    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Distribution.Simple.Command
    -- Copyright   :  Duncan Coutts 2007
    --
    -- Maintainer  :  cabal-devel@haskell.org
    -- Portability :  portable
    --
    -- This is to do with command line handling. The Cabal command line is
    -- organised into a number of named sub-commands (much like darcs). The
    -- 'CommandUI' abstraction represents one of these sub-commands, with a name,
    -- description, a set of flags. Commands can be associated with actions and
    -- run. It handles some common stuff automatically, like the @--help@ and
    -- command line completion flags. It is designed to allow other tools make
    -- derived commands. This feature is used heavily in @cabal-install@.

    {- All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are
    met:

        * Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

        * Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials provided
          with the distribution.

        * Neither the name of Isaac Jones nor the names of other
          contributors may be used to endorse or promote products derived
          from this software without specific prior written permission.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
    OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
    SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
    LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
    DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
    THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. -}

    module Distribution.Simple.Command (

      -- * Command interface
      CommandUI(..),
      commandShowOptions,

      -- ** Constructing commands
      ShowOrParseArgs(..),
      makeCommand,

      -- ** Associating actions with commands
      Command,
      commandAddAction,
      noExtraFlags,

      -- ** Running commands
      CommandParse(..),
      commandsRun,

    -- * Option Fields
      OptionField(..), Name,

    -- ** Constructing Option Fields
      option, multiOption,

    -- ** Liftings & Projections
      liftOption, viewAsFieldDescr,

    -- * Option Descriptions
      OptDescr(..), Description, SFlags, LFlags, OptFlags, ArgPlaceHolder,

    -- ** OptDescr 'smart' constructors
      MkOptDescr,
      reqArg, reqArg', optArg, optArg', noArg,
      boolOpt, boolOpt', choiceOpt, choiceOptFromEnum

      ) where

    import Control.Monad
    import Data.Char (isAlpha, toLower)
    import Data.List (sortBy)
    import Data.Maybe
    import Data.Monoid
    import qualified Distribution.GetOpt as GetOpt
    import Distribution.Text
             ( Text(disp, parse) )
    import Distribution.ParseUtils
    import Distribution.ReadE
    import Distribution.Simple.Utils (die, intercalate)
    import Text.PrettyPrint.HughesPJ    ( punctuate, cat, comma, text, empty)

    data CommandUI flags = CommandUI {
        -- | The name of the command as it would be entered on the command line.
        -- For example @\"build\"@.
        commandName        :: String,
        -- | A short, one line description of the command to use in help texts.
        commandSynopsis :: String,
        -- | The useage line summary for this command
        commandUsage    :: String -> String,
        -- | Additional explanation of the command to use in help texts.
        commandDescription :: Maybe (String -> String),
        -- | Initial \/ empty flags
        commandDefaultFlags :: flags,
        -- | All the Option fields for this command
        commandOptions     :: ShowOrParseArgs -> [OptionField flags]
      }

    data ShowOrParseArgs = ShowArgs | ParseArgs

    type Name        = String
    type Description = String

    -- | We usually have a datatype for storing configuration values, where
    --   every field stores a configuration option, and the user sets
    --   the value either via command line flags or a configuration file.
    --   An individual OptionField models such a field, and we usually
    --   build a list of options associated to a configuration datatype.
    data OptionField a = OptionField {
      optionName        :: Name,
      optionDescr       :: [OptDescr a] }

    -- | An OptionField takes one or more OptDescrs, describing the command line interface for the field.
    data OptDescr a  = ReqArg Description OptFlags ArgPlaceHolder (ReadE (a->a))         (a -> [String])
                     | OptArg Description OptFlags ArgPlaceHolder (ReadE (a->a)) (a->a)  (a -> [Maybe String])
                     | ChoiceOpt [(Description, OptFlags, a->a, a -> Bool)]
                     | BoolOpt Description OptFlags{-True-} OptFlags{-False-} (Bool -> a -> a) (a-> Maybe Bool)

    -- | Short command line option strings
    type SFlags   = [Char]
    -- | Long command line option strings
    type LFlags   = [String]
    type OptFlags = (SFlags,LFlags)
    type ArgPlaceHolder = String

