std.getopt - D Programming Language
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# std.getopt

Processing of command line options.
The getopt module implements a `getopt` function, which adheres to the POSIX syntax for command line options. GNU extensions are supported in the form of long options introduced by a double dash ("--"). Support for bundling of command line options, as was the case with the more traditional single-letter approach, is provided but not enabled by default.
License:
Boost License 1.0.
Authors:
Andrei Alexandrescu

Credits This module and its documentation are inspired by Perl's Getopt::Long module. The syntax of D's `getopt` is simpler than its Perl counterpart because getopt infers the expected parameter types from the static types of the passed-in pointers.

Source std/getopt.d

class `GetOptException`: object.Exception;
  Thrown on one of the following conditions:
  - An unrecognized command-line argument is passed, and std.getopt.config.passThrough was not present.
  - A command-line option was not found, and std.getopt.config.required was present.
  - A callback option is missing a value.
GetoptResult `getopt`(T...)(ref string[] `args`, T `opts`);
  Parse and remove command line options from a string array.

  Synopsis

      import std.getopt;

      string data = "file.dat";
      int length = 24;
      bool verbose;
      enum Color { no, yes };
      Color color;

      void main(string[] args)
      {
        auto helpInformation = getopt(
          args,
          "length",  &length,    // numeric
          "file",    &data,      // string
          "verbose", &verbose,   // flag
          "color", "Information about this color", &color);    // enum
        ...

        if (helpInformation.helpWanted)
        {
          defaultGetoptPrinter("Some information about the program.",
            helpInformation.options);
        }
      }

  The `getopt` function takes a reference to the command line (as received by main) as its first argument, and an unbounded number of pairs of strings and pointers. Each string is an option meant to "fill" the value referenced by the pointer to its right (the "bound" pointer). The option string in the call to `getopt` should not start with a dash.
  In all cases, the command-line options that were parsed and used by `getopt` are removed from `args`. Whatever in the arguments did not look like an option is left in `args` for further processing by the program. Values that were unaffected by the options are not touched, so a common idiom is to initialize options to their defaults and then invoke `getopt`. If a command-line argument is recognized as an option with a parameter and the parameter cannot be parsed properly (e.g., a number is expected but not present), a ConvException exception is thrown. If std.`getopt`.config.passThrough was not passed to `getopt` and an unrecognized command-line argument is found, or if a required argument is missing a GetOptException is thrown.
  Depending on the type of the pointer being bound, `getopt` recognizes the following kinds of options:
  1. Boolean options. A lone argument sets the option to true. Additionally true or false can be set within the option separated with an "=" sign:

           bool verbose = false, debugging = true;
           getopt(args, "verbose", &verbose, "debug", &debugging);

     To set verbose to true, invoke the program with either --verbose or --verbose=true.
     To set debugging to false, invoke the program with --debugging=false.
  2. Numeric options. If an option is bound to a numeric type, a number is expected as the next option, or right within the option separated with an "=" sign:

           uint timeout;
           getopt(args, "timeout", &timeout);

     To set timeout to 5, invoke the program with either --timeout=5 or --timeout 5.
  3. Incremental options. If an option name has a "+" suffix and is bound to a numeric type, then the option's value tracks the number of times the option occurred on the command line:

           uint paranoid;
           getopt(args, "paranoid+", &paranoid);

     Invoking the program with "--paranoid --paranoid --paranoid" will set paranoid to 3. Note that an incremental option never expects a parameter, e.g., in the command line "--paranoid 42 --paranoid", the "42" does not set paranoid to 42; instead, paranoid is set to 2 and "42" is not considered as part of the normal program arguments.
  4. Enum options. If an option is bound to an enum, an enum symbol as a string is expected as the next option, or right within the option separated with an "=" sign:

           enum Color { no, yes };
           Color color; // default initialized to Color.no
           getopt(args, "color", &color);

     To set color to Color.yes, invoke the program with either --color=yes or --color yes.
  5. String options. If an option is bound to a string, a string is expected as the next option, or right within the option separated with an "=" sign:

         string outputFile;
         getopt(args, "output", &outputFile);

     Invoking the program with "--output=myfile.txt" or "--output myfile.txt" will set outputFile to "myfile.txt". If you want to pass a string containing spaces, you need to use the quoting that is appropriate to your shell, e.g. --output='my file.txt'.
  6. Array options. If an option is bound to an array, a new element is appended to the array each time the option occurs:

         string[] outputFiles;
         getopt(args, "output", &outputFiles);

     Invoking the program with "--output=myfile.txt --output=yourfile.txt" or "--output myfile.txt --output yourfile.txt" will set outputFiles to [ "myfile.txt", "yourfile.txt" ].
     Alternatively you can set arraySep to allow multiple elements in one parameter.

