content: {-# OPTIONS_GHC -XFlexibleInstances -XTypeSynonymInstances -XStandaloneDeriving #-}
  
  {- Commands for HSH
  Copyright (C) 2004-2008 John Goerzen <jgoerzen@complete.org>
  Please see the COPYRIGHT file
  -}
  
  {- |
     Module     : HSH.Command
     Copyright  : Copyright (C) 2006-2009 John Goerzen
     License    : GNU LGPL, version 2.1 or above
  
     Maintainer : John Goerzen <jgoerzen@complete.org>
     Stability  : provisional
     Portability: portable
  
  Copyright (c) 2006-2009 John Goerzen, jgoerzen\@complete.org
  -}
  
  module HSH.Command (Environment,
                      ShellCommand(..),
                      PipeCommand(..),
                      (-|-),
                      RunResult,
                      run,
                      runIO,
                      runSL,
                      InvokeResult,
                      checkResults,
                      tryEC,
                      catchEC,
                      setenv,
                      unsetenv
                     ) where
  
  -- import System.IO.HVIO
  -- import System.IO.Utils
  import Prelude hiding (catch)
  import System.IO
  import System.Exit
  import System.Log.Logger
  import System.IO.Error (isUserError, ioeGetErrorString)
  import Data.Maybe.Utils
  import Data.Maybe
  import Data.List.Utils(uniq)
  import Control.Exception(try, evaluate, SomeException, catch)
  import Text.Regex.Posix
  import Control.Monad(when)
  import Data.String.Utils(rstrip)
  import Control.Concurrent
  import System.Process
  import System.Environment(getEnvironment)
  import qualified Data.ByteString.Lazy as BSL
  import qualified Data.ByteString as BS
  import HSH.Channel
  
  d, dr :: String -> IO ()
  d = debugM "HSH.Command"
  dr = debugM "HSH.Command.Run"
  em = errorM "HSH.Command"
  
  {- | Result type for shell commands.  The String is the text description of
  the command, not its output. -}
  type InvokeResult = (String, IO ExitCode)
  
  {- | Type for the environment. -}
  type Environment = Maybe [(String, String)]
  
  {- | A shell command is something we can invoke, pipe to, pipe from,
  or pipe in both directions.  All commands that can be run as shell
  commands must define these methods.
  
  Minimum implementation is 'fdInvoke'.
  
  Some pre-defined instances include:
  
   * A simple bare string, which is passed to the shell for execution. The shell
     will then typically expand wildcards, parse parameters, etc.
  
   * A @(String, [String])@ tuple.  The first item in the tuple gives
     the name of a program to run, and the second gives its arguments.
     The shell is never involved.  This is ideal for passing filenames,
     since there is no security risk involving special shell characters.
  
   * A @Handle -> Handle -> IO ()@ function, which reads from the first
     handle and write to the second.
  
   * Various functions.  These functions will accept input representing
     its standard input and output will go to standard output.  
  
  Some pre-defined instance functions include:
  
   * @(String -> String)@, @(String -> IO String)@, plus the same definitions
     for ByteStrings.
  
   * @([String] -> [String])@, @([String] -> IO [String])@, where each @String@
     in the list represents a single line
  
   * @(() -> String)@, @(() -> IO String)@, for commands that explicitly
     read no input.  Useful with closures.  Useful when you want to avoid
     reading stdin because something else already is.  These have the unit as
     part of the function because otherwise we would have conflicts with things
     such as bare Strings, which represent a command name.
  
  -}
  class (Show a) => ShellCommand a where
      {- | Invoke a command. -}
      fdInvoke :: a               -- ^ The command
               -> Environment     -- ^ The environment
               -> Channel         -- ^ Where to read input from
               -> IO (Channel, [InvokeResult]) -- ^ Returns an action that, when evaluated, waits for the process to finish and returns an exit code.
  