    -- | Create an option taking a single OptDescr.
    --   No explicit Name is given for the Option, the name is the first LFlag given.
    option :: SFlags -> LFlags -> Description -> get -> set -> MkOptDescr get set a -> OptionField a
    option sf lf@(n:_) d get set arg = OptionField n [arg sf lf d get set]
    option _ _ _ _ _ _ = error "Distribution.command.option: An OptionField must have at least one LFlag"

    -- | Create an option taking several OptDescrs.
    --   You will have to give the flags and description individually to the OptDescr constructor.
    multiOption :: Name -> get -> set
                -> [get -> set -> OptDescr a]  -- ^MkOptDescr constructors partially applied to flags and description.
                -> OptionField a
    multiOption n get set args = OptionField n [arg get set | arg <- args]

    type MkOptDescr get set a = SFlags -> LFlags -> Description -> get -> set -> OptDescr a

    -- | Create a string-valued command line interface.
    reqArg :: Monoid b => ArgPlaceHolder -> ReadE b -> (b -> [String])
                       -> MkOptDescr (a -> b) (b -> a -> a) a
    reqArg ad mkflag showflag sf lf d get set =
      ReqArg d (sf,lf) ad (fmap (\a b -> set (get b `mappend` a) b) mkflag) (showflag . get)

    -- | Create a string-valued command line interface with a default value.
    optArg :: Monoid b => ArgPlaceHolder -> ReadE b -> b -> (b -> [Maybe String])
                       -> MkOptDescr (a -> b) (b -> a -> a) a
    optArg ad mkflag def showflag sf lf d get set  =
      OptArg d (sf,lf) ad (fmap (\a b -> set (get b `mappend` a) b) mkflag)
                   (\b ->          set (get b `mappend` def) b)
                   (showflag . get)

    -- | (String -> a) variant of "reqArg"
    reqArg' :: Monoid b => ArgPlaceHolder -> (String -> b) -> (b -> [String])
                        -> MkOptDescr (a -> b) (b -> a -> a) a
    reqArg' ad mkflag showflag =
        reqArg ad (succeedReadE mkflag) showflag

    -- | (String -> a) variant of "optArg"
    optArg' :: Monoid b => ArgPlaceHolder -> (Maybe String -> b) -> (b -> [Maybe String])
                        -> MkOptDescr (a -> b) (b -> a -> a) a
    optArg' ad mkflag showflag =
        optArg ad (succeedReadE (mkflag . Just)) def showflag
          where def = mkflag Nothing

    noArg :: (Eq b, Monoid b) => b -> MkOptDescr (a -> b) (b -> a -> a) a
    noArg flag sf lf d = choiceOpt [(flag, (sf,lf), d)] sf lf d

    boolOpt :: (b -> Maybe Bool) -> (Bool -> b) -> SFlags -> SFlags -> MkOptDescr (a -> b) (b -> a -> a) a
    boolOpt g s sfT sfF _sf _lf@(n:_) d get set =
        BoolOpt d (sfT, ["enable-"++n]) (sfF, ["disable-"++n]) (set.s) (g.get)
    boolOpt _ _ _ _ _ _ _ _ _ = error "Distribution.Simple.Setup.boolOpt: unreachable"

    boolOpt' :: (b -> Maybe Bool) -> (Bool -> b) -> OptFlags -> OptFlags -> MkOptDescr (a -> b) (b -> a -> a) a
    boolOpt' g s ffT ffF _sf _lf d get set = BoolOpt d ffT ffF (set.s) (g . get)

    -- | create a Choice option
    choiceOpt :: Eq b => [(b,OptFlags,Description)] -> MkOptDescr (a -> b) (b -> a -> a) a
    choiceOpt aa_ff _sf _lf _d get set  = ChoiceOpt alts
        where alts = [(d,flags, set alt, (==alt) . get) | (alt,flags,d) <- aa_ff]

