value_of_t_or and value_of_t_or_else

[letheed]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

3.0.14

Describe your use case

The use case is the parsing of optional arguments where a default value is provided when there is none.

Currently I’m using this code:

const MY_ARG_DEFAULT = {…};

let my_val = match args.value_of_t("MY_ARG") {
        Err(error) if error.kind() == ErrorKind::ArgumentNotFound => Ok(MY_ARG_DEFAULT),
        result => result,
    }?;

Describe the solution you'd like

Ideally I would like something like:

let my_val = args.value_of_t_or("MY_ARG", MY_ARG_DEFAULT)?;

and for more expensive values:

let my_val = args.value_of_t_or_else("MY_ARG", || <expr>)?;

I’m actually a bit surprised something like that doesn’t exist because I though the derive feature would use something like that for default_value_t instead of a print/parse roundtrip.

Alternatives, if applicable

No response

Additional Context

No response

[epage]

I’m actually a bit surprised something like that doesn’t exist because I though the derive feature would use something like that for default_value_t instead of a print/parse roundtrip.

We've been tracking this with #2505

Keep in mind that value_of_t is new in clap 3.0, so our window of feedback has been small. We just didn't have time to change the API to how it was proposed in there. Your API proposals offer some alternatives that can be added in the mean time. Naming between an erroring vs an optional version is unclear (not really seen a pattern for that emerge).

We also will have #2683 impact some of these API choices.

I'm going to close this out in favor of continuing the conversation on #2505