  instance Show (Handle -> Handle -> IO ()) where
      show _ = "(Handle -> Handle -> IO ())"
  instance Show (Channel -> IO Channel) where
      show _ = "(Channel -> IO Channel)"
  instance Show (String -> String) where
      show _ = "(String -> String)"
  instance Show (() -> String) where
      show _ = "(() -> String)"
  instance Show (String -> IO String) where
      show _ = "(String -> IO String)"
  instance Show (() -> IO String) where
      show _ = "(() -> IO String)"
  instance Show (BSL.ByteString -> BSL.ByteString) where
      show _ = "(Data.ByteString.Lazy.ByteString -> Data.ByteString.Lazy.ByteString)"
  instance Show (() -> BSL.ByteString) where
      show _ = "(() -> Data.ByteString.Lazy.ByteString)"
  instance Show (BSL.ByteString -> IO BSL.ByteString) where
      show _ = "(Data.ByteString.Lazy.ByteString -> IO Data.ByteString.Lazy.ByteString)"
  instance Show (() -> IO BSL.ByteString) where
      show _ =  "(() -> IO BSL.ByteString)"
  instance Show (BS.ByteString -> BS.ByteString) where
      show _ = "(Data.ByteString.ByteString -> Data.ByteString.ByteString)"
  instance Show (() -> BS.ByteString) where
      show _ = "(() -> Data.ByteString.ByteString)"
  instance Show (BS.ByteString -> IO BS.ByteString) where
      show _ = "(Data.ByteString.ByteString -> IO Data.ByteString.ByteString)"
  instance Show (() -> IO BS.ByteString) where
      show _ = "(() -> IO Data.ByteString.ByteString)"
  
  instance ShellCommand (String -> IO String) where
      fdInvoke = genericStringlikeIO chanAsString
  
  {- | A user function that takes no input, and generates output.  We will deal
  with it using hPutStr to send the output on. -}
  instance ShellCommand (() -> IO String) where
      fdInvoke = genericStringlikeO
  
  instance ShellCommand (BSL.ByteString -> IO BSL.ByteString) where
      fdInvoke = genericStringlikeIO chanAsBSL
  
  instance ShellCommand (() -> IO BSL.ByteString) where
      fdInvoke = genericStringlikeO
  
  instance ShellCommand (BS.ByteString -> IO BS.ByteString) where
      fdInvoke = genericStringlikeIO chanAsBS
  
  instance ShellCommand (() -> IO BS.ByteString) where
      fdInvoke = genericStringlikeO
  
  {- | An instance of 'ShellCommand' for a pure Haskell function mapping
  String to String.  Implement in terms of (String -> IO String) for
  simplicity. -}
  instance ShellCommand (String -> String) where
      fdInvoke func =
          fdInvoke iofunc
              where iofunc :: String -> IO String
                    iofunc = return . func
  
  instance ShellCommand (() -> String) where
      fdInvoke func =
          fdInvoke iofunc
              where iofunc :: () -> IO String
                    iofunc = return . func
  
  instance ShellCommand (BSL.ByteString -> BSL.ByteString) where
      fdInvoke func =
          fdInvoke iofunc
              where iofunc :: BSL.ByteString -> IO BSL.ByteString
                    iofunc = return . func
  
  instance ShellCommand (() -> BSL.ByteString) where
      fdInvoke func =
          fdInvoke iofunc
              where iofunc :: () -> IO BSL.ByteString
                    iofunc = return . func
  
  instance ShellCommand (BS.ByteString -> BS.ByteString) where
      fdInvoke func =
          fdInvoke iofunc
              where iofunc :: BS.ByteString -> IO BS.ByteString
                    iofunc = return . func
  
  instance ShellCommand (() -> BS.ByteString) where
      fdInvoke func =
          fdInvoke iofunc
              where iofunc :: () -> IO BS.ByteString
                    iofunc = return . func
  
  instance ShellCommand (Channel -> IO Channel) where
      fdInvoke func _ cstdin =
          runInHandler (show func) (func cstdin)
  
  {-
  instance ShellCommand (Handle -> Handle -> IO ()) where
      fdInvoke func cstdin cstdout =
          runInHandler (show func) (func hstdin hstdout)
  -}
  
  genericStringlikeIO :: (Show (a -> IO a), Channelizable a) =>
                         (Channel -> IO a)
                      -> (a -> IO a)
                      -> Environment
                      -> Channel
                      -> IO (Channel, [InvokeResult])
  genericStringlikeIO dechanfunc userfunc _ cstdin =
      do contents <- dechanfunc cstdin
         runInHandler (show userfunc) (realfunc contents)
      where realfunc contents = do r <- userfunc contents
                                   return (toChannel r)
  
  genericStringlikeO :: (Show (() -> IO a), Channelizable a) =>
                        (() -> IO a)
                     -> Environment
                     -> Channel
                     -> IO (Channel, [InvokeResult])
  genericStringlikeO userfunc _ _ =
      runInHandler (show userfunc) realfunc
          where realfunc :: IO Channel
                realfunc = do r <- userfunc ()
                              return (toChannel r)
  
  instance Show ([String] -> [String]) where
      show _ = "([String] -> [String])"
  instance Show (() -> [String]) where
      show _ = "(() -> [String])"
  instance Show ([String] -> IO [String]) where
      show _ = "([String] -> IO [String])"
  instance Show (() -> IO [String]) where
      show _ = "(() -> IO [String])"
  
  {- | An instance of 'ShellCommand' for a pure Haskell function mapping
  [String] to [String].
  