    -- | create a Choice option out of an enumeration type.
    --   As long flags, the Show output is used. As short flags, the first character
    --   which does not conflict with a previous one is used.
    choiceOptFromEnum :: (Bounded b, Enum b, Show b, Eq b) => MkOptDescr (a -> b) (b -> a -> a) a
    choiceOptFromEnum _sf _lf d get = choiceOpt [ (x, (sf, [map toLower $ show x]), d')
                                                    | (x, sf) <- sflags'
                                                    , let d' = d ++ show x]
                                                _sf _lf d get
        where sflags' = foldl f [] [firstOne..]
              f prev x = let prevflags = concatMap snd prev in
                         prev ++ take 1 [(x, [toLower sf]) | sf <- show x, isAlpha sf
                                                           , toLower sf `notElem` prevflags]
              firstOne = minBound `asTypeOf` get undefined

    commandGetOpts :: ShowOrParseArgs -> CommandUI flags -> [GetOpt.OptDescr (flags -> flags)]
    commandGetOpts showOrParse command =
        concatMap viewAsGetOpt (commandOptions command showOrParse)

    viewAsGetOpt :: OptionField a -> [GetOpt.OptDescr (a->a)]
    viewAsGetOpt (OptionField _n aa) = concatMap optDescrToGetOpt aa
      where
        optDescrToGetOpt (ReqArg d (cs,ss) arg_desc set _) =
             [GetOpt.Option cs ss (GetOpt.ReqArg set' arg_desc) d]
                 where set' = readEOrFail set
        optDescrToGetOpt (OptArg d (cs,ss) arg_desc set def _) =
             [GetOpt.Option cs ss (GetOpt.OptArg set' arg_desc) d]
                 where set' Nothing    = def
                       set' (Just txt) = readEOrFail set txt
        optDescrToGetOpt (ChoiceOpt alts) =
             [GetOpt.Option sf lf (GetOpt.NoArg set) d | (d,(sf,lf),set,_) <- alts ]
        optDescrToGetOpt (BoolOpt d (sfT,lfT) (sfF, lfF) set _) =
             [ GetOpt.Option sfT lfT (GetOpt.NoArg (set True))  ("Enable " ++ d)
             , GetOpt.Option sfF lfF (GetOpt.NoArg (set False)) ("Disable " ++ d) ]

    -- | to view as a FieldDescr, we sort the list of interfaces (Req > Bool > Choice > Opt) and consider only the first one.
    viewAsFieldDescr :: OptionField a -> FieldDescr a
    viewAsFieldDescr (OptionField _n []) = error "Distribution.command.viewAsFieldDescr: unexpected"
    viewAsFieldDescr (OptionField n dd) = FieldDescr n get set
        where optDescr = head $ sortBy cmp dd
              ReqArg{}    `cmp` ReqArg{}    = EQ
              ReqArg{}    `cmp` _           = GT
              BoolOpt{}   `cmp` ReqArg{}    = LT
              BoolOpt{}   `cmp` BoolOpt{}   = EQ
              BoolOpt{}   `cmp` _           = GT
              ChoiceOpt{} `cmp` ReqArg{}    = LT
              ChoiceOpt{} `cmp` BoolOpt{}   = LT
              ChoiceOpt{} `cmp` ChoiceOpt{} = EQ
              ChoiceOpt{} `cmp` _           = GT
              OptArg{}    `cmp` OptArg{}    = EQ
              OptArg{}    `cmp` _           = LT
              get t = case optDescr of
                        ReqArg _ _ _ _ ppr ->
                         (cat . punctuate comma . map text . ppr) t
                        OptArg _ _ _ _ _ ppr ->
                         case ppr t of
                            []        -> empty
                            (Nothing : _) -> text "True"
                            (Just a  : _) -> text a
                        ChoiceOpt alts ->
                         fromMaybe empty $ listToMaybe
                             [ text lf | (_,(_,lf:_), _,enabled) <- alts, enabled t]
                        BoolOpt _ _ _ _ enabled -> (maybe empty disp . enabled) t
              set line val a =
                      case optDescr of
                        ReqArg _ _ _ readE _    -> ($ a) `liftM` runE line n readE val
                                                 -- We parse for a single value instead of a list,
                                                 -- as one can't really implement parseList :: ReadE a -> ReadE [a]
                                                 -- with the current ReadE definition
                        ChoiceOpt{}             -> case getChoiceByLongFlag optDescr val of
                                                     Just f -> return (f a)
                                                     _      -> syntaxError line val
                        BoolOpt _ _ _ setV _    -> (`setV` a) `liftM` runP line n parse val
                        OptArg _ _ _ _readE _ _ -> -- The behaviour in this case is not clear, and it has no use so far,
                                                   -- so we avoid future surprises by not implementing it.
                                                   error "Command.optionToFieldDescr: feature not implemented"

