    {-# LANGUAGE FlexibleInstances, TypeSynonymInstances, TypeOperators, ScopedTypeVariables, NamedFieldPuns #-}

    -- | This module provides functions for calling command line programs, primarily
    --   'command' and 'cmd'. As a simple example:
    --
    -- @
    -- 'command' [] \"gcc\" [\"-c\",myfile]
    -- @
    --
    --   The functions from this module are now available directly from "Development.Shake".
    --   You should only need to import this module if you are using the 'cmd' function in the 'IO' monad.
    module Development.Shake.Command(
        command, command_, cmd, unit, CmdArguments, (:->),
        Stdout(..), Stderr(..), Stdouterr(..), Exit(..), Process(..), CmdTime(..), CmdLine(..),
        CmdResult, CmdString, CmdOption(..),
        addPath, addEnv,
        ) where

    import Data.Tuple.Extra
    import Control.Exception.Extra
    import Control.Monad.Extra
    import Control.Monad.IO.Class
    import Data.Char
    import Data.Either.Extra
    import Data.List.Extra
    import Data.Maybe
    import System.Directory
    import System.Environment.Extra
    import System.Exit
    import System.IO.Extra hiding (withTempFile, withTempDir)
    import System.Process
    import System.Info.Extra
    import System.Time.Extra
    import System.IO.Unsafe(unsafeInterleaveIO)
    import qualified Data.ByteString as BS
    import qualified Data.ByteString.Lazy.Char8 as LBS
    import General.Process
    import Control.Applicative
    import Prelude

    import Development.Shake.CmdOption
    import Development.Shake.Core
    import Development.Shake.FilePath
    import Development.Shake.FilePattern
    import Development.Shake.Types
    import Development.Shake.Rules.File
    import Development.Shake.Derived

    ---------------------------------------------------------------------
    -- ACTUAL EXECUTION

    -- | /Deprecated:/ Use 'AddPath'. This function will be removed in a future version.
    --
    --   Add a prefix and suffix to the @$PATH@ environment variable. For example:
    --
    -- @
    -- opt <- 'addPath' [\"\/usr\/special\"] []
    -- 'cmd' opt \"userbinary --version\"
    -- @
    --
    --   Would prepend @\/usr\/special@ to the current @$PATH@, and the command would pick
    --   @\/usr\/special\/userbinary@, if it exists. To add other variables see 'addEnv'.
    addPath :: MonadIO m => [String] -> [String] -> m CmdOption
    addPath pre post = do
        args <- liftIO getEnvironment
        let (path,other) = partition ((== "PATH") . (if isWindows then upper else id) . fst) args
        return $ Env $
            [("PATH",intercalate [searchPathSeparator] $ pre ++ post) | null path] ++
            [(a,intercalate [searchPathSeparator] $ pre ++ [b | b /= ""] ++ post) | (a,b) <- path] ++
            other

    -- | /Deprecated:/ Use 'AddEnv'. This function will be removed in a future version.
    --
    --   Add a single variable to the environment. For example:
    --
    -- @
    -- opt <- 'addEnv' [(\"CFLAGS\",\"-O2\")]
    -- 'cmd' opt \"gcc -c main.c\"
    -- @
    --
    --   Would add the environment variable @$CFLAGS@ with value @-O2@. If the variable @$CFLAGS@
    --   was already defined it would be overwritten. If you wish to modify @$PATH@ see 'addPath'.
    addEnv :: MonadIO m => [(String, String)] -> m CmdOption
    addEnv extra = do
        args <- liftIO getEnvironment
        return $ Env $ extra ++ filter (\(a,_) -> a `notElem` map fst extra) args

    data Str = Str String | BS BS.ByteString | LBS LBS.ByteString | Unit deriving Eq

    data Result
        = ResultStdout Str
        | ResultStderr Str
        | ResultStdouterr Str
        | ResultCode ExitCode
        | ResultTime Double
        | ResultLine String
        | ResultProcess Pid
          deriving Eq

