Defining a Default Value Based on Another Argument

[marsfan]

Preface: I'm new to Rust, so my apologies if my understanding of something or my terminology is wrong.

Is there a way that I can define one argument's default value to be based on another argument (ideally with derive method)? I'm working on a tool that will take a single input file, and produces multiple outputs. I have additional optional arguments that allow customizing the output files. If not provided, they instead use the path of the input file, but with a different suffix.

As a minimal/contrived example, if the input file is test.txt, then the outputs would be test.data.bin and test.notes.md, unless the additional arguments are provided.

Is there a way I can configure this to be done automatically when parsing the arguments?

For example, if I was doing this in a language with inheritance, I could create a subclass of the parser with the additional logic at the subclassed method, but it seems this is not possible in Rust, so is there some way I can specify a post-processing function, or do I just need to write that myself and manually call it?

[epage]

I'd recommend checking out default_value_if. If you are using the derive API, any builder method may be used as a derive attribute. In this case #[arg(default_value_if = ...)] won't work and you'll only be able to use the #[arg(default_value_if(...))] syntax.

[marsfan]

Looking at that, I'm not quite sure it handles what I am looking for. Can I use it to make the default value of one argument be based on the value of another argument?

Example: if arg1 is foo, the default for arg2 is foo.txt. If arg1 is bar, then the default for arg2 is bar.txt

[epage]

Oh, you aren't wanting a straight "if" condition but a way to transform the user's arg1 value into a default for arg2?

No, that isn't supported. That seems like something best handled in the application logic.

[marsfan]

Yes. That is what I meant. Thank you for the answer though.

[habibalamin]

@epage it would be nice to be able to just set a value as the direct default of another pre-parse, though.

I'm writing an application which wants to take a required --email argument, and an optional --soa-email argument, where --email is parsed from String to EmailAddress, and the --soa-email is parsed from String to EmailAddress then from EmailAddress to domain::Name, and the pre-parsed default value for --soa-email is whatever value was given to --email.

The advantage of this would be that the Err<String> returned by the value_parser for --soa-email would be handled by clap.

[epage]

imo this feels too specialized to have built-in to clap especially when it can be handled on your side. With #3476, people could experiment with APIs for this and that is the path forward I would recommend within clap.

[habibalamin]

In the end, I did argument parsing by hand. It took a little under 250 lines.

Sadly, it means no automatic version/help flag or usage output or manpage generation or completions generation or error handling, etc, but none of the Rust CLI argument parsing options looked that appealing, and none of them seemed to offer this functionality.

What I'm writing is pretty simple and only for personal use, anyway (though open source), so it's not the biggest deal, but personally, I'm surprised none of the libraries handle this; it feels like it'd be a fairly common need, but 🤷‍♂️ what do I know?