enums - 如何在 structopt 中使用枚举？

Question

我想制作 StructOpt使用枚举，这样每次用户通过 -d sunday它被解析为 Day::Sunday :

#[macro_use]
extern crate structopt;

use std::path::PathBuf;
use structopt::StructOpt;

// My enum
enum Day {
    Sunday, Monday
}

#[derive(Debug, StructOpt)]
#[structopt(name = "example", about = "An example of StructOpt usage.")]
struct Opt {
    /// Set speed
    #[structopt(short = "s", long = "speed", default_value = "42")]
    speed: f64,
    /// Input file
    #[structopt(parse(from_os_str))]
    input: PathBuf,
    /// Day of the week
    #[structopt(short = "d", long = "day", default_value = Day::Monday)]
    day: Day,
}

fn main() {
    let opt = Opt::from_args();
    println!("{:?}", opt);
}

我目前的最佳解决方案是使用Option<String>作为类型并传递自定义 parse_day() :

fn parse_day(day: &str) -> Result<Day, ParseError> {
    match day {
        "sunday" => Ok(Day::Sunday),
        _ => Ok(Day::Monday)
    }
    Err("Could not parse a day")
}

Answer 1

Struct-opt 接受任何实现 FromStr 的类型，离你的 parse_day 函数不远:

use std::str::FromStr;

// any error type implementing Display is acceptable.
type ParseError = &'static str;

impl FromStr for Day {
    type Err = ParseError;
    fn from_str(day: &str) -> Result<Self, Self::Err> {
        match day {
            "sunday" => Ok(Day::Sunday),
            "monday" => Ok(Day::Monday),
            _ => Err("Could not parse a day"),
        }
    }
}

此外，default_value 应该是一个字符串，它将使用 from_str 解释为 Day。

#[structopt(short = "d", long = "day", default_value = "monday")]
day: Day,