How to set default to stdin?

[x10an14]

Hi!

I'm trying to use Clap in an app that takes only one (positional) argument.
And I'd like a default behavior for said argument to read from STDIN if nothing has been passed via the cmdline.

I've looked at the current documentation and derive reference, and didn't find any answer to my quest.

How can this be done?

I'm currently using:

$ grep -H clap Cargo.toml
> Cargo.toml:clap = {version = "3.0.14", features = ["derive"]}

use anyhow;
use clap::Parser;

#[derive(Debug, Parser)]
struct Foo {
    bar: String,
}

fn main() -> anyhow::Result<()> {
    let cli = Foo::parse();
    println!("Hello, world!\n\t{}", cli.bar);
    Ok(())
}

[epage]

You can declare the positional argument optional via bar: Option<String> and key off of that for reading from stdin.

If bar were a path, I would do

#[clap(value_name = "PATH", default_value = "-", parse(from_os_str))]
input: std::path::PathBuf,

[x10an14]

In your case, each line is a unique piece of data. That won't be the case everywhere.

I see your point, but I think it's entirely orthogonal to "allow using stdin as default if no other input is given" =)

[epage]

In considering expanding the API, we need to consider how generally applicable it is, what kind of customization it needs, etc. If everyone will wan something different from a stdin default, it makes it harder to support and lowers the value of doing so.

[alkeryn]

sorry for the bump but is there any fix for this.
i'd like to have one argument that is required only if there is nothing comming from stdin.
so that it still complain it is missing if nothing come from stdin, but doesn't if something comes from stdin.
whilst still allowing me to provide other optional arguments.

[epage]

As I said before, you should be able to implement this all with what clap today

• You could make the flag optional and check on your side
• You could check the status of stdin and pass that to require(...)

Anything closer to direct support is only vague talk right now, see #3476 (comment). Right now, the priorities for clap are

1. clap v4
2. making colors more extensible

after that we'll re-evaluate where we should put our focus in terms of extensibility to help us towards making clap powerful and easy to use while not severely impacting build times or binary size

[alkeryn]

thanks epage, i ended up using option, though the only issue i can find with it is that clap won't say : "argument missing" in case nor the option nor stdin was provided, other than that functionality wise it works great !

though, i can't find the require(...) you mentioned in the doc ^^