errs

This crate is for error handling in Rust programs, providing an Err struct which represents an error with a reason.

The type of this reason is any, but typically an enum variant is used. The name of this enum variant indicates the reason for the error, and its fields store contextual information about the situation in which the error occurred. Since the path of an enum variant, including its package, is unique within a program, the enum variant representing the reason is useful for identifying the specific error, locating where it occurred, or generating appropriate error messages, etc.

Optionally, by using notify feature and registering error handlers in advance, it is possible to receive notifications either synchronously or asynchronously at the time the error struct is created.

Installation

In Cargo.toml, write this crate as a dependency:

[dependencies]
errs = "0.7.0"

If you want to use error notification, specify the errs-notify or errs-notify-tokio in the dependency features. The errs-notify feature is for general use, while the errs-notify-tokio feature is for use with the Tokio runtime.

[dependencies]
errs = { version = "0.7.0", features = ["errs-notify"] }

If you are using Tokio, you should specify errs-notify-tokio:

[dependencies]
errs = { version = "0.7.0", features = ["errs-notify-tokio"] }

Usage

Err instantiation and identification of a reason

The Err struct can be instantiated with new<R>(reason: R) function or with_source<R, E>(reason: R, source: E) function.

Then, the reason can be identified with reason<R>(&self) method and a match statement, or match_reason<R>(&self, func fn(&R)) method.

The following code is an example which uses new<R>(reason: R) function for instantiation, and reason<R>(&self) method and a match statement for identifying a reason:

use errs::Err;

#[derive(Debug)]
enum Reasons {
    IllegalState { state: String },
    // ...
}

let err = Err::new(Reasons::IllegalState { state: "bad state".to_string() });

match err.reason::<Reasons>() {
    Ok(r) => match r {
        Reasons::IllegalState { state } => println!("state = {state}"),
        _ => { /* ... */ }
    }
    Err(err) => match err.reason::<String>() {
        Ok(s) => println!("string reason = {s}"),
        Err(err) => { /* ... */ }
    }
}

Function-based Error Handler Registration

This crate optionally provides a feature to notify pre-registered error handlers when an Err is instantiated. Multiple error handlers can be registered, and you can choose to receive notifications either synchronously or asynchronously.

To register handlers inside a function (like main), you can use the following functions:

• add_sync_err_handler: For synchronous handlers.
• add_async_err_handler: For general-purpose asynchronous handlers.
• add_tokio_async_err_handler: For Tokio-based asynchronous handlers.

Error notifications will not occur until the fix_err_handlers function is called. This function locks the current set of error handlers, preventing further additions and enabling notification processing.

// In your main function or initialization code:

#[cfg(feature = "errs-notify")]
errs::add_sync_err_handler(|err, tm| {
    println!("[Sync] {}:{}:{} - {}", tm, err.file(), err.line(), err);
});

#[cfg(feature = "errs-notify")]
errs::add_async_err_handler(|err, tm| {
    println!("[Async] {}:{}:{} - {}", tm, err.file(), err.line(), err);
});

#[cfg(feature = "errs-notify-tokio")]
errs::add_tokio_async_err_handler(async |err, tm| {
    println!("[Tokio Async] {}:{}:{} - {}", tm, err.file(), err.line(), err);
});

// Fix the handlers to start receiving notifications.
#[cfg(any(feature = "errs-notify", feature = "errs-notify-tokio"))]
errs::fix_err_handlers();

Macro-based Error Handler Registration

Alternatively, you can register handlers from a static context (outside a function body) using macros. These are useful for setting up global handlers that are compiled into your program.

• add_sync_err_handler!: Statically registers a synchronous handler.
• add_async_err_handler!: Statically registers a general-purpose asynchronous handler.
• add_tokio_async_err_handler!: Statically registers a Tokio-based asynchronous handler.

These macros require function pointers, not closures.

#[cfg(feature = "errs-notify")]
use errs::{add_async_err_handler, add_sync_err_handler};
#[cfg(feature = "errs-notify-tokio")]
use errs::{add_tokio_async_err_handler};
use errs::Err;
use chrono::{DateTime, Utc};
use std::sync::Arc;

// Define a static synchronous handler
fn my_sync_handler(err: &Err, tm: DateTime<Utc>) {
    println!("[Static Sync] Error at {}: {}", tm, err);
}
#[cfg(feature = "errs-notify")]
add_sync_err_handler!(my_sync_handler);

// Define a static asynchronous handler
fn my_async_handler(err: &Err, tm: DateTime<Utc>) {
    println!("[Static Async] Error at {}: {}", tm, err);
}
#[cfg(feature = "errs-notify")]
add_async_err_handler!(my_async_handler);

// Define a static Tokio-based asynchronous handler
#[cfg(feature = "errs-notify-tokio")]
add_tokio_async_err_handler!(async |err: Arc<Err>, tm: DateTime<Utc>| {
    println!("[Static Tokio Async] Error at {}: {}", tm, err);
});

// Later, in your main function, you still need to fix the handlers.
// errs::fix_err_handlers();

Supporting Rust versions

This crate supports Rust 1.80.1 or later.

% ./build.sh msrv
  [Meta]   cargo-msrv 0.18.4

Compatibility Check #1: Rust 1.74.1
  [FAIL]   Is incompatible

Compatibility Check #2: Rust 1.83.0
  [OK]     Is compatible

Compatibility Check #3: Rust 1.78.0
  [FAIL]   Is incompatible

Compatibility Check #4: Rust 1.80.1
  [OK]     Is compatible

Compatibility Check #5: Rust 1.79.0
  [FAIL]   Is incompatible

Result:
   Considered (min … max):   Rust 1.56.1 … Rust 1.92.0
   Search method:            bisect
   MSRV:                     1.80.1
   Target:                   x86_64-apple-darwin

License

Copyright (C) 2025 Takayuki Sato

This program is free software under MIT License.
See the file LICENSE in this distribution for more details.