    data Pid = Pid0 | Pid ProcessHandle
    instance Eq Pid where _ == _ = True

    ---------------------------------------------------------------------
    -- ACTION EXPLICIT OPERATION

    -- | Given explicit operations, apply the advance ones, like skip/trace/track/autodep
    commandExplicit :: String -> [CmdOption] -> [Result] -> String -> [String] -> Action [Result]
    commandExplicit funcName oopts results exe args = do
        ShakeOptions
            {shakeCommandOptions,shakeRunCommands
            ,shakeLint,shakeLintInside,shakeLintIgnore} <- getShakeOptions
        let fopts = shakeCommandOptions ++ oopts
        let useShell = Shell `elem` fopts
        let useLint = shakeLint == Just LintFSATrace
        let useAutoDeps = AutoDeps `elem` fopts
        let opts = filter (/= Shell) fopts

        let skipper act = if null results && not shakeRunCommands then return [] else act

        let verboser act = do
                let cwd = listToMaybe $ reverse [x | Cwd x <- opts]
                putLoud $ maybe "" (\x -> "cd " ++ x ++ "; ") cwd ++ showCommandForUser2 exe args
                verb <- getVerbosity
                (if verb >= Loud then quietly else id) act

        let tracer = case reverse [x | Traced x <- opts] of
                "":_ -> liftIO
                msg:_ -> traced msg
                [] -> traced (takeFileName exe)

        let tracker act
                | useLint = fsatrace act
                | useAutoDeps = autodeps act
                | useShell = shelled act
                | otherwise = act exe args

            shelled = runShell (unwords $ exe : args)

            ignore = map (?==) shakeLintIgnore
            ham cwd xs = [makeRelative cwd x | x <- map toStandard xs
                                             , any (`isPrefixOf` x) shakeLintInside
                                             , not $ any ($ x) ignore]

            fsaCmd act opts file
                | isMac = fsaCmdMac act opts file
                | useShell = runShell (unwords $ exe : args) $ \exe args -> act "fsatrace" $ opts : file : "--" : exe : args
                | otherwise = act "fsatrace" $ opts : file : "--" : exe : args

            fsaCmdMac act opts file = do
                let fakeExe e = liftIO $ do
                        me <- findExecutable e
                        case me of
                            Just re -> do
                                let isSystem = any (`isPrefixOf` re) [ "/bin"
                                                                     , "/usr"
                                                                     , "/sbin"
                                                                     ]
                                if isSystem
                                    then do
                                        tmpdir <- getTemporaryDirectory
                                        let fake = tmpdir ++ "fsatrace-fakes" ++ re
                                        unlessM (doesFileExist fake) $ do
                                            createDirectoryIfMissing True $ takeDirectory fake
                                            copyFile re fake
                                        return fake
                                    else return re
                            Nothing -> return e
                fexe <- fakeExe exe
                if useShell
                    then do
                        fsh <- fakeExe "/bin/sh"
                        act "fsatrace" $ opts : file : "--" : fsh : "-c" : [unwords $ fexe : args]
                    else act "fsatrace" $ opts : file : "--" : fexe : args

            fsatrace act = withTempFile $ \file -> do
                res <- fsaCmd act "rwm" file
                xs <- liftIO $ parseFSAT <$> readFileUTF8' file
                cwd <- liftIO getCurrentDirectory
                let reader (FSATRead x) = Just x; reader _ = Nothing
                    writer (FSATWrite x) = Just x; writer (FSATMove x _) = Just x; writer _ = Nothing
                    existing f = liftIO . filterM doesFileExist . nubOrd . mapMaybe f
                rs <- existing reader xs
                ws <- existing writer xs
                let reads = ham cwd rs
                    writes = ham cwd ws
                when useAutoDeps $
                    unsafeAllowApply $ needed reads
                trackRead reads
                trackWrite writes
                return res

            autodeps act = withTempFile $ \file -> do
                res <-  fsaCmd act "r" file
                pxs <- liftIO $ parseFSAT <$> readFileUTF8' file
                xs <- liftIO $ filterM doesFileExist [x | FSATRead x <- pxs]
                cwd <- liftIO getCurrentDirectory
                unsafeAllowApply $ needNorm $ ham cwd xs
                return res

        skipper $ tracker $ \exe args -> verboser $ tracer $ commandExplicitIO funcName opts results exe args

