/*!
\page gislib_cmdline_parsing Command Line Parsing

<!--
extracted form gislib.dox and improved by Vaclav Petras, 2013

Copyright 2004-2019 by the GRASS Development Team

Published under GNU Free Documentation License
-->

\tableofcontents


\section gislibcmd_Introduction Introduction

This section describes a standard mechanism for
command line parsing in GRASS. The system is usually referred as
<em>parser</em>, G_parser() or <em>g.parser</em> (because of the related
\gmod{g.parser} module).
Use of the provided set of functions will standardize
GRASS modules that expect command line arguments, creating a family of
GRASS modules that is easy for users to learn.

The standardization is important because as soon as a GRASS user
familiarizes himself with the general form of command line input as
defined by the parser, it will greatly simplify the necessity of
remembering or at least guessing the required command line arguments
for any GRASS command. It is strongly recommended, almost mandatory,
that GRASS programmers use this set of functions for all command line
parsing. With their use, the programmer is freed from the burden of
generating user interface code for every command. The parser will
limit the programmer to a pre-defined look and feel, but limiting the
interface is well worth the shortened user learning curve. Moreover,
system enables to generate module interface descriptions which can
be used by GUI to generate a graphical interface for a module.

\note There are also standard options and flags, which ensure even
better standardization of names, values and descriptions.


\section gislibcmd_Description Description

The GRASS parser is the collection of functions and
structures defined in the GRASS gis.h header file. These
structures and functions allow the programmer to define the options and
flags that make up the valid command line input of a GRASS command.

The parser functions behave in one of three ways:

<ul>
<li> If no command line arguments are entered by the user, the parser
searches for a completely interactive version of the command. If the
interactive version is found, control is passed over to that
version.

<li> If command line arguments are entered but they are a subset of the
options and flags that the programmer has defined as required
arguments, three things happen. The parser will pass an error message
to the user indicating which required options and/or flags were
missing from the command line, the parser will then display a complete
usage message for that command, and finally the parser will terminate
the command.

<li> If all necessary options and flags are entered on the command line
by the user, the parser executes the command with the given options
and flags.
</ul>

\note Python modules uses the same system but instead of functions
and structures, comments in files are used to define options and flags.


\section Parser_Interface Parser interface

The parser functions described below use two structures as defined in
the GRASS gis.h header file.

This is a basic list of members of the Option and Flag
structures. A comprehensive description of all elements of these two
structures and their possible values can be found in
\ref Complete_Structure_Members_Table.

\subsection Option_structure Option structure

The basic usage of the Option structure is as follows.
You declare a pointer to the Option structure.

\code
struct Option *opt;
\endcode

And then you call G_define_option() function, which allocates memory for
the Option structure and returns a pointer to it.

\code
opt = G_define_option();
\endcode

Then you set the structure members, basic members are:

 - <tt>key</tt> - The option name on the command line.
 - <tt>description</tt> - The option description to display to the user.
 - <tt>type</tt> - Variable type of the user's answer to the option.
 - <tt>required</tt> - Whether this option is mandatory or not.

For the full list of members see the Option structure documentation.


\subsection Flag_structure Flag structure

The basic usage of the Flag structure is as follows.
You declare a pointer to the Flag structure.

\code
struct Flag *flag;
\endcode

And then you call G_define_flag() function, which allocates memory for
the Flag structure and returns a pointer to it.

\code
flag = G_define_flag()
\endcode

Then you set the structure members, basic members are:

 - <tt>key</tt> - A single letter flag name on the command line.
 - <tt>description</tt> - The flag description to display to the user.

For the full list of members see the Flag structure documentation.


\subsection Running_Parser Running Parser

To process and check the command line parameters of your module,
you need to call the G_parser() function.

The command line parameters <i>argv</i> and the number of parameters
<i>argc</i> from the main() routine should be passed directly to the
G_parser() function. The function accepts the command line input
entered by the user, and parses this input according to the input
options and/or flags that were defined by the programmer.

