Default Argument values?

Related Projects ArgumentParser
1

I'm attempting to implement parameter parsing for a project of mine and I can't seem to get default values for arguments working. For instance:

struct Dragonfly: ParsableCommand {
    @Argument() var enableLogging: Bool
    
    func run() throws {
        DragonflyServer.run(enableLogging: enableLogging)
    }
}

Dragonfly.main()

This always requires a value to always be passed. Is this not the intended use for @Argument? I see that @Option can take a default, so is that what I should use?

Additionally, it seems that @Argument() is required instead of @Argument, is that just a byproduct of the wrapper implementation? Is it something that can be fixed?

2

I’m not entirely sure what you’re trying to do, but you might want to use @Flag var enableLogging: Bool instead of @Argument.

2 Likes
3

There are 2 ways you could do it @Option and @Flag

@Flag - for Bool defaults to false
@Option- you can specify the default value
Example below shows them and both of them can be omitted and will result in default values.

    //When using Flag, defaults to false
    //If user specifies --enableLogging, then it will be true, if user ignore it will be false
    //This can be omitted
    //Usage: [--enableLogging]
    @Flag()
    var enableLogging: Bool
    
    //When using Option, you can specify the default value
    //This can be omitted
    //Usage: [--flag2 <flag2>]
    @Option(default: true)
    var flag2: Bool
4

Generally, if the wrapper provides init() we can remove (). It doesn't seem to work with one with default values like what Argument provides: init(help: = default). Not sure if we should add init(), or add support to property wrapper.

5

You can drop the parens (i.e. just do @Foo) if the property wrapper has an init() or init(...) where all arguments have default values.

6

@Option does what I needed. I think I was confused about @Argument vs. @Option vs. @Flag. Still seems like @Argument should support default values.

7

It doesn't work. It is treated as not having default value. Which, for ParsableCommand, it forces me to write init() myself.

This fails to compile, Main.init() is missing.

import ArgumentParser

// Error: Type 'Main' does not conform to protocol 'ParsableArguments'
struct Main: ParsableCommand {
    @Argument var i: Int

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

Main.main(["33"])

This works

import ArgumentParser

struct Main: ParsableCommand {
    @Argument() var i: Int

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

Main.main(["33"])
8

I haven't looked at the code for this library, so I am not sure what's going on, but this works:

@propertyWrapper 
struct Foo {
  init(wrappedValue value: Int = 0) {
    self.wrappedValue = value
  }

  var wrappedValue: Int
}

struct Bar {
  @Foo var value: Int
}

let bar = Bar()
print(bar.value) // 0

To clarify, I was just talking in general that initialisers with all default args in property wrappers are supported now as of Swift 5.2 (I implemented them here), which allows you to do @Foo without the need to add parens.

9

I see what's going on now, current non-beta Xcode (11.3.1) still uses Swift 5.1.3. Ah well, I can wait :hugs:. Thanks for the fix btw :partying_face:.

1 Like
10

Since v0.0.3, it is now possible to define a default value for the @Argument.