Even Errors Have Standards...

Printing to stderr is something you, when making an app, most likely always do at least once. It's the place for errors, when things go unexpected (or expected, but failed) in your app.

INFO

stderr stands for Standard Error, and is an output stream dedicated for error messages and diagnostics, separate than stdout (which you most likely know). Why a separate output stream? stderr is typically unbuffered, which means error messages are displayed immediately even if the program crashes.

stdout or stderr?

Here's a good rule of thumb: if it's something the user asked for, it goes to stdout. If it's about what the program is doing (errors, warnings, diagnostics), it goes to stderr.

cprintln!("[green]Done![/] Output written to file."); // stdout: this is the result
ceprintln!("[red]error:[/] file not found.");         // stderr: this is a problem

This matters more than it might seem. Tools that pipe your program's output (like grep or >) only capture stdout by default, so keeping errors on stderr means they don't pollute the actual output.

Basic Usage

Farben's stderr macros mirror the stdout ones exactly, just with an e after the c:

use farben::prelude::*;

ceprint!("[red]error:[/] ");
ceprintln!("file not found.");

ceprintln!("[bold yellow]warning:[/] config missing, using defaults.");
ceprintln!(); // blank line for breathing room

TIP

ceprintln!() with no arguments prints a bare newline to stderr, same as eprintln!() in std. Useful for spacing out error output. This also applies to the other printing macros.

Bleed Variants

ceprintb! and ceprintbln! work the same as their stdout counterparts, styles carry forward without a trailing reset.

use farben::prelude::*;

ceprintb!("[bold red]fatal: ");
ceprintln!("something went very wrong."); // inherits bold red

See Purposefully Bleeding for a full breakdown of how bleed works.

Markdown on stderr

If you have the markdown feature enabled, mdeprint! and mdeprintln! bring inline markdown rendering to stderr too.

use farben::prelude::*;

mdeprintln!("**error:** the config file is *missing* or malformed.");

No reason your error messages can't look good.

Writer Variants

If you need to write to something other than stdout or stderr, Farben provides writer variants that work with any Write implementor.

use farben::prelude::*;
use std::io::Write;

let mut file = std::fs::File::create("log.txt")?;
cwriteln!(file, "[red]error:[/] {}", message);
cwriteb!(file, "[yellow]warning: ");
cwriteln!(file, "retrying...[/]");

All writer variants are available: cwrite!, cwriteln!, cwriteb!, and cwritebln!.