G_parser() returns 0 if successful. If not successful, it displays
a usage statement, which describes the expected and/or required
options and flags, and returns a non-zero value.


\subsection Parser_Additional_checks Additional checks of command line parameters

When the G_parser() function is not sufficient to check all the details
about the options and flags and their combinations, the code has to
implement the required logic through custom additional checks. The code will
typically call the G_usage() function if these checks are not successful.

G_usage() prints a usage message, which
explains the allowed and the required command line
input to the user. This description is given according to the
programmer's definitions for options and flags. This function becomes
useful when the user enters options and/or flags on the command line
that are syntactically valid to the parser, but functionally invalid
for the command (e.g. an invalid file name).

For example, the parser logic doesn't directly support grouping
options. If two options must be specified together or not at all, the
parser must be told that these options are not required and the
programmer must check that if one option is specified the other is as well.
If this additional check fails after G_parser() has succeeded, the
programmer can then call G_usage() to print the standard usage message
and then would separately need to print the additional information on how
the two options work together.


\subsection Parser_Displaying_multiple_answers Multiple answer default values

Providing multiple default values (answers) for an option that allows
multiple values is possible using:

\code
char *def[] = {"One", "Two", "Last", NULL};

opt->multiple = YES;
opt->answers  = def;
\endcode

The programmer must not forget the last NULL value.

<em>New in GRASS 5.</em>


\subsection Parser_Disabling_interactive Disabling interactive mode

This is mainly historical feature which enables to disable interactive
prompting in command line.

When a user runs a command with no arguments on the command line, the
parser will enter its own standardized interactive session and prompt the
user for all flags and options interactively. A call to
G_disable_interactive() disables this feature.


\section Parser_Programming_Examples Parser Programming Examples

The use of the parser in the programming process is demonstrated
here. Both a basic step by step example and a full code example are
presented.


\subsection Step_by_Step_Use_of_the_Parser Step by Step Use of the Parser


Below are four basic steps to use the GRASS parser in a GRASS command:

\subsubsection gislib_cmdline_Allocate Allocate memory for Flags and Options
Options and flags are pointers to structures (Option and Flag structures)
allocated through the parser functions G_define_option() and G_define_flag()
as described in \ref Parser_Interface.

\code
#include <grass/gis.h>;   /* The standard GRASS include file */

/* ... */

struct Option *opt;       /* Establish an Option pointer for each option */
struct Flag *flag;        /* Establish a Flag pointer for each option */

opt = G_define_option();  /* Request a pointer to memory for each option */

flag = G_define_flag();   /* Request a pointer to memory for each flag */
\endcode


\subsubsection gislib_cmdline_Define Define members of Flag and Option structures

Define the characteristics of each required option and flag, for example:

\code
opt->key = "option";                 /* The name of this option is "option". */
opt->description = _("Option test"); /* The option description is "Option test" */
opt->type = TYPE_STRING;             /* The data type of the answer to the option */
opt->required = YES;                 /* This option *is* required from the user */

flag->key = "t";                     /* Single letter name for flag */
flag->description = _("Flag test");  /* The flag description is "Flag test" */
\endcode

\note There are more options defined in \ref Complete_Structure_Members_Table.
You should for sure explore the <b>label</b>, the <b>options</b>
and the <b>multiple</b> structure members.


\subsubsection gislib_cmdline_Call Call the parser

\code
int main(int argc, char *argv[]); /* command line args passed into main() */

/* ... options and flags definitions */

if (G_parser(argc, argv))    /* Returns 0 if successful, non-zero otherwise */
      exit(EXIT_FAILURE);

/* ... additional checks */

/* ... module code */
\endcode


\subsubsection gislib_cmdline_Extracting Extracting information from the parser structures

The following code extracts the information about an option and a flag
and prints it to the standard output.

