Idea: support defaults in Opt functions

[isaacvando]

It would be nice to be able to supply a default value in the Opt functions directly to avoid passing around and unwrapping a result directly. This would allow the following:

Cli.build {
    inputDir: <-
        Opt.str {
            short: "i",
            long: "input-directory",
            help: "The directory containing the templates to be compiled",
            default: ".",
        },
}

instead of having to use Opt.maybeStr and defaulting the result manually.

What do you think @smores56?

[smores56]

I totally agree that this would be preferred, just not sure how to do it.

The above approach doesn't work IMO. Optional record values need to be resolvable by a value if they aren't provided. So if we want Opt.str to be able to take a default value or not, we either need to default to some sentinel value that when detected would mean a default was not passed. (We need to know whether a default was passed to determine whether we should error if a value was passed or not.) What if someone wants to use this default value? You couldn't use an empty string as a default. That makes this a weak option for me.

Maybe we can use a tag union. We can always default to a tag called NoDefault, and the default's type is Value T . We'd want a third variant like Generate ({} -> T) A bit clunky, but doesn't have the "cursed value" issue that the first option does.

We could also provide a Arg.withDefault function that does more clearly what I've done in the examples/ dir. You would call Opt.str { ... } |> Arg.withDefault "abc", or maybe Opt.generateDefault \{} -> "abc" for a thunk version. It's more generic/versatile interface, but is more verbose as well.

Do you have a preference between the two? I'm not sure myself.

[isaacvando]

The above approach doesn't work IMO.

Ah yes good point!

I like the tag approach, it seems simple and easily discoverable. I also like that it allows for a simple approach to using a thunks.

[smores56]

Okay, then let me make a branch and try it out. If I like how it works, I'll make a PR and ping you. Heads up, I'm about to go to Japan for a week, so even though I'll have a laptop, I'm not sure how long it'll take.

[isaacvando]

Sounds great. Enjoy your trip!

[smores56]

This was closed in #4

[isaacvando]

Thanks for doing this!