property wrappers

[tanner0101]

Property wrappers could be used to make the CommandSignature API more Swifty. Here's an idea for how this could look:

struct Cowsay: Command {
    struct Signature: CommandSignature {
        @Argument(help: "What the cow should say") 
        var message: String

        @Option(short: "e", help: "Change the cow's eyes")
        var eyes: String?

        @Option(short: "t", help: "Change the cow's tongue")
        var tongue: String?

        @Flag(help: "uses == for cow's eyes")
        var borg: Bool
    }

    let help = "Prints an ASCII cow with a message"

    func run(context: CommandContext, signature: Signature) throws {
        print(signature.message) // String
        print(signature.eyes) // String?
        print(signature.tongue) // String?
        print(signature.borg) // Bool
    }
}

Here's how this command could be used:

Usage: cowsay <message> [--eyes,-e eyes] [--tongue,-t tongue] [--borg]

Prints an ASCII cow with a message

Arguments:
    message What the cow should say

Options:
      --eyes, -e Change the cow's eyes
    --tongue, -t Change the cow's tongue

Flags:
    --borg uses == for cow's eyes