I don't like this that much, I think instead of parsing the arguments directly to Types it should probably just stay as strings and then any type specific parsing is done later. This is what src-self-hosted/arg.zig does and overall src-self-hosted/arg.zig is better execpt I like "name=value" pairs rather than "--flag value" syntax.
An interesting technique in src-self-host/arg.zig is creating a struct with different constructors initializing fields differently without using "comptime T: type". This allows straight forward creation of arrays without the need for tagged unions or a "common interface" and @fieldParentPtr.
ParseNumber seems pretty good with the optional "_" seperator.
Allow addition of different "types" at compile time and maybe runtime. Right now a tagged union ArgUnionFields is used to manage different types of data that can be parsed. For new "types" to be added ArgUnionFields and the appropriate code paths need to be updated. It would be nicer if the programmer could add them at comptime at a minimum and possibly at runtime.
in the use-ArgRecCommon I tried using the "common interface" and @fieldParentPtr technique but that really didn't help, at least I couldn't make it really extensible at compile time.
Parse the parameters passed on the command line and available in osArgIter: std.os.ArgIterator and well as my own ArgIteratorTest. Both are avaiable via the wrapper ArgIter:
ArgIter.initOsArgIter() Self
ArgIter.initTestArgIter(args: []const []const u8) Self
Besides the iterator with the command line arguments you need to create an ArrayList(ArgRec) and initialize the list with a "prototype" for each possible command line argument.
See the tests in parse_args.zig and test_app.zig for details, I'm not going to add doc's here yet, because I don't like the API. I need to make the improvments mentioned below.
$ zig test parse_args.zigRemove zig-cache/ directory
$ rm -rf test ./zig-cache/