Add support for customizing default value string displayed in Help

[griffin-stewie]

Sample code

Here is my sample code:

extension URL: ExpressibleByArgument {
    public init?(argument: String) {
        guard let url = URL(string: argument) else {
            return nil
        }
        self = url
    }
}

struct MySample: ParsableCommand {
    @Option(default: URL(string: FileManager.default.currentDirectoryPath)!, help: "Directory")
    var directory: URL
}

Produces output:

USAGE: sample [--directory <directory>]

OPTIONS:
  -v, --version           Print version 
  --directory <directory> Directory (default:
                          /Users/stewie/Library/Developer/Xcode/DerivedData/sample-blghzaivmncuwahbbafbmsweltpy/Build/Products/Debug)
  -h, --help              Show help information.

Expected behavior

I would expect the following

  --directory <directory> Directory (default: current directory)

Actual behavior

I get:

  --directory <directory> Directory (default:
                          /Users/stewie/Library/Developer/Xcode/DerivedData/sample-blghza

Current directory is common default value for file path like ls command. but 1294160 prints actual full path on help. Since defaultValueDescription is implemented 1294160 and it is not a protocol requirement.

Plan

Simply we can add defaultValueDescription in ExpressibleByArgument protocol

/// A type that can be expressed as a command-line argument.
public protocol ExpressibleByArgument {
  /// Creates a new instance of this type from a command-line-specified
  /// argument.
  init?(argument: String)

  /// Add this ↓
  var defaultValueDescription: String { get }
}

Source Impact

Default behavior is given by default implementation declared in ExpressibleByArgument as Protocol Extensions. Only users who want to customize need to override defaultValueDescription.

Environment

ArgumentParser: 1294160

Related Forum Thread

How can I change the default value string displayed in Help? - Related Projects / ArgumentParser - Swift Forums