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Bassam (Sam) Khouri
a78d98b92d
In some use cases, there is a need to have an option argument behave
like a flag.
This change introduced 4 new intialiazers to `Option` that accept a
`defaultAsFlag` value.
With the following usage:
```swift
struct Example: ParsableCommand {
@Option(defaultAsFlag: "default", help: "Set output format.")
var format: String?
func run() {
print("Format: \(format ?? "none")")
}
}
```
The `defaultAsFlag` parameter creates a hybrid that supports both patterns:
- **Flag behavior**: `--format` (sets format to "default")
- **Option behavior**: `--format json` (sets format to "json")
- **No usage**: format remains `nil`
As a user of the command line tool, the `--help` output clearly distinguishes
between the the hybrid and regular usages.
```
OPTIONS:
--format [<format>] Set output format. (default as flag: default)
````
Note the `(default as flag: ...)` text instead of regular `(default: ...)`,
and the optional value syntax `[<value>]` instead of required `<value>`.
Fixes: #829