\code
fprintf(stdout, "For the option "%s" you chose: <%s>\n", opt->description, opt->answer);
fprintf(stdout, "The flag "-%s" is %s.\n", flag->key, flag->answer ? "set" : "not set");
\endcode


\subsubsection gislib_cmdline_Running Running the example program

Once such a module has been compiled
(see \ref Compiling_and_Installing_GRASS_Modules), execution will result
in the following user interface scenarios. Lines that begin with '$' (dollar sign)
indicate a command line session with user commands.

\verbatim
$ r.mysample --help
\endverbatim

This is a standard user request for basic help information on the
module. The command line options (in this case, <tt>--help</tt>) are sent to
the parser via G_parser(). The parser recognizes the <tt>--help</tt>
command line option and returns the list of options and/or flags that
are applicable for the specific command. Note how the option and the flag
information specified above appears in the output.

\verbatim
r.mysample [-t] option=name

Flags:
-t Flag test

Parameters:
option Option test
\endverbatim

Now consider the following command:

\verbatim
$ r.mysample -t
\endverbatim

This command does not contain the required option. Note that the
output provides this information along with the standard usage message
(as already shown above):

\verbatim
Required parameter <option> not set (Option test).

Usage:

r.mysample [-t] option=name


Flags:
-t Flag test

Parameters:
option Option test
\endverbatim

The following commands are correct and equivalent:

\verbatim
$ r.mysample option=Hello -t
$ r.mysample -t option=Hello
\endverbatim

The parser provides no  error messages and the module executes normally:

\verbatim
For the option "Option test" you chose: Hello
The flag "-t" is set.
\endverbatim


\subsection gislibcmd_Full_Module_Example Full Module Example

The following code demonstrates some of the basic capabilities of the
parser. To compile this code, create this Makefile and run the
<tt>make</tt> command (see \ref Compiling_and_Installing_GRASS_Modules).

\verbatim
MODULE_TOPDIR = ../..

PGM = r.mysample

LIBES = $(GISLIB)
DEPENDENCIES = $(GISDEP)

include $(MODULE_TOPDIR)/include/Make/Module.make

default: cmd
\endverbatim

The sample C code follows (the usual name of the file with the main
function is <tt>%main.c</tt>). You can experiment with this code to
familiarize yourself with the parser.

\note This example includes some of the advanced structure
members described in \ref Complete_Structure_Members_Table.

\code
#include <stdlib.h>
#include <string.h>
#include <grass/gis.h>
#include <grass/glocale.h>

int main(int argc, char *argv[])
{
    struct Option *opt, *coor;
    struct Flag *flag;
    double X, Y;
    int n;

    opt = G_define_option();
    opt->key = "debug";
    opt->type = TYPE_STRING;
    opt->required = NO;
    opt->answer = "0";
    opt->description = _("Debug level");

    coor = G_define_option();
    coor->key = "coordinate";
    coor->key_desc = "x,y";
    coor->type = TYPE_STRING;
    coor->required = YES;
    coor->multiple = YES;
    coor->description = _("One or more coordinate(s)");

    /* Note that coor->answer is not given a default value. */
    flag = G_define_flag();
    flag->key = 'v';
    flag->description = _("Verbose execution");

    /* Note that flag->answer is not given a default value. */

    if (G_parser(argc, argv))
	exit (EXIT_FAILURE);

    G_message("For the option <%s> you chose: <%s>",
	      opt->description, opt->answer);
    G_message("The flag <%s> is: %s", flag->key,
	      flag->answer ? "set" : "not set");
    G_message("You specified the following coordinates:");

    for (n=0; coor->answers[n] != NULL; n+=2) {
	G_scan_easting(coor->answers[n], &X , G_projection());
	G_scan_northing(coor->answers[n+1], &Y , G_projection());
	fprintf(stdout, "%.15g,%.15g", X, Y);
    }
}
\endcode

\note This example defines the option for coordinates in its own way to
demonstrate usage of particular parser features. However, it must be noted
that there is a standardized G_OPT_M_COORDS option, which should be used for
coordinates.


