Allow normal Swift default property initialization syntax

[MPLew-is]

This change allows the normal var foo = "blah" default initialization syntax for Options, as a parallel initialization method as using the default parameter.

This syntax may feel more natural to some developers (myself included), but does not break any existing code using the original initialization method.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[MPLew-is]

I've tested this with this simple change, and it seems to work as expected (and all the test cases still pass locally); however, I want to make sure this is something that would be desired before continuing to extend this to the other parameter types.

[MPLew-is]

A simple implementation to test:

import ArgumentParser

struct SomeCli: ParsableCommand {
	@Option
	var data: String = "test"

	func run() throws {
		print(data)
	}
}

SomeCli.main()

$ swift run cli
test
$ swift run cli --data=test2
test2

[natecook1000]

@swift-ci Please test

[MPLew-is]

Looks like everything passed in CI, so is this something that would be desired for the remaining parameter types?

[natecook1000]

Thanks for starting this implementation! It’s been on my to-do list to investigate, but I apparently hadn’t opened an issue. I have a couple in-flight changes that you might want to wait for, most notably removing ExpressibleByArgument conformance for Optional, which will change the list of inits for @Argument and @Option. I’ll try to get that in so you can keep moving forward with this.

The biggest downside I see to this change is that it can effectively double the number of inits in some cases, where the default: Value? = nil parameter needs to be replaced by two separate inits (one with a wrappedValue parameter and one without). That said, I think the improved syntax is worth it as long as type checking/etc still works out.

[MPLew-is]

Given those concerns, would it maybe make sense to drop the current default: syntax entirely, assuming that this change works as I intend it to? I realize it would break a lot of existing usage, but this is also in very early development so it may be the best time to do it if you feel this syntax is an improvement over the current one.

And honestly I’ll probably push ahead with the development on this more just as an exercise for myself, so don’t rush your other changes on my account; once they’re done, I can always integrate them back into this.

[kylemacomber]

I think this is a great direction

[natecook1000]

Given those concerns, would it maybe make sense to drop the current default: syntax entirely, assuming that this change works as I intend it to? I realize it would break a lot of existing usage, but this is also in very early development so it may be the best time to do it if you feel this syntax is an improvement over the current one.

I definitely agree that we'd just want the one syntax long term. To help existing users transition, we'll want to go from the status quo to three different inits:

// current
init(default: Value? = nil, ...)

// new
init(wrappedValue: Value, ...)
init(...)
@available(*, deprecated, message: "Use regular property initialization for default values")
init(default: Value?, ...)

There's ample room for code deduplication in the initializers, but that can come after this change.

And honestly I’ll probably push ahead with the development on this more just as an exercise for myself, so don’t rush your other changes on my account; once they’re done, I can always integrate them back into this.

That change landed in #173, so you've got a clear field for this. Thanks again!

[MPLew-is]

I'll continue work on this tomorrow/over the next week, but I believe Options are now fully ready for review so please let me know if you have any feedback.

[john-mueller]

I just submitted #186, which adds the default parameter to the last two initializers for Option and Argument (when value is an array), so those would have to be reworked as well. 😬

I like this direction for initialization over the default parameters.

[natecook1000]

This is looking great, @MPLew-is! One note for you below, plus the documentation should read as if using the regular assignment syntax was always the way to do this — we'll handle the notes about old/new and migration in the changelog.

[MPLew-is]

Other than the issue I've pointed out in a line comment, this should be functionally complete. I'll work on the high-level documentation next, but please let me know if you see any other changes I should make!

[MPLew-is]

This should be ready for review, let me know if you find any issues!

[natecook1000]

This is looking great, @MPLew-is! I've reconciled changes with the array defaults addition in #186 — going to merge and then continue that integration. 🎉🎉

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test Linux platform