src/CommandLine.hs

   1 -- The CommandLine module handles parsing of the command-line options.
   2 -- It should more or less be a black box, providing Main with only the
   3 -- information it requires.
   4
   5 module CommandLine
   6 ( help_set,
   7   help_text,
   8   input_function,
   9   parse_errors
  10 ) where
  11
  12 import System.Console.GetOpt
  13 import System.Environment (getArgs)
  14
  15
  16 -- A record containing values for all available options.
  17 data Options = Options { opt_help  :: Bool,
  18                          opt_input :: IO String }
  19
  20 -- This constructs an instance of Options, with each of its members
  21 -- set to default values.
  22 default_options :: Options
  23 default_options = Options { opt_help = False,
  24                             opt_input = getContents }
  25
  26
  27 -- The options list that we construct associates a function with each
  28 -- option. This function is responsible for updating an Options record
  29 -- with the appropriate value.
  30 --
  31 -- For more information and an example of this idiom, see,
  32 --
  33 -- http://www.haskell.org/haskellwiki/High-level_option_handling_with_GetOpt
  34 --
  35 options :: [OptDescr (Options -> IO Options)]
  36 options =
  37   [ Option ['h'][] (NoArg set_help) "Prints this help message.",
  38     Option ['i'][] (ReqArg set_input "FILE") "Read FILE instead of stdin." ]
  39
  40 -- Takes an Options as an argument, and sets its opt_help member to
  41 -- True.
  42 set_help :: Options -> IO Options
  43 set_help opts = do
  44   return opts { opt_help = True }
  45
  46 -- If the input file option is set, this function will update the
  47 -- passed Options record with a new function for opt_input. The
  48 -- default opt_input is to read from stdin, but if this option is set,
  49 -- we replace that with readFile.
  50 set_input :: String -> Options -> IO Options
  51 set_input arg opts = do
  52   return opts { opt_input = readFile arg }
  53
  54 -- The usage header
  55 usage :: String
  56 usage = "Usage: hath [-h] [-i FILE]"
  57
  58
  59 -- The usage header, and all available flags (as generated by GetOpt)
  60 help_text :: String
  61 help_text = usageInfo usage options
  62
  63
  64 -- Return a tuple of options and errors.
  65 parse_options :: IO (Options, [String])
  66 parse_options = do
  67   argv <- getArgs
  68   let (actions, _, errors) = getOpt Permute options argv
  69
  70   -- This will execute each of the functions contained in our options
  71   -- list, one after another, on a default_options record. The end
  72   -- result should be an Options instance with all of its members set
  73   -- correctly.
  74   opts <- foldl (>>=) (return default_options) actions
  75
  76   return (opts, errors)
  77
  78
  79 -- Return just the errors from parse_options.
  80 parse_errors :: IO [String]
  81 parse_errors = do
  82     (_, errors) <- parse_options
  83     return errors
  84
  85
  86 -- Is the help option set?
  87 help_set :: IO Bool
  88 help_set = do
  89     (opts, _) <- parse_options
  90     return (opt_help opts)
  91
  92
  93 -- Return our input function, getContents by default, or readFile if
  94 -- the input file option was set.
  95 input_function :: IO (IO String)
  96 input_function = do
  97     (opts, _) <- parse_options
  98     return (opt_input opts)