         string[] outputFiles;
         arraySep = ",";  // defaults to "", meaning one element per parameter
         getopt(args, "output", &outputFiles);

     With the above code you can invoke the program with "--output=myfile.txt,yourfile.txt", or "--output myfile.txt,yourfile.txt".
  7. Hash options. If an option is bound to an associative array, a string of the form "name=value" is expected as the next option, or right within the option separated with an "=" sign:

         double[string] tuningParms;
         getopt(args, "tune", &tuningParms);

     Invoking the program with e.g. "--tune=alpha=0.5 --tune beta=0.6" will set tuningParms to [ "alpha" : 0.5, "beta" : 0.6 ].
     Alternatively you can set arraySep as the element separator:

         double[string] tuningParms;
         arraySep = ",";  // defaults to "", meaning one element per parameter
         getopt(args, "tune", &tuningParms);

     With the above code you can invoke the program with "--tune=alpha=0.5,beta=0.6", or "--tune alpha=0.5,beta=0.6".
     In general, the keys and values can be of any parsable types.
  8. Callback options. An option can be bound to a function or delegate with the signature void function(), void function(string option), void function(string option, string value), or their delegate equivalents.
     - If the callback doesn't take any arguments, the callback is invoked whenever the option is seen.
     - If the callback takes one string argument, the option string (without the leading dash(es)) is passed to the callback. After that, the option string is considered handled and removed from the options array.

           void main(string[] args)
           {
             uint verbosityLevel = 1;
             void myHandler(string option)
             {
               if (option == "quiet")
               {
                 verbosityLevel = 0;
               }
               else
               {
                 assert(option == "verbose");
                 verbosityLevel = 2;
               }
             }
             getopt(args, "verbose", &myHandler, "quiet", &myHandler);
           }

     - If the callback takes two string arguments, the option string is handled as an option with one argument, and parsed accordingly. The option and its value are passed to the callback. After that, whatever was passed to the callback is considered handled and removed from the list.

           int main(string[] args)
           {
             uint verbosityLevel = 1;
             bool handlerFailed = false;
             void myHandler(string option, string value)
             {
               switch (value)
               {
                 case "quiet": verbosityLevel = 0; break;
                 case "verbose": verbosityLevel = 2; break;
                 case "shouting": verbosityLevel = verbosityLevel.max; break;
                 default :
                   stderr.writeln("Unknown verbosity level ", value);
                   handlerFailed = true;
                   break;
               }
             }
             getopt(args, "verbosity", &myHandler);
             return handlerFailed ? 1 : 0;
           }

  Options with multiple names Sometimes option synonyms are desirable, e.g. "--verbose", "--loquacious", and "--garrulous" should have the same effect. Such alternate option names can be included in the option specification, using "|" as a separator:

      bool verbose;
      getopt(args, "verbose|loquacious|garrulous", &verbose);

  Case By default options are case-insensitive. You can change that behavior by passing `getopt` the caseSensitive directive like this:

      bool foo, bar;
      getopt(args,
          std.getopt.config.caseSensitive,
          "foo", &foo,
          "bar", &bar);

  In the example above, "--foo" and "--bar" are recognized, but "--Foo", "--Bar", "--FOo", "--bAr", etc. are rejected. The directive is active until the end of `getopt`, or until the converse directive caseInsensitive is encountered:

      bool foo, bar;
      getopt(args,
          std.getopt.config.caseSensitive,
          "foo", &foo,
          std.getopt.config.caseInsensitive,
          "bar", &bar);

  The option "--Foo" is rejected due to std.getopt.config.caseSensitive, but not "--Bar", "--bAr" etc. because the directive std.getopt.config.caseInsensitive turned sensitivity off before option "bar" was parsed.

  Short versus long options Traditionally, programs accepted single-letter options preceded by only one dash (e.g. -t). `getopt` accepts such parameters seamlessly. When used with a double-dash (e.g. --t), a single-letter option behaves the same as a multi-letter option. When used with a single dash, a single-letter option is accepted.

  To set timeout to 5, use either of the following: --timeout=5, --timeout 5, --t=5, --t 5, -t5, or -t 5. Forms such as -timeout=5 will be not accepted.
  For more details about short options, refer also to the next section.

  Bundling Single-letter options can be bundled together, i.e. "-abc" is the same as "-a -b -c". By default, this option is turned off. You can turn it on with the std.`getopt`.config.bundling directive:

      bool foo, bar;
      getopt(args,
          std.getopt.config.bundling,
          "foo|f", &foo,
          "bar|b", &bar);

  In case you want to only enable bundling for some of the parameters, bundling can be turned off with std.`getopt`.config.noBundling.