    -- | Given a shell command, call the continuation with the sanitised exec-style arguments
    runShell :: String -> (String -> [String] -> Action a) -> Action a
    runShell x act | not isWindows = act "/bin/sh" ["-c",x] -- do exactly what Haskell does
    runShell x act = withTempDir $ \dir -> do
        let file = dir </> "s.bat"
        writeFile' file x
        act "cmd.exe" ["/d/q/c",file]

    -- | Parse the FSATrace structure
    data FSAT
        = FSATWrite FilePath
        | FSATRead FilePath
        | FSATDelete FilePath
        | FSATMove FilePath FilePath

    -- | Parse the 'FSAT' entries, ignoring anything you don't understand.
    parseFSAT :: String -> [FSAT]
    parseFSAT = mapMaybe f . lines
        where f ('w':'|':xs) = Just $ FSATWrite xs
              f ('r':'|':xs) = Just $ FSATRead xs
              f ('d':'|':xs) = Just $ FSATDelete xs
              f ('m':'|':xs) | (xs,'|':ys) <- break (== '|') xs = Just $ FSATMove xs ys
              f _ = Nothing

    ---------------------------------------------------------------------
    -- IO EXPLICIT OPERATION

    -- | Given a very explicit set of CmdOption, translate them to a General.Process structure
    commandExplicitIO :: String -> [CmdOption] -> [Result] -> String -> [String] -> IO [Result]
    commandExplicitIO funcName opts results exe args = do
        let (grabStdout, grabStderr) = both or $ unzip $ for results $ \r -> case r of
                ResultStdout{} -> (True, False)
                ResultStderr{} -> (False, True)
                ResultStdouterr{} -> (True, True)
                _ -> (False, False)

        optEnv <- resolveEnv opts
        let optCwd = let x = last $ "" : [x | Cwd x <- opts] in if x == "" then Nothing else Just x
        let optStdin = flip mapMaybe opts $ \x -> case x of
                Stdin x -> Just $ SrcString x
                StdinBS x -> Just $ SrcBytes x
                FileStdin x -> Just $ SrcFile x
                _ -> Nothing
        let optShell = Shell `elem` opts
        let optBinary = BinaryPipes `elem` opts
        let optAsync = ResultProcess Pid0 `elem` results
        let optTimeout = listToMaybe $ reverse [x | Timeout x <- opts]
        let optWithStdout = last $ False : [x | WithStdout x <- opts]
        let optWithStderr = last $ True : [x | WithStderr x <- opts]
        let optFileStdout = [x | FileStdout x <- opts]
        let optFileStderr = [x | FileStderr x <- opts]
        let optEchoStdout = last $ (not grabStdout && null optFileStdout) : [x | EchoStdout x <- opts]
        let optEchoStderr = last $ (not grabStderr && null optFileStderr) : [x | EchoStderr x <- opts]

