Make it easier to set up custom argument parsers

[zoffixznet]

Based off http://colabti.org/irclogger/irclogger_log/perl6?date=2018-09-15#l316

13:30 | leont | Sure, but given that there isn't any non-internal way to do this,
      I would still like it to keep working.
-- | -- | --
13:30 | leont | Now working around it with a multi, but it's a bit unfortunate.
13:30 | Zoffix | leont: "do this" is do what?
13:30 | leont | Install a different argument parser
13:31 | Zoffix | leont: and basically replace the MAIN_HELPER with a new sub?
13:32 | leont | Yeah. I'm not even the first to do that.

I'm imagining a module that sets something in %*SUB-MAIN-OPTS (e.g. assigns a custom Callable to some key) affect how the arguments are parsed. I'm hoping that's possible to do in a way that the use of that module would just be able to use GetOpt::Whatever and not have to modify SUB-MAIN-OPTS themselves.

[Leont]

I'm imagining a module that sets something in %*SUB-MAIN-OPTS (e.g. assigns a custom Callable to some key) affect how the arguments are parsed.

That sounds sensible enough. Something that takes a Sub (and the options?) and returns a Capture

[Leont]

Is it possible to create a contextual variable in an outer scope though?

[zoffixznet]

a contextual variable

No idea what that is. Do you mean dynamic? One way would be to stick it into PROCESS::

$ perl6 -e 'sub foo { PROCESS::<$MEOWS> = 42 }; foo; say $*MEOWS'
42

[Leont]

Yeah I meant dynamic. I'm not sure that solution really is better than our current, at least MAIN_HELPER is scoped!

[zoffixznet]

Does that even matter? Isn't there only one MAIN (even if it's a bunch of multies) that would get processed? And %*SUB-MAIN-OPTS is the official hash with MAIN handling options.

In fact, it was global briefly before, but was removed so that we don't pay for its initialization when the user isn't gonna use it.

[zoffixznet]

Also, we could have something similar to how you can do use Foo:from<Perl5> without having to explicitly use Inline::Perl5, so that the user would specify which processor they want by declaring the %*SUB-MAIN-OPTS. Something like

    my %*SUB-MAIN-OPTS = :processor<Awesome>;

And that would automatically load GetOpt::Awesome module and use its callable from its predefined method or something (sort-of how we handle custom exeption handlers with Exceptions::BlahBlah and setting env var to BlahBlah string)

[Leont]

To be honest, the old (pre 2018.06) MAIN_HELPER still looks like the best interface I've seen so far.

[zoffixznet]

@Leont would you be able to take new MAIN helpers out for a spin and see if they resolve this ticket?

[Leont]

Yeah it looks sensible.