A Haskell port of python's docopt.

How about writing your help text first, and getting a parser for free!

Save your help text to a file (i.e. USAGE.txt):

Usage:
  myprog cat <file>
  myprog echo [--caps] <string>

Options:
  -c, --caps    Caps-lock the echoed argument

Then, in your Myprog.hs:

{-# LANGUAGE QuasiQuotes #-}
import Control.Monad (when)
import Data.Char (toUpper)
import System.Environment (getArgs)
import System.Console.Docopt

patterns :: Docopt
patterns = [docoptFile|USAGE.txt|]

getArgOrExit = getArgOrExitWith patterns

main = do
  args <- parseArgsOrExit patterns =<< getArgs

  when (args `isPresent` (command "cat")) $ do
    file <- args `getArgOrExit` (argument "file")
    putStr =<< readFile file

  when (args `isPresent` (command "echo")) $ do
    let charTransform = if args `isPresent` (longOption "caps")
                          then toUpper
                          else id
    string <- args `getArgOrExit` (argument "string")
    putStrLn $ map charTransform string

That's it! No Template Haskell, no unreadable syntax, no learning yet another finicky API. Write the usage patterns you support, and docopt builds the appropriate option parser for you (internally using parsec). If your user invokes your program correctly, you query for the arguments they provided. If the arguments provided do not match a supported usage pattern, you guessed it: docopt automatically prints the help text and exits!

cabal sandbox init
cabal install docopt

See the package on hackage

Docopt only cares about 2 parts of your help text:

• Usage patterns, e.g.:

      Usage:
        my_program [-hs] [-o=<file>] [--quiet | --verbose] [<input>...]
  

  These begin with Usage: (case-insensitive), and end with a blank line.

• Option descriptions, e.g.:

      Options:
        -h --help    show this
        -s --sorted  sorted output
        -o=<file>    specify output file
                     [default: ./test.txt]
        --quiet      print less text
        --verbose    print more text
  

  Any line after the usage patterns that begins with a - is treated as an option description (though an option's default may be on a different line).

• <argument> or ARGUMENT

  Positional arguments. Constructed via argument, i.e. argument "arg" matches an <arg> element in the help text, and argument "ARG" matches an ARG element.

• --flag or --option=<arg>

  Options are typically optional (though this is up to you), and can be either boolean (present/absent), as in --flag, or expect a trailing argument, as in --option=<arg> or --option=ARG. Arguments can be separated from the option name by an = or a single space, and can be specified as <arg> or ARG (consistency of style is recommended, but it is not enforced).

  Short-style options, as in -f or -f ARG or -f=<arg>, are also allowed. Synonyms between different spellings of the same option (e.g. -v and --verbose) can be established in the option descriptions (see below). Short-style options can also be stacked, as in -rfA. When options are stacked, -rfA is effectively equivalent to (-r | -f | -A)... to the argument parser.

  You can match a long-style option --option or --option=<arg> with longOption "option", and a short-style option -f or -f=<arg> with shortOption 'f'. Note that neither --option=<arg> nor -f=<arg> would be matched by argument "arg".

• command

  Anything not recognized as a positional argument or a short or long option is treated as a command (or subcommand, same thing to docopt). A command named pull can be matched with command "pull".

• [] (brackets) e.g. command [--option]

  Patterns inside brackets are optional.

• () (parens)

  Patterns inside parens are required (the same as patterns not in () are required). Parens are useful if you need to group some elements, either for use with | or ....

• | (pipe) e.g. command [--quiet | --verbose]

  A pipe | separates mutually exclusive elements in a group. A group could be elements inside [], (), or the whole usage line.

      Usage:
        myprog command [--opt1 | --opt2]  # valid
        myprog go (left | right)          # valid
        myprog -v | -h                    # valid
  

  When elements are separated by a pipe, the elements are tried from left to right until one succeeds. At least one of the elements are required unless in an eplicitly optional group surrounded by [].

• ... (ellipsis) e.g. command <file>...

  An ellipsis can trail any element or group to make it repeatable. Repeatable elements will be accumulated into a list of occurrences.

• [options] (case sensitive)

  The string [options] is a shortcut to match any options specified in your option descriptions.

• [-] and [--]

  Single hyphen - is used by convention to specify using stdin as input instead of reading a file. Double hyphen -- is typically used to manually separate leading options from trailing positional arguments. Both of these are treated as commands, and so are perfectly legal in usage patterns. They are typically optional elements, but can be required if you drop the []. These are treated as commands and can be matched with command "-" or command "--", whether they're wrapped [-] or not.

Option descriptions establish:

• which short and long options are synonymous
• whether an option expects an argument or is a simple flag
• if an option's argument has a default value

Rules:

• Any line after the usage patterns whose first non-space character is a - is treated as an option description. (Options: prefix line not required).

      Options: --help       # invalid: line does not start with '-'
               --verbose    # good
  

• Options on the same line will be treated by the parser as synonyms (everywhere interchangeable). Synonymous options are separated by a space (with optional comma):

      Usage:
        myprog --help | --verbose
  
      Options:
        -h, --help      Print help text
        -v --verbose    Print help text twice
  

  Here, myprog --help and myprog -h will both work the same, as will myprog --verbose and myprog -v.

• If any synonymous options are specified in the description with an argument, the option parser will expect an argument for all synonyms. If not, all synonyms will be treated as flags.

      Usage:
        myprog analyze [--verbose] <file>
  
      Options:
        --verbose, -v LEVEL   The level of output verbosity.
  

  Here, in the arguments myprog analyze --verbose ./file1.txt would be invalid, because -v and its synonyms expect an argument, so ./file1.txt is captured as the argument of --verbose, not as the positional argument <file>. Be careful!

  Options can be separated from arguments with a single space or a =, and arguments can have the form <arg> or ARG. Just be sure to separate synonyms and arguments from the beginning of the description by at least 2 spaces.

      --opt1 ARG1   Option 1.
      --opt2=<arg2> Option 2.        # BAD: use 2 spaces
      -a <arg3>     Option 3.
      -b=ARG4       Option 4.
  

• Options that expect arguments can be given a default value, in the form [default: <default-val>]. Default values do not need to be on the same line

      --host=NAME       Host to listen on. [default: localhost]
      --port=PORT       Port number [default: 8080]
      --directory=DIR   This option has an especially long description
                        explaining its meaning. [default: ./]
  

• does not automatically exclude from the [options] shortcut options that are already used elsewhere in the usage pattern (e.g. usage: prog [options] -a will try to parse -a twice).

• does not automatically resolve partially-specified arguments, e.g. --verb does not match where --verbose is expected. This is planned to be deprecated in future versions of docopt, and will likely not be implemented in docopt.hs

• is not insensitive to the ordering of adjacent options, e.g. usage: prog -a -b does not allow prog -b -a (reference implementation currently does).