Function bpaf::pure_with

pub fn pure_with<T, F, E>(val: F) -> ParsePureWith<T, F, E>
where
    F: Fn() -> Result<T, E>,
    E: ToString,

Wrap a calculated value into a Parser

This parser represents a possibly failing equivalent to pure. It produces T by invoking the provided callback without consuming anything from the command line, which can be useful with construct!. As with any parsers, T should be Clone and Debug.

Both pure and pure_with are designed to put values into structures, to generate fallback you should be using fallback and fallback_with.

See also pure for a pure computation that can’t fail.

Combinatoric example

#[derive(Debug, Clone)]
pub struct Options {
    name: String,
    money: u32,
}

fn starting_money() -> Result<u32, &'static str> {
    Ok(330)
}

pub fn options() -> OptionParser<Options> {
    // User can customise a name
    let name = long("name").help("Use a custom user name").argument("NAME");
    // but not starting amount of money
    let money = pure_with(starting_money);
    construct!(Options { name, money }).to_options()
}

fn main() {
    println!("{:?}", options().run())
}

Derive example

#[derive(Debug, Clone, Bpaf)]
#[bpaf(options)]
pub struct Options {
    #[bpaf(argument("NAME"))]
    /// Use a custom user name
    name: String,
    #[bpaf(pure_with(starting_money))]
    money: u32,
}

fn starting_money() -> Result<u32, &'static str> {
    Ok(330)
}

fn main() {
    println!("{:?}", options().run())
}

Output

pure does not show up in --help message

$ app --help

Usage: app --name=NAME

Available options:

--name=NAME

Use a custom user name

-h, --help

Prints help information

And there’s no way to alter the value from the command line

$ app --name Bob
Options { name: "Bob", money: 330 }

Any attempts to do so would result in an error :)

$ app --money 100000 --name Hackerman
Error: --money is not expected in this context