Optional arguments that don't require values

[abrisco]

I think I lack the vocabulary to effectively describe the issue or find a similar question but, does anyone know if it's possible with clap-derive to have an argument that's optional but if passed with no value uses some pre-defined default? The user experience would be something like:

my-binary --foo=bar
my-binary --foo  # Defaults to "baz"
my-binary  # valid and recognizes --foo was not passed

[epage]

The examples for default_missing_value are what you are looking for. Note that any of the builder methods can just be used as attributes within the derive API.

[abrisco]

Thank you! That's exactly what I was looking for!

[sk8ersteve]

This isn't possible for other types of values is it? For example, I want my Option arg to be None if the arg is omitted, but I want it to default to 0 if it's included with no value.

[epage]

While we don't (yet) have a default_missing_value_t, you can still using default_missing_value as that is substituted in early in the process before your types value_parser is run. In fact, default_value_t roughly maps to default_value(&value.to_string()).

[ip1981]

Can we have something like that for environment variables too? Like env_missing_value?

[epage]

So if the env var is empty, apply this other value instead? Would be worth discussing. Feel free to create an issue.

[ip1981]

No, if the argument's value is missed, get it from the env.

[epage]

For myself, that feels too specialized to support natively and would want to wait on any opening up of clap's design to allow people to extend it with features like that