  A [String] is generated from a Handle via the 'lines' function, and the
  reverse occurs via 'unlines'.
  
  So, this function is intended to operate upon lines of input and produce
  lines of output. -}
  instance ShellCommand ([String] -> [String]) where
      fdInvoke func = fdInvoke (unlines . func . lines)
  
  instance ShellCommand (() -> [String]) where
      fdInvoke func = fdInvoke (unlines . func)
  
  {- | The same for an IO function -}
  instance ShellCommand ([String] -> IO [String]) where
      fdInvoke func = fdInvoke iofunc
          where iofunc input = do r <- func (lines input)
                                  return (unlines r)
  
  instance ShellCommand (() -> IO [String]) where
      fdInvoke func = fdInvoke iofunc
          where iofunc :: (() -> IO String)
                iofunc () = do r <- func ()
                               return (unlines r)
  
  
  {- | An instance of 'ShellCommand' for an external command.  The
  first String is the command to run, and the list of Strings represents the
  arguments to the program, if any. -}
  instance ShellCommand (String, [String]) where
      fdInvoke (fp, args) = genericCommand (RawCommand fp args)
  
  {- | An instance of 'ShellCommand' for an external command.  The
  String is split using words to the command to run, and the arguments, if any. -}
  instance ShellCommand String where
      fdInvoke cmd = genericCommand (ShellCommand cmd)
  
  {- | How to we handle and external command. -}
  genericCommand :: CmdSpec 
                 -> Environment
                 -> Channel
                 -> IO (Channel, [InvokeResult])
  
  -- Handling external command when stdin channel is a Handle
  genericCommand c environ (ChanHandle ih) =
      let cp = CreateProcess {cmdspec = c,
                              cwd = Nothing,
                              env = environ,
                              std_in = UseHandle ih,
                              std_out = CreatePipe,
                              std_err = Inherit,
                              close_fds = True
  #if MIN_VERSION_process(1,1,0)
  -- Or use GHC version as a proxy:  __GLASGOW_HASKELL__ >= 720
  			    -- Added field in process 1.1.0.0:
  			    , create_group = False
  #endif
  #if MIN_VERSION_process(1,2,0)
  			    , delegate_ctlc = False
  #endif
  #if MIN_VERSION_process(1,3,0)
            , detach_console = False
            , create_new_console = False
            , new_session = False
  #endif
  #if MIN_VERSION_process(1,4,0)
            , child_group = Nothing
            , child_user = Nothing
  #endif
  #if MIN_VERSION_process(1,5,0)
            , use_process_jobs = False
  #endif
  			   }
      in do (_, oh', _, ph) <- createProcess cp
            let oh = fromJust oh'
            return (ChanHandle oh, [(printCmdSpec c, waitForProcess ph)])
  genericCommand cspec environ ichan = 
      let cp = CreateProcess {cmdspec = cspec,
                              cwd = Nothing,
                              env = environ,
                              std_in = CreatePipe,
                              std_out = CreatePipe,
                              std_err = Inherit,
                              close_fds = True
  #if MIN_VERSION_process(1,1,0)
  			    -- Added field in process 1.1.0.0:
  			    , create_group = False
  #endif
  #if MIN_VERSION_process(1,2,0)
  			    , delegate_ctlc = False
  #endif
  #if MIN_VERSION_process(1,3,0)
            , detach_console = False
            , create_new_console = False
            , new_session = False
  #endif
  #if MIN_VERSION_process(1,4,0)
            , child_group = Nothing
            , child_user = Nothing
  #endif
  #if MIN_VERSION_process(1,5,0)
            , use_process_jobs = False
  #endif
  			   }
      in do (ih', oh', _, ph) <- createProcess cp
            let ih = fromJust ih'
            let oh = fromJust oh'
            chanToHandle True ichan ih
            return (ChanHandle oh, [(printCmdSpec cspec, waitForProcess ph)])
  
  printCmdSpec :: CmdSpec -> String
  printCmdSpec (ShellCommand s) = s
  printCmdSpec (RawCommand fp args) = show (fp, args)
  