        let cmdline = showCommandForUser2 exe args
        let bufLBS f = do (a,b) <- buf $ LBS LBS.empty; return (a, (\(LBS x) -> f x) <$> b)
            buf Str{} | optBinary = bufLBS (Str . LBS.unpack)
            buf Str{} = do x <- newBuffer; return ([DestString x | not optAsync], Str . concat <$> readBuffer x)
            buf LBS{} = do x <- newBuffer; return ([DestBytes x | not optAsync], LBS . LBS.fromChunks <$> readBuffer x)
            buf BS {} = bufLBS (BS . BS.concat . LBS.toChunks)
            buf Unit  = return ([], return Unit)
        (dStdout, dStderr, resultBuild) :: ([[Destination]], [[Destination]], [Double -> ProcessHandle -> ExitCode -> IO Result]) <-
            fmap unzip3 $ forM results $ \r -> case r of
                ResultCode _ -> return ([], [], \_ _ ex -> return $ ResultCode ex)
                ResultTime _ -> return ([], [], \dur _ _ -> return $ ResultTime dur)
                ResultLine _ -> return ([], [], \_ _ _ -> return $ ResultLine cmdline)
                ResultProcess _ -> return ([], [], \_ pid _ -> return $ ResultProcess $ Pid pid)
                ResultStdout    s -> do (a,b) <- buf s; return (a , [], \_ _ _ -> fmap ResultStdout b)
                ResultStderr    s -> do (a,b) <- buf s; return ([], a , \_ _ _ -> fmap ResultStderr b)
                ResultStdouterr s -> do (a,b) <- buf s; return (a , a , \_ _ _ -> fmap ResultStdouterr b)

        exceptionBuffer <- newBuffer
        po <- resolvePath ProcessOpts
            {poCommand = if optShell then ShellCommand $ unwords $ exe:args else RawCommand exe args
            ,poCwd = optCwd, poEnv = optEnv, poTimeout = optTimeout
            ,poStdin = [SrcBytes LBS.empty | optBinary && not (null optStdin)] ++ optStdin
            ,poStdout = [DestEcho | optEchoStdout] ++ map DestFile optFileStdout ++ [DestString exceptionBuffer | optWithStdout && not optAsync] ++ concat dStdout
            ,poStderr = [DestEcho | optEchoStderr] ++ map DestFile optFileStderr ++ [DestString exceptionBuffer | optWithStderr && not optAsync] ++ concat dStderr
            ,poAsync = optAsync
            }
        res <- try_ $ duration $ process po

        let failure extra = do
                cwd <- case optCwd of
                    Nothing -> return ""
                    Just v -> do
                        v <- canonicalizePath v `catch_` const (return v)
                        return $ "Current directory: " ++ v ++ "\n"
                fail $
                    "Development.Shake." ++ funcName ++ ", system command failed\n" ++
                    "Command: " ++ cmdline ++ "\n" ++
                    cwd ++ extra
        case res of
            Left err -> failure $ show err
            Right (_,(_,ex)) | ex /= ExitSuccess && ResultCode ExitSuccess `notElem` results -> do
                exceptionBuffer <- readBuffer exceptionBuffer
                let captured = ["Stderr" | optWithStderr] ++ ["Stdout" | optWithStdout]
                failure $
                    "Exit code: " ++ show (case ex of ExitFailure i -> i; _ -> 0) ++ "\n" ++
                    if null captured then "Stderr not captured because WithStderr False was used\n"
                    else if null exceptionBuffer then intercalate " and " captured ++ " " ++ (if length captured == 1 then "was" else "were") ++ " empty"
                    else intercalate " and " captured ++ ":\n" ++ unlines (dropWhile null $ lines $ concat exceptionBuffer)
            Right (dur,(pid,ex)) -> mapM (\f -> f dur pid ex) resultBuild

    -- | Apply all environment operations, to produce a new environment to use.
    resolveEnv :: [CmdOption] -> IO (Maybe [(String, String)])
    resolveEnv opts
        | null env, null addEnv, null addPath, null remEnv = return Nothing
        | otherwise = Just . unique . tweakPath . (++ addEnv) . filter (flip notElem remEnv . fst) <$>
                      if null env then getEnvironment else return (concat env)
        where
            env = [x | Env x <- opts]
            addEnv = [(x,y) | AddEnv x y <- opts]
            remEnv = [x | RemEnv x <- opts]
            addPath = [(x,y) | AddPath x y <- opts]

