Derive does not automatically capture multiple parameters

[chrisduerr]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Clap Version

3

Describe your use case

With structopt when defining a type as a Vec<T>, it would automatically capture multiple values. Generally I'd say this makes sense because rarely would one want to specify an array differently on the command line. However the new clap derive does not do this, which I'd assume might also cause some regressions for people migrating that aren't careful about their CLI.

While it is currently possible to just specify multiple_values = true, it seems unnecessary.

Describe the solution you'd like

A field annotated with #[clap(...)] of the type Vec<T> for automatic derive should imply multiple_values = true.

Alternatives, if applicable

No response

Additional Context

No response

[chrisduerr]

Looking at what seems to be the relevant code (https://github.com/clap-rs/clap/blob/master/clap_derive/src/derives/args.rs#L311-L320), it might be intentional that a vec only implies multiple_occurences, not necessarily multiple_values?

I'd still really like to see this because I think only having multiple_occurences mostly makes sense for flags, but for a Vec<T> which actually takes a string as an argument it should pretty much always be convenient to the user to not have to specify --option again just to pass another thing when --option x y z would come at probably no disadvantage?

[epage]

It was intentional that we deviated from structopt. See #1772

Most applications either take

• Multiple occurrences (cargo build --package foo --package bar)
• Multiple delimited values (cargo build --features "derive wrap_help")
• Or both (cargo build --features "derive wrap_help" --features suggestions)

Making multiple occurrences by default

• Is a solid base to then build up other behavior (usually if you do delimited values, you also do multiple occurrences)
• Is less surprising since it is what most developers are expecting (personal anecdote: I wrote many applications with Vec in structopt thinking I was getting multiple occurrences only but I was also getting multiple values)
• Has fewer gotchas (see Arg::multiple_values)

For the cases when users want the older / structopt experience, they can still easily get it by opting in with multiple_values(true).

[epage]

btw this was called out in the breaking changes from structopt: https://github.com/clap-rs/clap/blob/master/CHANGELOG.md#breaking-changes

[chrisduerr]

I can't say I agree with that choice, but I suppose multiple_values can still be specified manually.

[tdudz]

@chrisduerr is there a way i can use derive to get clap v3 to capture multiple values?

[chrisduerr]

I'm a bit confused because this issue has multiple comments outlining how to do this already.

But just to make it 100% clear: https://github.com/alacritty/alacritty/blob/master/alacritty/src/cli.rs#L61

[epage]

You can also look at our tests for ideas of what can be done
https://github.com/clap-rs/clap/blob/master/tests/derive/options.rs#L297