How to detect when String option was specified without value

[pditommaso]

I'm wondering if picocli allows the support for implicit value parameter having an arity of 0..1 and not value is specified.

To make it clear consider the following definition:

@Command(name='sample')
class Sample {
  @Option(names="--foo", aritiy="0..1")
  String foo
}

Now, the use case requires that when the user specifies the --foo parameter w/o a value an implicit value is assigned to foo. The property default is not a valid workaround because it would not allow to distinguish the absence of that parameter.

How to handle this use case with picocli ?

[remkop]

Can you explain a bit more about your use case? I have trouble understanding when it would be useful to distinguish between

# --foo option not specified:
<command> --bar

and

# --foo option specified without value 
<command> --foo --bar

Won’t the application use the default value in both cases?

[pditommaso]

Yes sure, the need for this is to allow the user to enable a feature and optionally to provide an extra parameter value.

So for example:

# --foo option not specified:
<command> --bar

Attribute foo is null therefore the associated feature disabled.

<command> --foo --bar

The attribute foo is set with the Option implicit value. Finally:

<command> --foo hello --bar

This allows the user to provide a specific value for foo eg hello.

In a nutshell this allow to allows to implement a flag attribute that by default has no value ie. null, it can be enabled with a implicit (ie. default) value when the user specifies only the parameters name and finally, allows the user to provide a specific value as an optional value.

I haven't found a way to implement this requirement with picocli.

[remkop]

Thanks for the clarification. To be honest, I don't see an easy way to accomplish this with annotations. Do you have any suggestions?

This may be easier with the programmatic API, although the API for the parse results (#257) is still in a very early stage. I could imagine your use case would look something like this:

String[] args = { "--foo" };
ParseResult parseResult = new CommandLine(new Sample()).parseArgs(args);
if (parseResult.hasOption("--foo")) {
    enableFooFeature();
    File fooFile = parseResult.optionValue("--foo");
    setFooFile(fooFile);
}

One idea is to inject the ParseResult object into your command with the planned @Inject annotation (#259).

[pditommaso]

I was think about having an implicit attribute on the Option annotation eg.

class Sample {
  @Option(names="--foo", aritiy="0..1", implicit="hello")
  String foo
}

And use it when the user does not provide a specific value. Would not that be possible?

[remkop]

There is some work in progress (#261) to add a defaultValue = "..." attribute to @Option and @Parameters. I think it would be confusing to have an additional implicit attribute that is very close in meaning but subtly different.

One thing I can think of is to set the annotated field to the empty string when the option is specified without a value. That would give:

1. --foo option not specified: foo field is set to the default value (null in the above example)
2. --foo option specified without value: foo field is set to the empty String
3. --foo option is specified with value: foo field is set to the specified value

I need to think about this some more, but would this work for you?

[pditommaso]

This would provide a workaround, but I found it a suboptimal solution.

[remkop]

All right, how about this? Let’s split this ticket into parts:

• How to detect when String option was specified without value
• Customize value assigned when option specified without value

When I finish with the first part I will close this ticket. Can you raise a separate ticket for the second feature?

[remkop]

Pushed to master. Please verify.

[pditommaso]

Is there a snapshot to which I can give a try ?

[remkop]

Nothing published. Can you check out master and do gradle build?

[pditommaso]

Just done! Nice, it work as expected.

So the plan is to be able to customise the implicit value with #280, right? Also, my understanding is that this works only with String fields, the #280 would allow the use on fields of any type ?

[remkop]

Merged into the 2.x branch. This will be included in the upcoming 2.3 release.