    getChoiceByLongFlag :: OptDescr b -> String -> Maybe (b->b)
    getChoiceByLongFlag (ChoiceOpt alts) val = listToMaybe [ set | (_,(_sf,lf:_), set, _) <- alts
                                                                 , lf == val]

    getChoiceByLongFlag _ _ = error "Distribution.command.getChoiceByLongFlag: expected a choice option"

    getCurrentChoice :: OptDescr a -> a -> [String]
    getCurrentChoice (ChoiceOpt alts) a =
        [ lf | (_,(_sf,lf:_), _, currentChoice) <- alts, currentChoice a]

    getCurrentChoice _ _ = error "Command.getChoice: expected a Choice OptDescr"

    liftOption :: (b -> a) -> (a -> (b -> b)) -> OptionField a -> OptionField b
    liftOption get' set' opt = opt { optionDescr = liftOptDescr get' set' `map` optionDescr opt}

    liftOptDescr :: (b -> a) -> (a -> (b -> b)) -> OptDescr a -> OptDescr b
    liftOptDescr get' set' (ChoiceOpt opts) =
        ChoiceOpt [ (d, ff, liftSet get' set' set , (get . get'))
                  | (d, ff, set, get) <- opts]

    liftOptDescr get' set' (OptArg d ff ad set def get) =
        OptArg d ff ad (liftSet get' set' `fmap` set) (liftSet get' set' def) (get . get')

    liftOptDescr get' set' (ReqArg d ff ad set get) =
        ReqArg d ff ad (liftSet get' set' `fmap` set) (get . get')

    liftOptDescr get' set' (BoolOpt d ffT ffF set get) =
        BoolOpt d ffT ffF (liftSet get' set' . set) (get . get')

    liftSet :: (b -> a) -> (a -> (b -> b)) -> (a -> a) -> b -> b
    liftSet get' set' set x = set' (set $ get' x) x

    -- | Show flags in the standard long option command line format
    commandShowOptions :: CommandUI flags -> flags -> [String]
    commandShowOptions command v = concat
      [ showOptDescr v  od | o <- commandOptions command ParseArgs
                           , od <- optionDescr o]
      where
        showOptDescr :: a -> OptDescr a -> [String]
        showOptDescr x (BoolOpt _ (_,lfT:_) (_,lfF:_) _ enabled)
          = case enabled x of
              Nothing -> []
              Just True  -> ["--" ++ lfT]
              Just False -> ["--" ++ lfF]
        showOptDescr x c@ChoiceOpt{}
          = ["--" ++ val | val <- getCurrentChoice c x]
        showOptDescr x (ReqArg _ (_ssff,lf:_) _ _ showflag)
          = [ "--"++lf++"="++flag
            | flag <- showflag x ]
        showOptDescr x (OptArg _ (_ssff,lf:_) _ _ _ showflag)
          = [ case flag of
                Just s  -> "--"++lf++"="++s
                Nothing -> "--"++lf
            | flag <- showflag x ]
        showOptDescr _ _
          = error "Distribution.Simple.Command.showOptDescr: unreachable"

    commandListOptions :: CommandUI flags -> [String]
    commandListOptions command =
      concatMap listOption $
        addCommonFlags ShowArgs $ -- This is a slight hack, we don't want
                                  -- "--list-options" showing up in the
                                  -- list options output, so use ShowArgs
          commandGetOpts ShowArgs command
      where
        listOption (GetOpt.Option shortNames longNames _ _) =
             [ "-"  ++ [name] | name <- shortNames ]
          ++ [ "--" ++  name  | name <- longNames ]

    -- | The help text for this command with descriptions of all the options.
    commandHelp :: CommandUI flags -> String -> String
    commandHelp command pname =
        commandUsage command pname
     ++ (GetOpt.usageInfo ""
      . addCommonFlags ShowArgs
      $ commandGetOpts ShowArgs command)
     ++ case commandDescription command of
          Nothing   -> ""
          Just desc -> '\n': desc pname

