unset default value if some other Arg is present (or has a specific value)

[delan]

Rust Version

rustc 1.32.0 (9fda7c223 2019-01-16)

Affected Version of clap

2.32.0

Feature Request Summary

Some way to unset the default value for an Arg based on the presence or value of some other Arg. One alternative the current API allows is to pass the empty string (or some other value that means nothing) to Arg::default_value_if (at the expense of additional code to treat Some("") like None):

.default_value_if("other", None, "").default_value("foo")

The signature I’m imagining looks something like:

fn no_default_value_if(self, arg: &'a str, val: Option<&'b str>) -> Self

[delan]

@Moongoodboy-K suggests default_value_unless over no_default_value_if

[patrickelectric]

That's a really important feature!

[stevenengler]

Instead of creating a new method, could the signature of default_value_if() be changed in version 3 to support an Option<&str> for the default rather than just a &str? This way the default can be unset without needing to add more functions to the API.

[delan]

Instead of creating a new method, could the signature of default_value_if() be changed in version 3 to support an Option<&str> for the default rather than just a &str?

Good idea (fwiw). The parameter type could even change without adding to the version 3 migration burden with impl Into<Option<&str>>, which allows you to continue passing &str values without wrapping them in Some.

[pksunkara]

@logansquirel And then you can take this.