Prompt for missing values in Clap

Cargo.toml

[package]
name = "clap_prompt_example"
version = "0.1.0"
edition = "2021"

[dependencies]
clap = { version = "3.1", features = ["derive"] }
dialoguer = "0.10"

main.rs

use std::{fmt::Debug, str::FromStr};

use clap::Parser;
use dialoguer::{theme::ColorfulTheme, Input};

/// Simple program to greet a person
#[derive(Parser, Debug)]
#[clap(author, version, about, long_about = None)]
struct InputArgs {
    /// Name of the person to greet
    #[clap(short, long)]
    name: Option<String>,

    /// Number of times to greet
    #[clap(short, long)]
    count: Option<u8>,
}

/// Internal representation matching the InputArgs without the nullable properties
struct Args {
    name: String,
    count: u8,
}

/// Transform input arguments into validated arguments by prompting for missing information
impl From<InputArgs> for Args {
    fn from(input: InputArgs) -> Self {
        let name = match input.name {
            Some(name) => name,
            None => prompt("Who would you like to greet", None),
        };
        let count = match input.count {
            Some(count) => count,
            None => prompt("How many times", Some(1)),
        };
        Self { name, count }
    }
}

fn main() {
    let input = InputArgs::parse();
    let args = Args::from(input);

    for _ in 0..args.count {
        println!("Hello {}!", args.name)
    }
}

/// Prompt for user input with the ability to provide a default value
fn prompt<T>(prompt: &str, default: Option<T>) -> T
where
    T: Clone + ToString + FromStr,
    <T as FromStr>::Err: Debug + ToString,
{
    let theme = &ColorfulTheme::default();
    let mut builder = Input::<T>::with_theme(theme);
    builder.with_prompt(prompt);

    if let Some(default) = default {
        builder.default(default);
    }

    builder.interact_text().unwrap()
}