\section Complete_Structure_Members_Table Complete Structure Members Table


\subsection memtabFlag struct Flag

<table>
<tr>
 <th>structure member</th>
 <th>C type</th>
 <th>required</th>
 <th>default</th>
 <th> description and example</th>
</tr><tr>
 <td>key</td>
 <td> char</td>
 <td> YES</td>
 <td> none</td>
 <td> Key char used on command line.<br>
      <tt>flag->key = 'f' ;</tt></td>
</tr><tr>
 <td>Description</td>
 <td> char *</td>
 <td> YES</td>
 <td> none</td>
 <td> A string describing the flag meaning.<br>
      <tt>flag->description = _("run in fast mode") ;</tt></td>
</tr><tr>
 <td>answer</td>
 <td> char</td>
 <td> NO</td>
 <td> NULL</td>
 <td> Default and parser-returned
     flag states.</td>
</tr>
</table>

\subsection memtabOption struct Option

<table>
<tr>
 <th>structure member</th>
 <th>C type </th>
 <th>required </th>
 <th>default </th>
 <th>description and example</th>
</tr>
<tr>
 <td>key </td>
 <td>char * </td>
 <td>YES </td>
 <td>none </td>
 <td>Key word used on command line.<br>
 <tt>opt->key = "map" ;</tt></td>
</tr>
<tr>
 <td>type </td>
 <td>int </td>
 <td>YES </td>
 <td>none </td>
 <td>%Option type: <br>
 TYPE_STRING <br>
 TYPE_INTEGER <br>
 TYPE_DOUBLE <br>
 <tt>opt->type = TYPE_STRING ;</tt></td>
</tr>
<tr>
 <td>Description </td>
 <td>char * </td>
 <td>YES </td>
 <td>none </td>
 <td>A string describing the option along with the gettext macro for internationalization.<br>
 <tt>opt->description = _("Map name") ;</tt></td>
</tr>
<tr>
 <td>answer </td>
 <td>char * </td>
 <td>NO </td>
 <td>NULL </td>
 <td>The default and the parser-returned answer to the option.<br>
 <tt>opt->answer = "defaultmap" ;</tt></td>
</tr>
<tr>
 <td>key_desc </td>
 <td>char * </td>
 <td>NO </td>
 <td>NULL </td>
 <td>Single word describing the key. Commas in this string denote 
 to the parser that several comma-separated arguments are expected 
 from the user as one answer. For example, if a pair of coordinates 
 is desired, this element should be defined as follows.<br>
  <tt>opt->key_desc = "x,y" ;</tt></td>
</tr>
<tr>
 <th>structure member</th>
 <th>C type </th>
 <th>required </th>
 <th>default </th>
 <th>description and example</th>
</tr>
<tr>
 <td>multiple </td>
 <td>int </td>
 <td>NO </td>
 <td>NO </td>
 <td>Indicates whether the user can provide multiple answers or not. 
 YES and NO are defined in "gis.h" and should be used (NO is 
 the default.) <b>multiple</b> is used in conjunction with the <b>answers</b>
 structure member below.<br>
 <tt>opt->multiple = NO ;</tt></td>
</tr>
<tr>
 <td>answers </td>
 <td>  </td>
 <td>NO </td>
 <td>NULL </td>
 <td>Multiple parser-returned answers to an option.<br>
 N/A</td>
</tr>
<tr>
 <td>required </td>
 <td>int </td>
 <td>NO </td>
 <td>NO </td>
 <td>Indicates whether the user MUST provide the option on the command 
 line. YES and NO are defined in "gis.h" and should be used (NO 
 is the default).<br>
 <tt>opt->required = YES ;</tt></td>
</tr>
<tr>
 <td>options </td>
 <td>char * </td>
 <td>NO </td>
 <td>NULL </td>
 <td>Approved values or range of values. <br>
  <tt>opt->options = "red,blue,white" ;</tt><br>
  For integers and doubles, the following format is available: <br>
 <tt>opt->options = "0-1000" ;</tt></td>
</tr>
<tr>
 <td>gisprompt</td>
 <td>char *</td>
 <td>NO</td>
 <td>NULL</td>
 <td>Interactive prompt guidance. There are three comma-separated 
 parts to this argument, which guide the standard GRASS 
 file name prompting routines.<br>
 <tt>opt->gisprompt = "old,cell,raster" ;</tt></td>
</tr>
<tr>
 <td>checker</td>
 <td>char *()</td>
 <td>NO</td>
 <td>NULL</td>
 <td>A routine to check an answer to the option.<br>
 <tt>opt->checker = my_routine() ;</tt></td>
</tr>
</table>


