error-rail

use error_rail::prelude::*;

fn load_config() -> BoxedResult<String, std::io::Error> {
    std::fs::read_to_string("config.toml")
        .ctx("loading configuration")
}

Why error-rail?

Most error handling libraries format context eagerly—even on success paths where the context is never used. error-rail uses lazy evaluation, deferring string formatting until an error actually occurs.

Benchmark Results (full details):

Since most operations succeed (95%+), lazy evaluation provides significant real-world performance gains.

Installation

cargo add error-rail

30-Second Quick Start

use error_rail::prelude::*;

// Add context to any Result with .ctx()
fn read_config() -> BoxedResult<String, std::io::Error> {
    std::fs::read_to_string("config.toml")
        .ctx("loading configuration")
}

// Chain multiple contexts
fn process() -> BoxedResult<(), std::io::Error> {
    let config = read_config()?;
    parse_config(&config)
        .ctx("parsing configuration")
}

fn main() {
    if let Err(e) = process() {
        eprintln!("{}", e.error_chain());
        // Output: parsing configuration -> loading configuration -> No such file...
    }
}

New to error-rail? See the Quick Start Guide for step-by-step examples.

Key Features

1. Structured Error Context

Wrap any error in ComposableError and attach layered metadata.

use error_rail::{ComposableError, context, group};

let err = ComposableError::<&str>::new("connection failed")
    .with_context(context!("retry attempt {}", 3))
    .with_context(group!(
        tag("database"),
        location(file!(), line!()),
        metadata("host", "localhost:5432")
    ))
    .set_code(500);

println!("{}", err.error_chain());
// Output: retry attempt 3 -> [database] at src/main.rs:7 (host=localhost:5432) -> connection failed (code: 500)

2. Error Pipeline

Chain context and transformations with a fluent builder API.

use error_rail::{ErrorPipeline, context, group};

fn fetch_user(id: u64) -> Result<String, &'static str> {
    Err("user not found")
}

let result = ErrorPipeline::new(fetch_user(42))
    .with_context(group!(
        tag("user-service"),
        location(file!(), line!())
    ))
    .with_context(context!("user_id: {}", 42))
    .map_error(|e| format!("Fetch failed: {}", e))  // Transform error type
    .finish_boxed();

if let Err(e) = result {
    eprintln!("{}", e.error_chain());
}

3. Validation Accumulation

Collect multiple errors instead of failing fast—ideal for form validation.

use error_rail::Validation;

fn validate_age(age: i32) -> Validation<&'static str, i32> {
    if age >= 0 && age <= 150 {
        Validation::Valid(age)
    } else {
        Validation::invalid("age must be between 0 and 150")
    }
}

fn validate_name(name: &str) -> Validation<&'static str, &str> {
    if !name.is_empty() {
        Validation::Valid(name)
    } else {
        Validation::invalid("name cannot be empty")
    }
}

// Collect all validation results
let results: Validation<&str, Vec<_>> = vec![
    validate_age(-5).map(|v| v.to_string()),
    validate_name("").map(|v| v.to_string()),
].into_iter().collect();

match results {
    Validation::Valid(values) => println!("All valid: {:?}", values),
    Validation::Invalid(errors) => {
        for err in errors {
            println!("Error: {}", err);
        }
    }
}
// Output:
// Error: age must be between 0 and 150
// Error: name cannot be empty

4. Zero-Cost Lazy Context

The context! macro defers string formatting until an error actually occurs.

use error_rail::{ErrorPipeline, context};

#[derive(Debug)]
struct LargePayload { /* ... */ }

fn process(data: &LargePayload) -> Result<(), &'static str> {
    Ok(()) // Success path
}

let payload = LargePayload { /* ... */ };

// format!() is NEVER called on success path
let result = ErrorPipeline::new(process(&payload))
    .with_context(context!("processing payload: {:?}", payload))
    .finish_boxed();

Performance: In benchmarks, lazy context adds near-zero overhead on success. Eager formatting can be orders of magnitude slower.

5. Convenient Macros

use error_rail::{rail, ComposableError};

// rail! is shorthand for ErrorPipeline::new(...).finish_boxed()
let result = rail!(std::fs::read_to_string("config.toml"));

6. Type Aliases for Ergonomics

use error_rail::{ComposableResult, BoxedComposableResult};

// Instead of Result<T, ComposableError<E>>
fn parse_config() -> ComposableResult<Config, ParseError> { /* ... */ }

// Instead of Result<T, Box<ComposableError<E>>>
fn load_file() -> BoxedComposableResult<String, std::io::Error> { /* ... */ }

When to Use What?

Quick Reference

Validation vs Result

Common Pitfalls

1. Forgetting to Box for Return Types

// ❌ Large stack size (48+ bytes per Result)
fn process() -> Result<Data, ComposableError<MyError>> { ... }

// ✅ Reduced stack size (8 bytes pointer)
fn process() -> BoxedComposableResult<Data, MyError> { ... }

2. Excessive Context Depth

// ❌ Adding context at every layer (O(n) performance)
db_call()
    .with_context(ctx1)
    .and_then(|x| validate(x).with_context(ctx2))
    .and_then(|x| transform(x).with_context(ctx3))
    // ... 20 more layers

// ✅ Add context at boundaries only
let result = db_call()
    .and_then(validate)
    .and_then(transform);

ErrorPipeline::new(result)
    .with_context(context!("user_id: {}", id))
    .finish_boxed()

3. Eager vs Lazy Context

// ❌ Eager: format! runs even on success
.with_context(ErrorContext::new(format!("data: {:?}", large_struct)))

// ✅ Lazy: format! only runs on error
.with_context(context!("data: {:?}", large_struct))

Module Reference

no_std Support

error-rail is no_std compatible by default. It requires only the alloc crate.

[dependencies]
error-rail = { version = "0.4", default-features = false }

Enable std features when needed:

[dependencies]
error-rail = { version = "0.4", features = ["std"] }  # Enables backtrace! macro

Examples

cargo run --example quick_start       # Basic usage patterns
cargo run --example readme_features   # All features from this README
cargo run --example pipeline          # Error pipeline chaining
cargo run --example validation_collect # Validation accumulation

Integration Guides

• Quick Start Guide - Step-by-step tutorial

License

Apache-2.0