    -- | Make a Command from standard 'GetOpt' options.
    makeCommand :: String                         -- ^ name
                -> String                         -- ^ short description
                -> Maybe (String -> String)       -- ^ long description
                -> flags                          -- ^ initial\/empty flags
                -> (ShowOrParseArgs -> [OptionField flags]) -- ^ options
                -> CommandUI flags
    makeCommand name shortDesc longDesc defaultFlags options =
      CommandUI {
        commandName         = name,
        commandSynopsis     = shortDesc,
        commandDescription  = longDesc,
        commandUsage        = usage,
        commandDefaultFlags = defaultFlags,
        commandOptions      = options
      }
      where usage pname = "Usage: " ++ pname ++ " " ++ name ++ " [FLAGS]\n\n"
                       ++ "Flags for " ++ name ++ ":"

    -- | Common flags that apply to every command
    data CommonFlag = HelpFlag | ListOptionsFlag

    commonFlags :: ShowOrParseArgs -> [GetOpt.OptDescr CommonFlag]
    commonFlags showOrParseArgs = case showOrParseArgs of
      ShowArgs  -> [help]
      ParseArgs -> [help, list]
      where
        help = GetOpt.Option helpShortFlags ["help"] (GetOpt.NoArg HelpFlag)
                 "Show this help text"
        helpShortFlags = case showOrParseArgs of
          ShowArgs  -> ['h']
          ParseArgs -> ['h', '?']
        list = GetOpt.Option [] ["list-options"] (GetOpt.NoArg ListOptionsFlag)
                 "Print a list of command line flags"

    addCommonFlags :: ShowOrParseArgs
                   -> [GetOpt.OptDescr a]
                   -> [GetOpt.OptDescr (Either CommonFlag a)]
    addCommonFlags showOrParseArgs options =
         map (fmapOptDesc Left)  (commonFlags showOrParseArgs)
      ++ map (fmapOptDesc Right) options
      where fmapOptDesc f (GetOpt.Option s l d m) =
                           GetOpt.Option s l (fmapArgDesc f d) m
            fmapArgDesc f (GetOpt.NoArg a)    = GetOpt.NoArg (f a)
            fmapArgDesc f (GetOpt.ReqArg s d) = GetOpt.ReqArg (f . s) d
            fmapArgDesc f (GetOpt.OptArg s d) = GetOpt.OptArg (f . s) d

    commandParseArgs :: CommandUI flags -> Bool -> [String]
                     -> CommandParse (flags -> flags, [String])
    commandParseArgs command global args =
      let options = addCommonFlags ParseArgs
                  $ commandGetOpts ParseArgs command
          order | global    = GetOpt.RequireOrder
                | otherwise = GetOpt.Permute
      in case GetOpt.getOpt' order options args of
        (flags, _, _,  _)
          | any listFlag flags -> CommandList (commandListOptions command)
          | any helpFlag flags -> CommandHelp (commandHelp command)
          where listFlag (Left ListOptionsFlag) = True; listFlag _ = False
                helpFlag (Left HelpFlag)        = True; helpFlag _ = False
        (flags, opts, opts', [])
          | global || null opts' -> CommandReadyToGo (accum flags, mix opts opts')
          | otherwise            -> CommandErrors (unrecognised opts')
        (_, _, _, errs)          -> CommandErrors errs

      where -- Note: It is crucial to use reverse function composition here or to
            -- reverse the flags here as we want to process the flags left to right
            -- but data flow in function compsition is right to left.
            accum flags = foldr (flip (.)) id [ f | Right f <- flags ]
            unrecognised opts = [ "unrecognized option `" ++ opt ++ "'\n"
                                | opt <- opts ]
            -- For unrecognised global flags we put them in the position just after
            -- the command, if there is one. This gives us a chance to parse them
            -- as sub-command rather than global flags.
            mix []     ys = ys
            mix (x:xs) ys = x:ys++xs

    data CommandParse flags = CommandHelp (String -> String)
                            | CommandList [String]
                            | CommandErrors [String]
                            | CommandReadyToGo flags
    instance Functor CommandParse where
      fmap _ (CommandHelp help)       = CommandHelp help
      fmap _ (CommandList opts)       = CommandList opts
      fmap _ (CommandErrors errs)     = CommandErrors errs
      fmap f (CommandReadyToGo flags) = CommandReadyToGo (f flags)

    data Command action = Command String String ([String] -> CommandParse action)