            newPath mid = intercalate [searchPathSeparator] $
                concat (reverse $ map fst addPath) ++ [mid | mid /= ""] ++ concatMap snd addPath
            isPath x = (if isWindows then upper else id) x == "PATH"
            tweakPath xs | not $ any (isPath . fst) xs = ("PATH", newPath "") : xs
                         | otherwise = map (\(a,b) -> (a, if isPath a then newPath b else b)) xs

            unique = reverse . nubOrdOn (if isWindows then upper . fst else fst) . reverse

    -- | If the user specifies a custom $PATH, and not Shell, then try and resolve their exe ourselves.
    --   Tricky, because on Windows it doesn't look in the $PATH first.
    resolvePath :: ProcessOpts -> IO ProcessOpts
    resolvePath po
        | Just e <- poEnv po
        , Just (_, path) <- find ((==) "PATH" . (if isWindows then upper else id) . fst) e
        , RawCommand prog args <- poCommand po
        = do
        let progExe = if prog == prog -<.> exe then prog else prog <.> exe
        -- use unsafeInterleaveIO to allow laziness to skip the queries we don't use
        pathOld <- unsafeInterleaveIO $ fromMaybe "" <$> lookupEnv "PATH"
        old <- unsafeInterleaveIO $ findExecutable prog
        new <- unsafeInterleaveIO $ findExecutableWith (splitSearchPath path) progExe
        old2 <- unsafeInterleaveIO $ findExecutableWith (splitSearchPath pathOld) progExe

        switch <- return $ case () of
            _ | path == pathOld -> False -- The state I can see hasn't changed
              | Nothing <- new -> False -- I have nothing to offer
              | Nothing <- old -> True -- I failed last time, so this must be an improvement
              | Just old <- old, Just new <- new, equalFilePath old new -> False -- no different
              | Just old <- old, Just old2 <- old2, equalFilePath old old2 -> True -- I could predict last time
              | otherwise -> False
        return $ case new of
            Just new | switch -> po{poCommand = RawCommand new args}
            _ -> po
    resolvePath po = return po

    -- | Given a list of directories, and a file name, return the complete path if you can find it.
    --   Like findExecutable, but with a custom PATH.
    findExecutableWith :: [FilePath] -> String -> IO (Maybe FilePath)
    findExecutableWith path x = flip firstJustM (map (</> x) path) $ \s ->
        ifM (doesFileExist s) (return $ Just s) (return Nothing)

    ---------------------------------------------------------------------
    -- FIXED ARGUMENT WRAPPER

    -- | Collect the @stdout@ of the process.
    --   If used, the @stdout@ will not be echoed to the terminal, unless you include 'EchoStdout'.
    --   The value type may be either 'String', or either lazy or strict 'ByteString'.
    newtype Stdout a = Stdout {fromStdout :: a}

    -- | Collect the @stderr@ of the process.
    --   If used, the @stderr@ will not be echoed to the terminal, unless you include 'EchoStderr'.
    --   The value type may be either 'String', or either lazy or strict 'ByteString'.
    newtype Stderr a = Stderr {fromStderr :: a}

    -- | Collect the @stdout@ and @stderr@ of the process.
    --   If used, the @stderr@ and @stdout@ will not be echoed to the terminal, unless you include 'EchoStdout' and 'EchoStderr'.
    --   The value type may be either 'String', or either lazy or strict 'ByteString'.
    newtype Stdouterr a = Stdouterr {fromStdouterr :: a}

    -- | Collect the 'ExitCode' of the process.
    --   If you do not collect the exit code, any 'ExitFailure' will cause an exception.
    newtype Exit = Exit {fromExit :: ExitCode}

    -- | Collect the 'ProcessHandle' of the process.
    --   If you do collect the process handle, the command will run asyncronously and the call to 'cmd'/'command'
    --   will return as soon as the process is spawned. Any 'Stdout'\/'Stderr' captures will return empty strings.
    newtype Process = Process {fromProcess :: ProcessHandle}

