serde-like default value behavior

[epage]

Maintainer's notes

• We can use the field's Default::default with #[clap(default_value_t)]
• Design work is needed for using the container's Default::default

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Issue by epage
Thursday May 03, 2018 at 13:17 GMT
Originally opened as TeXitoi/structopt#105

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It'd be nice to have annotations like serde for default values

• Use the struct's Default impl
• Use the field's Default impl

[epage]

Comment by TeXitoi
Thursday May 03, 2018 at 13:38 GMT

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The problem is that, most of the time, clap will not be able to show the default value (because just a few types implement Display).

[epage]

default_value_t allows you to do the type's Default impl or a custom value of the type, so long as the type implements Display.

[epage]

I missed the case of using the container's Default::default. The question is how to specify this. Serde does this by having a container-level default attribute but for us to do that, we would make all fields optional. This would require a weird handling of structs to use flatten to leverage it or a new way to specify it at the field level