[Feature Request] default value from function (or closure)

[pbzweihander]

It would be great if I can make default values from functions, not literals.

For example, if I want to get some options from args, but if not exists, I want to fallback to environment variables, and if not exists either, I want to panic. Like this:

Arg::with_name("some-arg").default_from(|| {
    std::env::var("SOME_ARG").expect("Some arg is not found neither opt nor envvar")
));

If this is possible, I can make the 'environment variable part' lazily evaluated!

[ananthp]

I thought it's already possible to generate a value from a function and use it as default value for an argument:

Arg::with_name("path")
    .default_value(generate_default_path())

The problem is:

• generate_default_path() dynamically constructs a value. It has to return String, not &str, but default_value() expects &str
• .default_value(&generate_default_path()) has a lifetime issue. The borrowed value will expire by the time get_matches() returns the value. So, the parsed arguments can not be used.

[frol]

Another problem of .default_value(generate_default_path()) is laziness, i.e. generate_default_path() will be executed even if the value was provided.

[CreepySkeleton]

Does anybody really need this? I doubt this calculations can ever be complex.

[frol]

@CreepySkeleton The calculations could be non-complex, but they may involve some remote resource fetching or file reading, which you would prefer to avoid.

[pksunkara]

I think this can be done with the hook feature that was described in #1634 (comment). Do you guys agree?

[CreepySkeleton]

Yes, indeed. Closing in favor of #1634 (comment)