  ------------------------------------------------------------
  -- Pipes
  ------------------------------------------------------------
  
  data PipeCommand a b = (ShellCommand a, ShellCommand b) => PipeCommand a b
  
  deriving instance Show (PipeCommand a b)
  
  {- | An instance of 'ShellCommand' represeting a pipeline. -}
  instance (ShellCommand a, ShellCommand b) => ShellCommand (PipeCommand a b) where
      fdInvoke (PipeCommand cmd1 cmd2) env ichan =
          do (chan1, res1) <- fdInvoke cmd1 env ichan
             (chan2, res2) <- fdInvoke cmd2 env chan1
             return (chan2, res1 ++ res2)
  
  {- | Pipe the output of the first command into the input of the second. -}
  (-|-) :: (ShellCommand a, ShellCommand b) => a -> b -> PipeCommand a b
  (-|-) = PipeCommand
  
  {- | Different ways to get data from 'run'.
  
   * IO () runs, throws an exception on error, and sends stdout to stdout
  
   * IO String runs, throws an exception on error, reads stdout into
     a buffer, and returns it as a string.  Note: This output is not lazy.
  
   * IO [String] is same as IO String, but returns the results as lines.
     Note: this output is not lazy.
  
   * IO ExitCode runs and returns an ExitCode with the exit
     information.  stdout is sent to stdout.  Exceptions are not thrown.
  
   * IO (String, ExitCode) is like IO ExitCode, but also
     includes a description of the last command in the pipe to have
     an error (or the last command, if there was no error).
  
   * IO ByteString and are similar to their String counterparts.
  
   * IO (String, IO (String, ExitCode)) returns a String read lazily
     and an IO action that, when evaluated, finishes up the process and
     results in its exit status.  This command returns immediately.
  
   * IO (IO (String, ExitCode)) sends stdout to stdout but returns
     immediately.  It forks off the child but does not wait for it to finish.
     You can use 'checkResults' to wait for the finish.
  
   * IO Int returns the exit code from a program directly.  If a signal
     caused the command to be reaped, returns 128 + SIGNUM.
  
   * IO Bool returns True if the program exited normally (exit code 0,
     not stopped by a signal) and False otherwise.
  
  To address insufficient laziness, you can process anything that needs to be
  processed lazily within the pipeline itself.
  -}
  class RunResult a where
      {- | Runs a command (or pipe of commands), with results presented
         in any number of different ways. -}
      run :: (ShellCommand b) => b -> a
  
  instance RunResult (IO ()) where
      run cmd = run cmd >>= checkResults
  
  instance RunResult (IO (String, ExitCode)) where
      run cmd =
          do (ochan, r) <- fdInvoke cmd Nothing (ChanHandle stdin)
             chanToHandle False ochan stdout
             processResults r
  
  instance RunResult (IO ExitCode) where
      run cmd = ((run cmd)::IO (String, ExitCode)) >>= return . snd
  
  instance RunResult (IO Int) where
      run cmd = do rc <- run cmd
                   case rc of
                     ExitSuccess -> return 0
                     ExitFailure x -> return x
  
  instance RunResult (IO Bool) where
      run cmd = do rc <- run cmd
                   return ((rc::Int) == 0)
  
  instance RunResult (IO [String]) where
      run cmd = do r <- run cmd
                   return (lines r)
  
  instance RunResult (IO String) where
      run cmd = genericStringlikeResult chanAsString (\c -> evaluate (length c))
                cmd
  
  instance RunResult (IO BSL.ByteString) where
      run cmd = genericStringlikeResult chanAsBSL
                (\c -> evaluate (BSL.length c))
                cmd
  
  instance RunResult (IO BS.ByteString) where
      run cmd = genericStringlikeResult chanAsBS
                (\c -> evaluate (BS.length c))
                cmd
  
  instance RunResult (IO (String, IO (String, ExitCode))) where
      run cmd = intermediateStringlikeResult chanAsString cmd
  
  instance RunResult (IO (BSL.ByteString, IO (String, ExitCode))) where
      run cmd = intermediateStringlikeResult chanAsBSL cmd
  