    -- | Collect the time taken to execute the process. Can be used in conjunction with 'CmdLine' to
    --   write helper functions that print out the time of a result.
    --
    -- @
    -- timer :: ('CmdResult' r, MonadIO m) => (forall r . 'CmdResult' r => m r) -> m r
    -- timer act = do
    --     ('CmdTime' t, 'CmdLine' x, r) <- act
    --     liftIO $ putStrLn $ \"Command \" ++ x ++ \" took \" ++ show t ++ \" seconds\"
    --     return r
    --
    -- run :: IO ()
    -- run = timer $ 'cmd' \"ghc --version\"
    -- @
    newtype CmdTime = CmdTime {fromCmdTime :: Double}

    -- | Collect the command line used for the process. This command line will be approximate -
    --   suitable for user diagnostics, but not for direct execution.
    newtype CmdLine = CmdLine {fromCmdLine :: String}

    -- | The allowable 'String'-like values that can be captured.
    class CmdString a where cmdString :: (Str, Str -> a)
    instance CmdString () where cmdString = (Unit, \Unit -> ())
    instance CmdString String where cmdString = (Str "", \(Str x) -> x)
    instance CmdString BS.ByteString where cmdString = (BS BS.empty, \(BS x) -> x)
    instance CmdString LBS.ByteString where cmdString = (LBS LBS.empty, \(LBS x) -> x)

    -- | A class for specifying what results you want to collect from a process.
    --   Values are formed of 'Stdout', 'Stderr', 'Exit' and tuples of those.
    class CmdResult a where
        -- Return a list of results (with the right type but dummy data)
        -- and a function to transform a populated set of results into a value
        cmdResult :: ([Result], [Result] -> a)

    instance CmdResult Exit where
        cmdResult = ([ResultCode ExitSuccess], \[ResultCode x] -> Exit x)

    instance CmdResult ExitCode where
        cmdResult = ([ResultCode ExitSuccess], \[ResultCode x] -> x)

    instance CmdResult Process where
        cmdResult = ([ResultProcess Pid0], \[ResultProcess (Pid x)] -> Process x)

    instance CmdResult ProcessHandle where
        cmdResult = ([ResultProcess Pid0], \[ResultProcess (Pid x)] -> x)

    instance CmdResult CmdLine where
        cmdResult = ([ResultLine ""], \[ResultLine x] -> CmdLine x)

    instance CmdResult CmdTime where
        cmdResult = ([ResultTime 0], \[ResultTime x] -> CmdTime x)

    instance CmdString a => CmdResult (Stdout a) where
        cmdResult = let (a,b) = cmdString in ([ResultStdout a], \[ResultStdout x] -> Stdout $ b x)

    instance CmdString a => CmdResult (Stderr a) where
        cmdResult = let (a,b) = cmdString in ([ResultStderr a], \[ResultStderr x] -> Stderr $ b x)

    instance CmdString a => CmdResult (Stdouterr a) where
        cmdResult = let (a,b) = cmdString in ([ResultStdouterr a], \[ResultStdouterr x] -> Stdouterr $ b x)

    instance CmdResult () where
        cmdResult = ([], \[] -> ())

    instance (CmdResult x1, CmdResult x2) => CmdResult (x1,x2) where
        cmdResult = (a1++a2, \rs -> let (r1,r2) = splitAt (length a1) rs in (b1 r1, b2 r2))
            where (a1,b1) = cmdResult
                  (a2,b2) = cmdResult

    cmdResultWith :: forall b c. CmdResult b => (b -> c) -> ([Result], [Result] -> c)
    cmdResultWith f = second (f .) cmdResult

    instance (CmdResult x1, CmdResult x2, CmdResult x3) => CmdResult (x1,x2,x3) where
        cmdResult = cmdResultWith $ \(a,(b,c)) -> (a,b,c)

    instance (CmdResult x1, CmdResult x2, CmdResult x3, CmdResult x4) => CmdResult (x1,x2,x3,x4) where
        cmdResult = cmdResultWith $ \(a,(b,c,d)) -> (a,b,c,d)

    instance (CmdResult x1, CmdResult x2, CmdResult x3, CmdResult x4, CmdResult x5) => CmdResult (x1,x2,x3,x4,x5) where
        cmdResult = cmdResultWith $ \(a,(b,c,d,e)) -> (a,b,c,d,e)

