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System.Console.GetOpt

Portability | portable
Stability | experimental
Maintainer | libraries@haskell.org

Contents

GetOpt
Examples
  Interpreting flags as concrete values
  Interpreting flags as transformations of an options record

Description
This library provides facilities for parsing the command-line options in a standalone program. It is essentially a Haskell port of the GNU `getopt` library.
Synopsis

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

data ArgOrder a

= RequireOrder
| Permute
| ReturnInOrder (String -> a)

data OptDescr a = Option [Char] [String] (ArgDescr a) String

data ArgDescr a

= NoArg a
| ReqArg (String -> a) String
| OptArg (Maybe String -> a) String

GetOpt

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

Process the command-line, and return the list of values that matched (and those that didn't). The arguments are:

- The order requirements (see `ArgOrder`)
- The option descriptions (see `OptDescr`)
- The actual command line arguments (presumably got from System.Environment.getArgs).

`getOpt` returns a triple consisting of the option arguments, a list of non-options, and a list of error messages.

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

This is almost the same as `getOpt`, but returns a quadruple consisting of the option arguments, a list of non-options, a list of unrecognized options, and a list of error messages.

usageInfo :: String -> [OptDescr a] -> String | Source

Return a string describing the usage of a command, derived from the header (first argument) and the options described by the second argument.

data ArgOrder a | Source

What to do with options following non-options
Constructors

RequireOrder | no option processing after first non-option
Permute | freely intersperse options and non-options
ReturnInOrder (String -> a) | wrap non-options into options

data OptDescr a | Source

Each `OptDescr` describes a single option.

The arguments to `Option` are:

- list of short option characters
- list of long option strings (without "--")
- argument descriptor
- explanation of option for user

Constructors

Option [Char] [String] (ArgDescr a) String |

data ArgDescr a | Source

Describes whether an option takes an argument or not, and if so how the argument is injected into a value of type `a`.
Constructors

NoArg a | no argument expected
ReqArg (String -> a) String | option requires argument
OptArg (Maybe String -> a) String | optional argument

Examples
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 of `getOpt` is parameterized by the type of values derived from flags.
Interpreting flags as concrete values

A simple choice for the type associated with flags is to define a type `Flag` as an algebraic type representing the possible flags and their arguments:

        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 flags to determine it's behaviour.

Interpreting flags as transformations of an options record

A different approach is to group the option values in a record of type `Options`, and have each flag yield a function of type `Options -> 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 type `Options -> IO Options` (or any other monad), allowing option processing to perform actions of the chosen monad, e.g. printing help or version messages, checking that file arguments exist, etc.

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