This library provides facilities for parsing the command-line options in a standalone program. It is essentially a Haskell port of the GNUgetopt library.

getOpt ::ArgOrder a -> [OptDescr a] -> [String] -> ([a], [String], [String])Source#

getOpt' ::ArgOrder a -> [OptDescr a] -> [String] -> ([a], [String], [String], [String])Source#

usageInfo ::String -> [OptDescr a] ->StringSource#

dataArgOrder aSource#

dataOptDescr aSource#

dataArgDescr aSource#

To hopefully illuminate the role of the different data structures, here are the command-line options for a (very simple) compiler, done in two different ways. The difference arises because the type ofgetOpt is parameterized by the type of values derived from flags.

A simple choice for the type associated with flags is to define a typeFlag as an algebraic type representing the possible flags and theirarguments:

   module Opts1 where      import System.Console.GetOpt   import Data.Maybe ( fromMaybe )      data Flag     = Verbose  | Version     | Input String | Output String | LibDir String      deriving Show      options :: [OptDescr Flag]   options =    [ Option ['v']     ["verbose"] (NoArg Verbose)       "chatty output on stderr"    , Option ['V','?'] ["version"] (NoArg Version)       "show version number"    , Option ['o']     ["output"]  (OptArg outp "FILE")  "output FILE"    , Option ['c']     []          (OptArg inp  "FILE")  "input FILE"    , Option ['L']     ["libdir"]  (ReqArg LibDir "DIR") "library directory"    ]      inp,outp :: Maybe String -> Flag   outp = Output . fromMaybe "stdout"   inp  = Input  . fromMaybe "stdin"      compilerOpts :: [String] -> IO ([Flag], [String])   compilerOpts argv =       case getOpt Permute options argv of         (o,n,[]  ) -> return (o,n)         (_,_,errs) -> ioError (userError (concat errs ++ usageInfo header options))     where header = "Usage: ic [OPTION...] files..."

Then the rest of the program will use the constructed list of flagsto determine it's behaviour.

A different approach is to group the option values in a record of typeOptions, and have each flag yield a function of typeOptions -> Options transforming this record.

   module Opts2 where   import System.Console.GetOpt   import Data.Maybe ( fromMaybe )   data Options = Options    { optVerbose     :: Bool    , optShowVersion :: Bool    , optOutput      :: Maybe FilePath    , optInput       :: Maybe FilePath    , optLibDirs     :: [FilePath]    } deriving Show   defaultOptions    = Options    { optVerbose     = False    , optShowVersion = False    , optOutput      = Nothing    , optInput       = Nothing    , optLibDirs     = []    }   options :: [OptDescr (Options -> Options)]   options =    [ Option ['v']     ["verbose"]        (NoArg (\ opts -> opts { optVerbose = True }))        "chatty output on stderr"    , Option ['V','?'] ["version"]        (NoArg (\ opts -> opts { optShowVersion = True }))        "show version number"    , Option ['o']     ["output"]        (OptArg ((\ f opts -> opts { optOutput = Just f }) . fromMaybe "output")                "FILE")        "output FILE"    , Option ['c']     []        (OptArg ((\ f opts -> opts { optInput = Just f }) . fromMaybe "input")                "FILE")        "input FILE"    , Option ['L']     ["libdir"]        (ReqArg (\ d opts -> opts { optLibDirs = optLibDirs opts ++ [d] }) "DIR")        "library directory"    ]   compilerOpts :: [String] -> IO (Options, [String])   compilerOpts argv =      case getOpt Permute options argv of         (o,n,[]  ) -> return (foldl (flip id) defaultOptions o, n)         (_,_,errs) -> ioError (userError (concat errs ++ usageInfo header options))     where header = "Usage: ic [OPTION...] files..."

Similarly, each flag could yield a monadic function transforming a record,of typeOptions -> IO Options (or any other monad), allowing optionprocessing to perform actions of the chosen monad, e.g. printing help orversion messages, checking that file arguments exist, etc.