    -- | Execute a system command. Before running 'command' make sure you 'Development.Shake.need' any files
    --   that are used by the command.
    --
    --   This function takes a list of options (often just @[]@, see 'CmdOption' for the available
    --   options), the name of the executable (either a full name, or a program on the @$PATH@) and
    --   a list of arguments. The result is often @()@, but can be a tuple containg any of 'Stdout',
    --   'Stderr' and 'Exit'. Some examples:
    --
    -- @
    -- 'command_' [] \"gcc\" [\"-c\",\"myfile.c\"]                          -- compile a file, throwing an exception on failure
    -- 'Exit' c <- 'command' [] \"gcc\" [\"-c\",myfile]                     -- run a command, recording the exit code
    -- ('Exit' c, 'Stderr' err) <- 'command' [] \"gcc\" [\"-c\",\"myfile.c\"]   -- run a command, recording the exit code and error output
    -- 'Stdout' out <- 'command' [] \"gcc\" [\"-MM\",\"myfile.c\"]            -- run a command, recording the output
    -- 'command_' ['Cwd' \"generated\"] \"gcc\" [\"-c\",myfile]               -- run a command in a directory
    -- @
    --
    --   Unless you retrieve the 'ExitCode' using 'Exit', any 'ExitFailure' will throw an error, including
    --   the 'Stderr' in the exception message. If you capture the 'Stdout' or 'Stderr', that stream will not be echoed to the console,
    --   unless you use the option 'EchoStdout' or 'EchoStderr'.
    --
    --   If you use 'command' inside a @do@ block and do not use the result, you may get a compile-time error about being
    --   unable to deduce 'CmdResult'. To avoid this error, use 'command_'.
    --
    --   By default the @stderr@ stream will be captured for use in error messages, and also echoed. To only echo
    --   pass @'WithStderr' 'False'@, which causes no streams to be captured by Shake, and certain programs (e.g. @gcc@)
    --   to detect they are running in a terminal.
    command :: CmdResult r => [CmdOption] -> String -> [String] -> Action r
    command opts x xs = b <$> commandExplicit "command" opts a x xs
        where (a,b) = cmdResult

    -- | A version of 'command' where you do not require any results, used to avoid errors about being unable
    --   to deduce 'CmdResult'.
    command_ :: [CmdOption] -> String -> [String] -> Action ()
    command_ opts x xs = void $ commandExplicit "command_" opts [] x xs

    ---------------------------------------------------------------------
    -- VARIABLE ARGUMENT WRAPPER

    -- | A type annotation, equivalent to the first argument, but in variable argument contexts,
    --   gives a clue as to what return type is expected (not actually enforced).
    type a :-> t = a