\section Description_of_Complex_Structure_Members Description of Complex Structure Members 

What follows are explanations of possibly confusing structure
members. They are intended to clarify and supplement the structures table
above.

\subsection Answer_member_of_the_Flag_and_Option_structures Answer member of the Flag and Option structures

The answer structure member serves two functions for GRASS commands
that use the parser.

<b>(1) To set the default answer to an option:</b>

If a default state is desired for a programmer-defined option, the
programmer may define the Option structure member <b>answer</b> before
calling G_parser() in his module. After the G_parser() call, the
<b>answer</b> member will hold this preset default value if the user did
<i>not</i> enter an option that has the default answer member value.

<b>(2) To obtain the command-line answer to an option or a flag:</b>

After a call to G_parser(), the <b>answer</b> member will contain one of the two
values:

 - (a) If the user provided an option and answered this option on the
command line, the user's input will replace the default value of the
<b>answer</b> member as described above.

 - (b) If the user provided an option, but did <i>not</i> answer this
option on the command line, the default is not used. The user may use
the default answer to an option by withholding mention of the option
on the command line. But if the user enters an option without an
answer, G_parser() will replace the default <b>answer</b> member value with
NULL.

As an example, please review the use of <b>answer</b> members in the structures 
implemented in \ref gislibcmd_Full_Module_Example.


\subsection Multiple_and_Answers_Members Multiple and Answers Members

The functionality of the <b>answers</b> structure member is reliant on the 
programmer's definition of the <b>multiple</b> structure member. If the <b>multiple</b>
member is set to NO, the <b>answer</b> member is used to obtain the answer to an 
option as described above.

If the <b>multiple</b> structure member is set to YES, it
tells G_parser() to capture multiple answers. Multiple answers are
separated by <em>commas</em> on the command line after an option.

<b>Note:</b> G_parser() does not recognize any character other than a
comma to delimit multiple answers.

After the programmer has set up an option to receive multiple answers,
these answers are stored in the <b>answers</b> member of the Option
structure. The <b>answers</b> member is an array that contains each
individual user-entered answer. The elements of this array are the
type specified by the programmer using the <b>type</b> member. The <b>answers</b>
array contains however many comma-delimited answers the user entered,
followed (terminated) by a NULL array element.

Below is an example definition of an Option using the <b>multiple</b>
and the <b>answers</b> structure members:

\code
opt->key = "option";
opt->description = _("option example");
opt->type = TYPE_INTEGER;
opt->required = NO;
opt->multiple = YES;
\endcode

The above definition would ask the user for multiple integer answers
to the option. If in response the user entered "option=1,3,8,15" on the
command line, the <b>answers</b> array would contain the following values:

\code
answers[0] == "1"
answers[1] == "3"
answers[2] == "8"
answers[3] == "15"
answers[4] == NULL
\endcode


\subsection key_desc_Member key_desc Member

The <b>key_desc</b> structure member is used to define the format of a single 
command line answer to an option. A programmer may wish to ask for one 
answer to an option, but this answer may not be a single argument of a type 
set by the type structure member. If the programmer wants the user to enter 
a coordinate, for example, the programmer might define an Option as follows:

\code
opt->key ="coordinate";
opt->description = _("Specified Coordinate");
opt->type = TYPE_INTEGER;
opt->required = NO;
opt->key_desc = "x,y"
opt->multiple = NO;
\endcode

The answer to this option would <i>not</i> be stored in the answer
member, but in the answers member. If the user entered
"coordinate=112,225" on the command line in response to a routine that
contains the above option definition, the <b>answers</b> array would have the
following values after the call to G_parser():

\code
answers[0] == "112"
answers[1] == "225"
answers[2] == NULL
\endcode

Note that "coordinate=112" would not be valid, as it does not contain both 
components of an answer as defined by the <b>key_desc</b> structure member.

If the <b>multiple</b> structure member was set to YES instead of NO in the
example above, the answers would be stored sequentially in the <b>answers</b>
member.  For example, if the user wanted to enter the coordinates
(112,225), (142,155), and (43,201), his response on the command line
would be "coordinate=112,225,142,155,43,201". Note that G_parser()
recognizes only a comma for both the <b>key_desc</b> member, and multiple
answers.

The <b>answers</b> array would have the following values after a call to 
G_parser():

\code
answers[0] == "112" answers[1] == "225"
answers[2] == "142" answers[3] == "155"
answers[4] == "43" answers[5] == "201"
answers[6] == NULL
\endcode

<B>Note.</B> In this case as well, neither "coordinate=112" nor
"coordinate=112,225,142" would be valid command line arguments, as
they do not contain even pairs of coordinates. Each answer's format
(as described by the <b>key_desc</b> member) must be fulfilled completely.

The overall function of the <b>key_desc</b> and the <b>multiple</b> structure members is
very similar. The <b>key_desc</b> member is used to specify the number of
<i>required</i> components of a single option answer (e.g. a
multi-valued coordinate.) The <b>multiple</b> member tells G_parser() to ask
the user for multiple instances of the compound answer as defined by
the format in the <b>key_desc</b> structure member.

Another function of the <b>key_desc</b> structure member is to explain to the
user the type of information expected as an answer. The coordinate
example is explained above.

The usage message that is displayed by G_parser() in case of an error,
or by G_usage() on programmer demand, is shown below. The Option
"option" for the command <tt>a.out</tt> does not have its <b>key_desc</b>
structure member defined.

\verbatim
Usage:

a.out option=name
\endverbatim

The use of "name" is a G_parser() standard. If the programmer defines
the <b>key_desc</b> structure member before a call to G_parser(), the
value of the <b>key_desc</b> member replaces "name". Thus, if the <b>key_desc</b>
member is set to "x,y" as was used in an example above, the following
usage message would be displayed:

\verbatim
Usage:

a.out option=x,y
\endverbatim

The <b>key_desc</b> structure member can be used by the programmer to clarify
the usage message as well as to specify single or multiple required
components of a single option answer.

\subsection gisprompt_Member gisprompt Member

The <b>gisprompt</b> Option structure member requires a bit more
description. The three comma-separated (no spaces allowed)
sub-arguments are defined as follows:

 - The first argument: "old" results in a call to the GRASS library
subroutine G_open_old(), "new" to G_open_new(), otherwise "any" or
"mapset".

  - If any option has "new" as the first component, the <tt>--o</tt>
(overwrite) flag will be listed in the module's interface
(<tt>--help</tt> output, manual page, GUI dialog, etc).

  - If an option that has "new" as the first component is given, the
parser checks whether the entity (map, etc.) already exists.

 - The second argument: this is identical to the "element" argument in the
above subroutine calls. It specifies a directory inside the mapset
that may contain the user's response. In other words, the second field
is used to determine where to look for the file (i.e. if the option
has "new,cell,...", it will look in the "cell" directory). The second
field should be the name of one of the standard subdirectories of the
mapset, as listed in $GISBASE/etc/element_list.

