[Issue 20948] New: std.getopt: Support std.getop.config.positional

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20948

          Issue ID: 20948
           Summary: std.getopt: Support std.getop.config.positional
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: andre at s-e-a-p.de

std.getopt supports optional arguments (starting with --) but does not support
mandatory arguments (positional arguments). 

Please see the very first example how this is solved in python library argparse
https://docs.python.org/3/library/argparse.html

In D, positional arguments could also be supported using a new config value
`positional`:

string foo;
string bar;

getopt(args,
    std.getopt.config.positional,
    "foo", &foo, // positional argument
    "bar", &bar  // non positional argument
);

You can call the application with
> myapp ABC --bar DEF

Value ABC would be assigned to string variable `foo`.
The string "foo" serves for documentation purposes while
calling the getopt help.

Supporting positional arguments in getopt would be a lot more convenient
and would provide a better user experience (getopt help)
instead of having custom parsing/validation logic implemented in each
application different.

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