    -- | Execute a system command. Before running 'cmd' make sure you 'Development.Shake.need' any files
    --   that are used by the command.
    --
    -- * @String@ arguments are treated as whitespace separated arguments.
    --
    -- * @[String]@ arguments are treated as literal arguments.
    --
    -- * 'CmdOption' arguments are used as options.
    --
    --   As some examples, here are some calls, and the resulting command string:
    --
    -- @
    -- 'unit' $ 'cmd' \"git log --pretty=\" \"oneline\"           -- git log --pretty= oneline
    -- 'unit' $ 'cmd' \"git log --pretty=\" [\"oneline\"]         -- git log --pretty= oneline
    -- 'unit' $ 'cmd' \"git log\" (\"--pretty=\" ++ \"oneline\")    -- git log --pretty=oneline
    -- 'unit' $ 'cmd' \"git log\" (\"--pretty=\" ++ \"one line\")   -- git log --pretty=one line
    -- 'unit' $ 'cmd' \"git log\" [\"--pretty=\" ++ \"one line\"]   -- git log "--pretty=one line"
    -- @
    --
    --   More examples, including return values, see this translation of the examples given for the 'command' function:
    --
    -- @
    -- () <- 'cmd' \"gcc -c myfile.c\"                                  -- compile a file, throwing an exception on failure
    -- 'unit' $ 'cmd' \"gcc -c myfile.c\"                                 -- alternative to () <- binding.
    -- 'Exit' c <- 'cmd' \"gcc -c\" [myfile]                              -- run a command, recording the exit code
    -- ('Exit' c, 'Stderr' err) <- 'cmd' \"gcc -c myfile.c\"                -- run a command, recording the exit code and error output
    -- 'Stdout' out <- 'cmd' \"gcc -MM myfile.c\"                         -- run a command, recording the output
    -- 'cmd' ('Cwd' \"generated\") \"gcc -c\" [myfile] :: 'Action' ()         -- run a command in a directory
    -- @
    --
    --   When passing file arguments we use @[myfile]@ so that if the @myfile@ variable contains spaces they are properly escaped.
    --
    --   If you use 'cmd' inside a @do@ block and do not use the result, you may get a compile-time error about being
    --   unable to deduce 'CmdResult'. To avoid this error, bind the result to @()@, or include a type signature, or use
    --   the 'unit' function.
    --
    --   The 'cmd' function can also be run in the 'IO' monad, but then 'Traced' is ignored and command lines are not echoed.
    --   As an example:
    --
    -- @
    -- 'cmd' ('Cwd' \"generated\") 'Shell' \"gcc -c myfile.c\" :: IO ()
    -- @
    cmd :: CmdArguments args => args :-> Action r
    cmd = cmdArguments []

    -- | The arguments to 'cmd' - see 'cmd' for examples and semantics.
    class CmdArguments t where cmdArguments :: [Either CmdOption String] -> t
    instance (Arg a, CmdArguments r) => CmdArguments (a -> r) where
        cmdArguments xs x = cmdArguments $ xs ++ arg x
    instance CmdResult r => CmdArguments (Action r) where
        cmdArguments x = case partitionEithers x of
            (opts, x:xs) -> let (a,b) = cmdResult in b <$> commandExplicit "cmd" opts a x xs
            _ -> error "Error, no executable or arguments given to Development.Shake.cmd"
    instance CmdResult r => CmdArguments (IO r) where
        cmdArguments x = case partitionEithers x of
            (opts, x:xs) -> let (a,b) = cmdResult in b <$> commandExplicitIO "cmd" opts a x xs
            _ -> error "Error, no executable or arguments given to Development.Shake.cmd"

    instance CmdArguments [Either CmdOption String] where
        cmdArguments = id

    class Arg a where arg :: a -> [Either CmdOption String]
    instance Arg String where arg = map Right . words
    instance Arg [String] where arg = map Right
    instance Arg CmdOption where arg = return . Left
    instance Arg [CmdOption] where arg = map Left
    instance Arg a => Arg (Maybe a) where arg = maybe [] arg

    ---------------------------------------------------------------------
    -- UTILITIES

    -- A better version of showCommandForUser, which doesn't escape so much on Windows
    showCommandForUser2 :: FilePath -> [String] -> String
    showCommandForUser2 cmd args = unwords $ map (\x -> if safe x then x else showCommandForUser x []) $ cmd : args
        where
            safe xs = not (null xs) && not (any bad xs)
            bad x = isSpace x || (x == '\\' && not isWindows) || x `elem` "\"\'"