    commandAddAction :: CommandUI flags
                     -> (flags -> [String] -> action)
                     -> Command action
    commandAddAction command action =
      Command (commandName command)
              (commandSynopsis command)
              (fmap (uncurry applyDefaultArgs)
             . commandParseArgs command False)

      where applyDefaultArgs mkflags args =
              let flags = mkflags (commandDefaultFlags command)
               in action flags args

    commandsRun :: CommandUI a
                -> [Command action]
                -> [String]
                -> CommandParse (a, CommandParse action)
    commandsRun globalCommand commands args =
      case commandParseArgs globalCommand' True args of
        CommandHelp      help          -> CommandHelp help
        CommandList      opts          -> CommandList (opts ++ commandNames)
        CommandErrors    errs          -> CommandErrors errs
        CommandReadyToGo (mkflags, args') -> case args' of
          ("help":cmdArgs) -> handleHelpCommand cmdArgs
          (name:cmdArgs) -> case lookupCommand name of
            [Command _ _ action] -> CommandReadyToGo (flags, action cmdArgs)
            _                    -> CommandReadyToGo (flags, badCommand name)
          []                     -> CommandReadyToGo (flags, noCommand)
          where flags = mkflags (commandDefaultFlags globalCommand)

      where
        lookupCommand cname = [ cmd | cmd@(Command cname' _ _) <- commands'
                              , cname'==cname ]
        noCommand        = CommandErrors ["no command given (try --help)\n"]
        badCommand cname = CommandErrors ["unrecognised command: " ++ cname
                                       ++ " (try --help)\n"]
        commands'      = commands ++ [commandAddAction helpCommandUI undefined]
        commandNames   = [ name | Command name _ _ <- commands' ]
        globalCommand' = globalCommand {
          commandUsage = \pname ->
               (case commandUsage globalCommand pname of
                 ""       -> ""
                 original -> original ++ "\n")
            ++ "Usage: " ++ pname ++ " COMMAND [FLAGS]\n"
            ++ "   or: " ++ pname ++ " [GLOBAL FLAGS]\n\n"
            ++ "Global flags:",
          commandDescription = Just $ \pname ->
               "Commands:\n"
            ++ unlines [ "  " ++ align name ++ "    " ++ description
                       | Command name description _ <- commands' ]
            ++ case commandDescription globalCommand of
                 Nothing   -> ""
                 Just desc -> '\n': desc pname
        }
          where maxlen = maximum [ length name | Command name _ _ <- commands' ]
                align str = str ++ replicate (maxlen - length str) ' '

        -- A bit of a hack: support "prog help" as a synonym of "prog --help"
        -- furthermore, support "prog help command" as "prog command --help"
        handleHelpCommand cmdArgs =
          case commandParseArgs helpCommandUI True cmdArgs of
            CommandHelp      help    -> CommandHelp help
            CommandList      list    -> CommandList (list ++ commandNames)
            CommandErrors    _       -> CommandHelp globalHelp
            CommandReadyToGo (_,[])  -> CommandHelp globalHelp
            CommandReadyToGo (_,(name:cmdArgs')) ->
              case lookupCommand name of
                [Command _ _ action] ->
                  case action ("--help":cmdArgs') of
                    CommandHelp help -> CommandHelp help
                    CommandList _    -> CommandList []
                    _                -> CommandHelp globalHelp
                _                    -> badCommand name

          where globalHelp = commandHelp globalCommand'
        helpCommandUI =
          (makeCommand "help" "Help about commands" Nothing () (const [])) {
            commandUsage = \pname ->
                 "Usage: " ++ pname ++ " help [FLAGS]\n"
              ++ "   or: " ++ pname ++ " help COMMAND [FLAGS]\n\n"
              ++ "Flags for help:"
          }

    -- | Utility function, many commands do not accept additional flags. This
    -- action fails with a helpful error message if the user supplies any extra.
    --
    noExtraFlags :: [String] -> IO ()
    noExtraFlags [] = return ()
    noExtraFlags extraFlags =
      die $ "Unrecognised flags: " ++ intercalate ", " extraFlags
    --TODO: eliminate this function and turn it into a variant on commandAddAction
    --      instead like commandAddActionNoArgs that doesn't supply the [String]