Why do enums need a FromStr impl, but subcommands don’t?

[loewenheim]

Say i have a configuration struct

#[derive(Clap)]
struct Config {
    #[clap(subcommand)]
    cmd: SubCommand,

    #[clap(long = "foobar", value_name = "FOOBAR")]
    foobar: FooBar,
}

I can then simply add names to the subcommands to make them parse:

#[derive(Clap)]
enum SubCommand {
    #[clap(name ="shake")]
    Shake,

    #[clap(name = "stir")]
    Stir,
}

But if I try to do the same thing with the FooBar enum …

#[derive(Clap)]
enum FooBar {
    #[clap(name = "foo")]
    Foo,

    #[clap(name = "bar")]
    Bar,
}

… the compiler complains about a missing FromStr impl for FooBar. Why is this needed here when it isn’t for the subcommand?

[pksunkara]

FooBar needs a FromStr impl so that the arguments provided to the clap program will be auto parsed and filled to it. What are you trying to do here?

[pksunkara]

It can work for enum, but you need something like the following:

    #[clap(arg_enum, long = "foobar", value_name = "FOOBAR")]
    foobar: FooBar,

[loewenheim]

Thank you, that worked. Unfortunately, it doesn’t seem to work with optional arguments. If I try

#[clap(arg_enum, long = "foobar", value_name = "FOOBAR")]
foobar: Option<FooBar>,

I get the same error about the FromStr impl as before.

[pksunkara]

Yup, it is still an ongoing work. We only recently implemented that.

[loewenheim]

I see, thanks. In the meantime, I’m using the parse-display crate as a workaround, so it’s not a huge problem. Thank you for the explanations!

[CreepySkeleton]

@loewenheim You don't need the name = "...", here. It has nothing to do with parsing, it's about what the subcommand/option will be in your CLI. See https://docs.rs/structopt/0.3.14/structopt/index.html#specifying-argument-types

@pksunkara
This kind of thing is exactly why I prefer separate #[derive(IntoApp, Subcommand, ArgEnum...)] over #[derive(Clap)]. It does work in simple cases, but when it doesn't, users are left with cryptic does not implement FromStr instead of telling does not implement ArgEnum.