 - The third argument: identical to the "prompt" argument in the above
subroutine calls. This is a string presented to the user that
describes the type of data element being requested.

Here are two examples:

\verbatim
"new,cell,raster"   G_open_new("cell", "map")
"old,vector,vector" G_open_old("vector", "map")
\endverbatim

The gisprompt values are passed to any GUI code, both self-contained
dialogs generated by the parser for the <tt>--ui</tt> option, and
stand-alone GUIs (wxGUI) which use the <tt>--xml-description</tt>
flags to obtain a machine-readable description of the module's
interface. How the GUI interprets this is up to the GUI.


\section gislibcmd_Standard_options Standard options and flags

There are standard options and standard flags, they ensure consistency
in names and values.

Standard options are defined by the G_define_standard_option() function
and standard flags are defined by the G_define_standard_flag() function.
Both the options and the flags are defined dynamically, so to see
the values of all members you need to open the file parser_standard_options.c.

The function G_define_standard_option() accepts one value of the STD_OPT
enum defined in the gis.h file. The G_define_standard_option() function
calls the G_define_option() function, so there is no need to call
the latter separately. The same convention applies to standard flags too,
which use the G_define_standard_flag() function and the STD_OPT enum.

Besides a name and a value, standard options also define a label, a description,
allowed values, their descriptions etc. Standard flags use a similar approach.
After defining a standard option or a standard flag you can still assign custom
values to individual structure members as required instead of the default values.


\section gislibcmd_FAQ Command line parsing FAQ


\par Can the user mix options and flags?

Yes. Options and flags can be given in any order.


\par In what order does the parser present options and flags?

Flags and options appear in the usage message according to the order
of the G_define_option() and the G_define_flag() function calls.


\par Is the user required to use full option names?

No. Users are required to type in only as many characters of an option name 
as is necessary to make the option choice unambiguous. If, for example, 
there are two options, "input=" and "output=", the following would be 
valid command line arguments:

\verbatim
$ command i=map1 o=map2

$ command in=map1 out=map2
\endverbatim


\par Are options standardized at all?

Yes. There are a few conventions. Options that identify a single input map 
are usually "map=", not "raster=" or "vector=". In the case of an 
input and an output map the convention is "input=xx output=yy". By passing 
the '--help' flag to existing GRASS commands, it is likely that you will 
find other conventions. The intent is to make it as easy as possible for the 
user to remember (or to guess correctly) what the command line syntax is for a 
given command.

To ensure maximal consistency, the most common options such as the options
named above are defined as standard options and are available through
G_define_standard_option() function. For flags you can use the
G_define_standard_flag() function.

\par How does a programmer query for coordinates?

There is the standardized G_OPT_M_COORDS option, which should be used for
coordinates.

See the source code for the GRASS commands <tt>r.drain</tt> or
<tt>r.cost</tt> for examples.


\par How does a programmer define that the option is a set of values?

For any user input that requires a tuple of values (like a pair of
map coordinates) the programmer specifies the number of arguments in
the <b>key_desc</b> member of the Option structure. For example, if
<b>key_desc</b> was set to "x,y", the parser would require that the
user enters a pair of arguments separated only with a comma.

\note There is the standardized G_OPT_M_COORDS option, which should be used for
coordinates.


\par How is automatic prompting turned off?

GRASS 4.0 introduced a new method for driving GRASS interactive and
non-interactive modules as described in \ref
Compiling_and_Installing_GRASS_Modules. Here is a short overview.

For most modules a user runs a front-end module out of the GRASS bin
directory, which in turn looks for the existence of interactive and
non-interactive versions of the module. If an interactive version
exists and the user provided no command line arguments, then that
version is executed.

In such a situation, the parser's default interaction will never be
seen by the user. A programmer using the parser is able to avoid the
front-end's default search for a fully interactive version of the
command by placing a call to G_disable_interactive() before
calling G_parser() (see \ref Parser_Interface for details).

*/