  instance RunResult (IO (BS.ByteString, IO (String, ExitCode))) where
      run cmd = intermediateStringlikeResult chanAsBS cmd
  
  instance RunResult (IO (IO (String, ExitCode))) where
      run cmd = do (ochan, r) <- fdInvoke cmd Nothing (ChanHandle stdin)
                   chanToHandle False ochan stdout
                   return (processResults r)
  
  intermediateStringlikeResult :: ShellCommand b =>
                                  (Channel -> IO a)
                               -> b
                               -> IO (a, IO (String, ExitCode))
  intermediateStringlikeResult chanfunc cmd =
          do (ochan, r) <- fdInvoke cmd Nothing (ChanHandle stdin)
             c <- chanfunc ochan
             return (c, processResults r)
  
  genericStringlikeResult :: ShellCommand b => 
                             (Channel -> IO a)
                          -> (a -> IO c)
                          -> b 
                          -> IO a
  genericStringlikeResult chanfunc evalfunc cmd =
          do (c, r) <- intermediateStringlikeResult chanfunc cmd
             evalfunc c
             --evaluate (length c)
             -- d "runS 6"
             -- d "runS 7"
             r >>= checkResults
             -- d "runS 8"
             return c
  
  {- | Evaluates the result codes and returns an overall status -}
  processResults :: [InvokeResult] -> IO (String, ExitCode)
  processResults r =
      do rc <- mapM procresult r
         case catMaybes rc of
           [] -> return (fst (last r), ExitSuccess)
           x -> return (last x)
      where procresult :: InvokeResult -> IO (Maybe (String, ExitCode))
            procresult (cmd, action) =
                do rc <- action
                   return $ case rc of
                     ExitSuccess -> Nothing
                     x -> Just (cmd, x)
  
  {- | Evaluates result codes and raises an error for any bad ones it finds. -}
  checkResults :: (String, ExitCode) -> IO ()
  checkResults (cmd, ps) =
         case ps of
           ExitSuccess -> return ()
           ExitFailure x ->
               fail $ cmd ++ ": exited with code " ++ show x
  {- FIXME: generate these again 
           Terminated sig ->
               fail $ cmd ++ ": terminated by signal " ++ show sig
           Stopped sig ->
               fail $ cmd ++ ": stopped by signal " ++ show sig
  -}
  
  {- | Handle an exception derived from a program exiting abnormally -}
  tryEC :: IO a -> IO (Either ExitCode a)
  tryEC action =
      do r <-  Control.Exception.try action
         case r of
           Left ioe ->
            if isUserError ioe then
                case (ioeGetErrorString ioe =~~ pat) of
                  Nothing -> ioError ioe -- not ours; re-raise it
                  Just e -> return . Left . procit $ e
            else ioError ioe      -- not ours; re-raise it
           Right result -> return (Right result)
      where pat = ": exited with code [0-9]+$|: terminated by signal ([0-9]+)$|: stopped by signal [0-9]+"
            procit :: String -> ExitCode
            procit e
                | e =~ "^: exited" = ExitFailure (str2ec e)
  --              | e =~ "^: terminated by signal" = Terminated (str2ec e)
  --              | e =~ "^: stopped by signal" = Stopped (str2ec e)
                | otherwise = error "Internal error in tryEC"
            str2ec e =
                read (e =~ "[0-9]+$")
  
  {- | Catch an exception derived from a program exiting abnormally -}
  catchEC :: IO a -> (ExitCode -> IO a) -> IO a
  catchEC action handler =
      do r <- tryEC action
         case r of
           Left ec -> handler ec
           Right result -> return result
  
  {- | A convenience function.  Refers only to the version of 'run'
  that returns @IO ()@.  This prevents you from having to cast to it
  all the time when you do not care about the result of 'run'.
  
  The implementation is simply:
  
  >runIO :: (ShellCommand a) => a -> IO ()
  >runIO = run
  -}
  runIO :: (ShellCommand a) => a -> IO ()
  runIO = run
  
  {- | Another convenience function.  This returns the first line of the output,
  with any trailing newlines or whitespace stripped off.  No leading whitespace
  is stripped.  This function will raise an exception if there is not at least
  one line of output.  Mnemonic: runSL means \"run single line\".
  
