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8. GetOpt

The GetOpt library contains Sven Panne's Haskell implementation of getopt, providing features nigh-on identical to GNU getopt:

module GetOpt where

-- representing a single option:
data OptDescr a
 = Option [Char]         --    list of short option characters
          [String]       --    list of long option strings (without "--")
          (ArgDescr a)   --    argument descriptor
          String         --    explanation of option for user

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

usageInfo :: String          -- header
          -> [OptDescr a]    -- options recognised 
          -> String          -- nicely formatted decription of options

getOpt :: ArgOrder a    -- non-option handling
       -> [OptDescr a]  -- options recognised
       -> [String]      -- the command-line
       -> ( [a]         -- options
          , [String]    -- non-options
          ,[String]     -- error messages
          )

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

To hopefully illuminate the role of the different GetOpt data structures, here's the command-line options for a (very simple) compiler:

module Opts where

import GetOpt
import 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 "stdout"

compilerOpts :: [String] -> IO ([Flag], [String])
compilerOpts argv = 
   case (getOpt Permute options argv) of
      (o,n,[]  ) -> return (o,n)
      (_,_,errs) -> fail (userError (concat errs ++ usageInfo header options))
  where header = "Usage: ic [OPTION...] files..."


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