ArgMatches::try_get_flag

[Lucretiel]

Please complete the following tasks

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

Clap Version

4.5

Describe your use case

I'm dynamically populating a Command, based on the contents of a struct. Because it's happening dynamically, I'm using the try versions of the getters on ArgMatches, to handle the case where things happened incorrectly. However, there's no try version of get_flag, which I use for my bool fields.

Describe the solution you'd like

Would like to have a fallible try_get_flag on ArgMatches, to compliment all of the other try_ versions of things. Currently there's no panic-free way to get an argument that you're expecting to be a SetTrue.

Alternatives, if applicable

No response

Additional Context

No response

[epage]

Is there a reason you aren't using try_get_one? get_flag is just a convenience helper for get_one. I lean towards people using try_get_one when they need more rather than adding many variants of helpers (also not a fan of all of the get_matches / parse helpers and been considering reducing those)

[Lucretiel]

I thought (because of the one) that try_get_one returns the value for an option that takes a single argument; that is, it returns Some when --arg value, None when --arg is omitted entirely, and an error otherwise. I haven't been able to find any precise documentation about the underlying "data model" used by ArgMatches; how it's populated from a Command, and specifically how the various get_* and try_get_* methods interpret the stored data.

[epage]

In clap v3, flags were a special case without a value in ArgMatches, instead users having to check for its presence. In clap v4, we transition to always setting a value in ArgMatches, hence the names like SetTrue and SetFalse for flag ArgAction.

In fact, SetTrue is just a convenience helper for Set that implies

arg
  .value_parser(BoolValueParser::new())
  .default_missing_value("true")
  .default_value("false")
  .num_args(0)

so get_flag is a helper for get_one (used for Set) which is a helper for get_many (used for Append, num_args)