualified System.Process.Typed as P
 import qualified XMonad.Core          as X
 import qualified XMonad.Util.Run      as XR
 
---------------------------------------------------------------------------------
--- | Opening subshell
--- https://github.com/xmonad/xmonad/issues/113
+-- | Fork a new process and wait for its exit code.
+--
+-- This function will work despite xmonad ignoring SIGCHLD.
+--
+-- A few facts about xmonad (and window managers in general):
+-- 1) It is single-threaded (since X is single threaded)
+-- 2) Because of (1), it ignores SIGCHLD, which means any subprocess started
+--    by xmonad will instantly be reaped after spawning. This guarantees the
+--    main thread running the WM will never be blocked.
+--
+-- In general, this means I can't wait for exit codes (since wait() doesn't
+-- work) See https://github.com/xmonad/xmonad/issues/113.
+--
+-- If I want an exit code, The best solution (I can come up with), is to use
+-- bracket to uninstall handlers, run process (with wait), and then reinstall
+-- handlers. I can use this with a much higher-level interface which will make
+-- things easier. This obviously means that if the process is running in the
+-- main thread, it needs to be almost instantaneous. Note if using a high-level
+-- API for this, the process needs to spawn, finish, and be reaped by the
+-- xmonad process all while the signal handlers are 'disabled' (which limits
+-- the functions I can use to those that call waitForProcess).
+--
+-- XMonad and contrib use their own method of spawning subprocesses using the
+-- extremely low-level 'System.Process.Posix' API. See the code for
+-- 'XMonad.Core.spawn' or 'XMonad.Util.Run.safeSpawn'. Specifically, the
+-- sequence is (in terms of the low level Linux API):
+-- 1) call fork()
+-- 2) uninstall signal handlers (to allow wait() to work in subprocesses)
+-- 3) call setsid() (so killing the child will kill its children, if any)
+-- 4) start new thing with exec()
+--
+-- In contrast with high-level APIs like 'System.Process', this will leave no
+-- trailing data structures to clean up, at the cost of being gross to look at
+-- and possibly more error-prone.
+runProcess :: P.ProcessConfig a b c -> IO ExitCode
+runProcess = withDefaultSignalHandlers . P.runProcess
 
+-- | Run an action without xmonad's signal handlers.
 withDefaultSignalHandlers :: IO a -> IO a
 withDefaultSignalHandlers =
   bracket_ X.uninstallSignalHandlers X.installSignalHandlers
 
+-- | Set a child process to create a new group and session
 addGroupSession :: P.ProcessConfig x y z -> P.ProcessConfig x y z
 addGroupSession = P.setCreateGroup True . P.setNewSession True
 
-runProcess :: P.ProcessConfig a b c -> IO ExitCode
-runProcess = withDefaultSignalHandlers . P.runProcess
-
+-- | Create a 'ProcessConfig' for a shell command
 shell :: T.Text -> P.ProcessConfig () () ()
 shell = addGroupSession . P.shell . T.unpack
 
+-- | Create a 'ProcessConfig' for a command with arguments
 proc :: FilePath -> [T.Text] -> P.ProcessConfig () () ()
 proc cmd args = addGroupSession $ P.proc cmd (T.unpack <$> args)
 
+-- | Run 'XMonad.Core.spawn' with 'Text' input.
 spawn :: MonadIO m => T.Text -> m ()
 spawn = X.spawn . T.unpack
 
--- spawnAt :: MonadIO m => FilePath -> T.Text -> m ()
--- spawnAt fp = liftIO . void . startProcess . P.setWorkingDir fp . shell
-
+-- | Run 'XMonad.Run.Utils.spawnPipe' with 'Text' input.
 spawnPipe :: MonadIO m => T.Text -> m Handle
 spawnPipe = XR.spawnPipe . T.unpack
 
+-- | Run 'XMonad.Core.spawn' with a command and arguments
 spawnCmd :: MonadIO m => FilePath -> [T.Text] -> m ()
 spawnCmd cmd = spawn . fmtCmd cmd
 
---------------------------------------------------------------------------------
--- | Formatting commands
-
+-- | Format a command and list of arguments as 'Text'
 fmtCmd :: FilePath -> [T.Text] -> T.Text
 fmtCmd cmd args = T.unwords $ T.pack cmd : args
 
 op :: T.Text -> T.Text -> T.Text -> T.Text
 op a x b = T.unwords [a, x, b]
 
+-- | Format two shell expressions separated by "&&"
 (#!&&) :: T.Text -> T.Text -> T.Text
 cmdA #!&& cmdB = op cmdA "&&" cmdB
 
 infixr 0 #!&&
 
+-- | Format two shell expressions separated by "|"
 (#!|) :: T.Text -> T.Text -> T.Text
 cmdA #!| cmdB = op cmdA "|" cmdB
 
 infixr 0 #!|
 
+-- | Format two shell expressions separated by "||"
 (#!||) :: T.Text -> T.Text -> T.Text
 cmdA #!|| cmdB = op cmdA "||" cmdB
 
 infixr 0 #!||
 
+-- | Format two shell expressions separated by ";"
 (#!>>) :: T.Text -> T.Text -> T.Text
 cmdA #!>> cmdB = op cmdA ";" cmdB
 
 infixr 0 #!>>
 
+-- | Wrap input in double quotes
 doubleQuote :: T.Text -> T.Text
 doubleQuote s = T.concat ["\"", s, "\""]
 
+-- | Wrap input in single quotes
 singleQuote :: T.Text -> T.Text
 singleQuote s = T.concat ["'", s, "'"]