  Required An option can be marked as required. If that option is not present in the arguments an exception will be thrown.

      bool foo, bar;
      getopt(args,
          std.getopt.config.required,
          "foo|f", &foo,
          "bar|b", &bar);

  Only the option directly following std.`getopt`.config.required is required.

  Passing unrecognized options through If an application needs to do its own processing of whichever arguments `getopt` did not understand, it can pass the std.`getopt`.config.passThrough directive to `getopt`:

      bool foo, bar;
      getopt(args,
          std.getopt.config.passThrough,
          "foo", &foo,
          "bar", &bar);

  An unrecognized option such as "--baz" will be found untouched in `args` after `getopt` returns.

  Help Information Generation If an option string is followed by another string, this string serves as a description for this option. The `getopt` function returns a struct of type GetoptResult. This return value contains information about all passed options as well a bool GetoptResult.helpWanted flag indicating whether information about these options was requested. The `getopt` function always adds an option for --help|-h to set the flag if the option is seen on the command line.

  Options Terminator A lone double-dash terminates `getopt` gathering. It is used to separate program options from other parameters (e.g., options to be passed to another program). Invoking the example above with "--foo -- --bar" parses foo but leaves "--bar" in `args`. The double-dash itself is removed from the argument array unless the std.`getopt`.config.keepEndOfOptions directive is given.

  Examples:

      auto args = ["prog", "--foo", "-b"];

      bool foo;
      bool bar;
      auto rslt = getopt(args, "foo|f", "Some information about foo.", &foo, "bar|b",
          "Some help message about bar.", &bar);

      if (rslt.helpWanted)
      {
          defaultGetoptPrinter("Some information about the program.",
              rslt.options);
      }

enum `config`: int;
  Configuration options for getopt.
  You can pass them to getopt in any position, except in between an option string and its bound pointer.
  `caseSensitive`
    Turn case sensitivity on
  `caseInsensitive`
    Turn case sensitivity off (default)
  `bundling`
    Turn bundling on
  `noBundling`
    Turn bundling off (default)
  `passThrough`
    Pass unrecognized arguments through
  `noPassThrough`
    Signal unrecognized arguments as errors (default)
  `stopOnFirstNonOption`
    Stop at first argument that does not look like an option
  `keepEndOfOptions`
    Do not erase the endOfOptions separator from args
  `required`
    Make the next option a required option
struct `GetoptResult`;
  The result of the getopt function.
  helpWanted is set if the option --help or -h was passed to the option parser.
  bool `helpWanted`;
    Flag indicating if help was requested
  Option[] `options`;
    All possible options
struct `Option`;
  Information about an option.
  string `optShort`;
    The short symbol for this option
  string `optLong`;
    The long symbol for this option
  string `help`;
    The description of this option
  bool `required`;
    If a option is required, not passing it will result in an error
dchar `optionChar`;
  The option character (default '-').
  Defaults to '-' but it can be assigned to prior to calling getopt.
string `endOfOptions`;
  The string that conventionally marks the end of all options (default '--').
  Defaults to "--" but can be assigned to prior to calling getopt. Assigning an empty string to `endOfOptions` effectively disables it.
dchar `assignChar`;
  The assignment character used in options with parameters (default '=').
  Defaults to '=' but can be assigned to prior to calling getopt.
string `arraySep`;
  When set to "", parameters to array and associative array receivers are treated as an individual argument. That is, only one argument is appended or inserted per appearance of the option switch. If `arraySep` is set to something else, then each parameter is first split by the separator, and the individual pieces are treated as arguments to the same option.
  Defaults to "" but can be assigned to prior to calling getopt.
@safe void `defaultGetoptPrinter`(string `text`, Option[] `opt`);
  This function prints the passed Options and text in an aligned manner on stdout.
  The passed text will be printed first, followed by a newline, then the short and long version of every option will be printed. The short and long version will be aligned to the longest option of every Option passed. If the option is required, then "Required:" will be printed after the long version of the Option. If a help message is present it will be printed next. The format is illustrated by this code:

      foreach (it; opt)
      {
          writefln("%*s %*s%s%s", lengthOfLongestShortOption, it.optShort,
              lengthOfLongestLongOption, it.optLong,
              it.required ? " Required: " : " ", it.help);
      }

  Parameters:

  string `text` | The text to printed at the beginning of the help output.
  Option[] `opt` | The Option extracted from the getopt parameter.

void `defaultGetoptFormatter`(Output)(Output `output`, string `text`, Option[] `opt`, string `style` = "%*s %*s%*s%s\n");
  This function writes the passed text and Option into an output range in the manner described in the documentation of function defaultGetoptPrinter, unless the style option is used.
  Parameters:

  Output `output` | The output range used to write the help information.
  string `text` | The text to print at the beginning of the help output.
  Option[] `opt` | The Option extracted from the getopt parameter.
  string `style` | The manner in which to display the output of each Option.

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