  This command exists separately from 'run' because there is already a
  'run' instance that returns a String, though that instance returns the
  entirety of the output in that String. -}
  runSL :: (ShellCommand a) => a -> IO String
  runSL cmd =
      do r <- run cmd
         when (r == []) $ fail $ "runSL: no output received from " ++ show cmd
         return (rstrip . head $ r)
  
  
  {- | Convenience function to wrap a child thread.  Kicks off the thread, handles
  running the code, traps execptions, the works.
  
  Note that if func is lazy, such as a getContents sort of thing,
  the exception may go uncaught here.
  
  NOTE: expects func to be lazy!
   -}
  runInHandler :: String           -- ^ Description of this function
              -> (IO Channel)     -- ^ The action to run in the thread
              -> IO (Channel, [InvokeResult])
  runInHandler descrip func =
      catch (realfunc) (exchandler)
      where realfunc = do r <- func
                          return (r, [(descrip, return ExitSuccess)])
            exchandler :: SomeException -> IO (Channel, [InvokeResult])
            exchandler e = do em $ "runInHandler/" ++ descrip ++ ": " ++ show e
                              return (ChanString "", [(descrip, return (ExitFailure 1))])
  
  
  ------------------------------------------------------------
  -- Environment
  ------------------------------------------------------------
  {- | An environment variable filter function.
  
  This is a low-level interface; see 'setenv' and 'unsetenv' for more convenient
  interfaces. -}
  type EnvironFilter = [(String, String)] -> [(String, String)]
  
  instance Show EnvironFilter where
      show _ = "EnvironFilter"
  
  
  {- | A command that carries environment variable information with it.
  
  This is a low-level interface; see 'setenv' and 'unsetenv' for more
  convenient interfaces. -}
  data EnvironCommand a = (ShellCommand a) => EnvironCommand EnvironFilter a
  
  deriving instance Show (EnvironCommand a)
  
  instance (ShellCommand a) => ShellCommand (EnvironCommand a) where
      fdInvoke (EnvironCommand efilter cmd) Nothing ichan =
          do -- No incoming environment; initialize from system default.
             e <- getEnvironment
             fdInvoke cmd (Just (efilter e)) ichan
      fdInvoke (EnvironCommand efilter cmd) (Just ienv) ichan =
          fdInvoke cmd (Just (efilter ienv)) ichan
  
  {- | Sets an environment variable, replacing an existing one if it exists.
  
  Here's a sample ghci session to illustrate.  First, let's see the defaults for
  some variables:
  
  > Prelude HSH> runIO $ "echo $TERM, $LANG"
  > xterm, en_US.UTF-8
  
  Now, let's set one:
  
  > Prelude HSH> runIO $ setenv [("TERM", "foo")] $ "echo $TERM, $LANG"
  > foo, en_US.UTF-8
  
  Or two:
  
  > Prelude HSH> runIO $ setenv [("TERM", "foo")] $ setenv [("LANG", "de_DE.UTF-8")] $ "echo $TERM, $LANG"
  > foo, de_DE.UTF-8
  
  We could also do it easier, like this:
  
  > Prelude HSH> runIO $ setenv [("TERM", "foo"), ("LANG", "de_DE.UTF-8")] $ "echo $TERM, $LANG"
  > foo, de_DE.UTF-8
  
  It can be combined with unsetenv:
  
  > Prelude HSH> runIO $ setenv [("TERM", "foo")] $ unsetenv ["LANG"] $ "echo $TERM, $LANG"
  > foo,
  
  And used with pipes:
  
  > Prelude HSH> runIO $ setenv [("TERM", "foo")] $ "echo $TERM, $LANG" -|- "tr a-z A-Z"
  > FOO, EN_US.UTF-8
  
  See also 'unsetenv'.
  -}
  setenv :: (ShellCommand cmd) => [(String, String)] -> cmd -> EnvironCommand cmd
  setenv items cmd =
      EnvironCommand efilter cmd
      where efilter ienv = foldr efilter' ienv items
            efilter' (key, val) ienv = 
                (key, val) : (filter (\(k, _) -> k /= key) ienv)
  
  {- | Removes an environment variable if it exists; does nothing otherwise.
  
  See also 'setenv', which has a more extensive example.
  -}
  unsetenv :: (ShellCommand cmd) => [String] -> cmd -> EnvironCommand cmd
  unsetenv keys cmd =
      EnvironCommand efilter cmd
      where efilter ienv = foldr efilter' ienv keys
            efilter' key = filter (\(k, _) -> k /= key)
  