Error Conversion Guide

Error Conversion Guide Skill

You are an expert at Rust error conversion patterns. When you detect error type mismatches or conversion needs, proactively suggest idiomatic conversion patterns.

When to Activate

Activate this skill when you notice:

• Multiple error types in a single function
• Manual error conversion with map_err
• Type mismatch errors with the ? operator
• Questions about From/Into traits for errors
• Need to combine different error types

Error Conversion Patterns

Pattern 1: Automatic Conversion with #[from]

What to Look For:

• Manual map_err calls that could be automatic
• Repetitive error conversions

Before:

#[derive(Debug)]
pub enum AppError {
    Io(std::io::Error),
    Parse(std::num::ParseIntError),
}

fn process() -> Result<i32, AppError> {
    let content = std::fs::read_to_string("data.txt")
        .map_err(|e| AppError::Io(e))?;  // ❌ Manual conversion

    let num = content.trim().parse::<i32>()
        .map_err(|e| AppError::Parse(e))?;  // ❌ Manual conversion

    Ok(num)
}

After:

use thiserror::Error;

#[derive(Error, Debug)]
pub enum AppError {
    #[error("IO error")]
    Io(#[from] std::io::Error),  // ✅ Automatic From impl

    #[error("Parse error")]
    Parse(#[from] std::num::ParseIntError),  // ✅ Automatic From impl
}

fn process() -> Result<i32, AppError> {
    let content = std::fs::read_to_string("data.txt")?;  // ✅ Auto-converts
    let num = content.trim().parse::<i32>()?;  // ✅ Auto-converts
    Ok(num)
}

Suggestion Template:

Use #[from] in your error enum to enable automatic conversion:

#[derive(Error, Debug)]
pub enum AppError {
    #[error("IO error")]
    Io(#[from] std::io::Error),
}

This implements From<std::io::Error> for AppError, allowing ? to automatically convert.

Pattern 2: Manual From Implementation

What to Look For:

• Custom error types that need conversion
• Complex conversion logic

Pattern:

use thiserror::Error;

#[derive(Error, Debug)]
pub enum AppError {
    #[error("Database error: {message}")]
    Database { message: String },

    #[error("Validation error: {0}")]
    Validation(String),
}

// Manual From for custom conversion logic
impl From<sqlx::Error> for AppError {
    fn from(err: sqlx::Error) -> Self {
        AppError::Database {
            message: format!("Database operation failed: {}", err),
        }
    }
}

// Convert with context
impl From<validator::ValidationErrors> for AppError {
    fn from(err: validator::ValidationErrors) -> Self {
        let messages: Vec<String> = err
            .field_errors()
            .iter()
            .map(|(field, errors)| {
                format!("{}: {:?}", field, errors)
            })
            .collect();

        AppError::Validation(messages.join(", "))
    }
}

Suggestion Template:

When you need custom conversion logic, implement From manually:

impl From<SourceError> for AppError {
    fn from(err: SourceError) -> Self {
        AppError::Variant {
            field: extract_info(&err),
        }
    }
}

Pattern 3: Converting Between Result Types

What to Look For:

• Calling functions with different error types
• Need to unify error types

Pattern:

use thiserror::Error;

#[derive(Error, Debug)]
pub enum ServiceError {
    #[error("Repository error")]
    Repository(#[from] RepositoryError),

    #[error("External API error")]
    Api(#[from] ApiError),

    #[error("Validation error")]
    Validation(#[from] ValidationError),
}

// All these errors convert automatically
fn service_operation() -> Result<Data, ServiceError> {
    // Returns Result<_, RepositoryError>
    let data = repository.fetch()?;  // ✅ Auto-converts to ServiceError

    // Returns Result<_, ValidationError>
    validate(&data)?;  // ✅ Auto-converts to ServiceError

    // Returns Result<_, ApiError>
    let enriched = api_client.enrich(data)?;  // ✅ Auto-converts to ServiceError

    Ok(enriched)
}

Suggestion Template:

Create a unified error type that can convert from all the errors you need:

#[derive(Error, Debug)]
pub enum UnifiedError {
    #[error("Database error")]
    Database(#[from] DbError),

    #[error("Network error")]
    Network(#[from] NetworkError),
}

fn operation() -> Result<(), UnifiedError> {
    db_operation()?;  // Auto-converts
    network_operation()?;  // Auto-converts
    Ok(())
}

Pattern 4: map_err for One-Off Conversions

What to Look For:

• Single conversion that doesn't justify From impl
• Adding context during conversion

Pattern:

use anyhow::Context;

fn process(id: &str) -> anyhow::Result<Data> {
    // One-off conversion with context
    let config = load_config()
        .map_err(|e| anyhow::anyhow!("Failed to load config: {}", e))?;

    // Better: use context
    let config = load_config()
        .context("Failed to load config")?;

    // map_err for type conversion without From impl
    let data = fetch_data(id)
        .map_err(|e| format!("Fetch failed for {}: {}", id, e))?;

    Ok(data)
}

When to Use:

• One-off conversions
• Adding context to specific call sites
• Converting to types that don't have From impl

Suggestion Template:

For one-off conversions or adding context, use map_err or anyhow's context:

// With map_err
operation().map_err(|e| MyError::Custom(format!("Failed: {}", e)))?;

// With anyhow (preferred)
operation().context("Operation failed")?;

Pattern 5: Error Type Aliases

What to Look For:

• Repetitive Result<T, MyError> types
• Complex error type signatures

Pattern:

use thiserror::Error;

#[derive(Error, Debug)]
pub enum AppError {
    #[error("IO error")]
    Io(#[from] std::io::Error),

    #[error("Parse error")]
    Parse(#[from] serde_json::Error),
}

// Type alias for cleaner signatures
pub type Result<T> = std::result::Result<T, AppError>;

// Now use it everywhere
pub fn load_config() -> Result<Config> {  // ✅ Clean
    let bytes = std::fs::read("config.json")?;
    let config = serde_json::from_slice(&bytes)?;
    Ok(config)
}

// Instead of
pub fn load_config_verbose() -> std::result::Result<Config, AppError> {  // ❌ Verbose
    // ...
}

Suggestion Template:

Create a type alias for your Result type:

pub type Result<T> = std::result::Result<T, AppError>;

Then use it in function signatures:

pub fn operation() -> Result<Data> {
    Ok(data)
}

Pattern 6: Boxing Errors

What to Look For:

• Need for dynamic error types
• Functions that can return multiple error types

Pattern:

// For libraries that need flexibility
type BoxError = Box<dyn std::error::Error + Send + Sync>;

fn flexible_function() -> Result<Data, BoxError> {
    let data1 = io_operation()?;  // std::io::Error auto-boxes
    let data2 = parse_operation()?;  // ParseError auto-boxes
    Ok(combine(data1, data2))
}

// Or use anyhow for applications
use anyhow::Result;

fn application_function() -> Result<Data> {
    let data1 = io_operation()?;
    let data2 = parse_operation()?;
    Ok(combine(data1, data2))
}

Trade-offs:

• ✅ Flexible: Can return any error type
• ✅ Simple: No need to define custom error enum
• ❌ Dynamic: Error type not known at compile time
• ❌ Harder to match on specific errors

Suggestion Template:

For flexible error handling, use Box<dyn Error> or anyhow:

// Libraries: Box<dyn Error>
type BoxError = Box<dyn std::error::Error + Send + Sync>;
fn operation() -> Result<T, BoxError> { ... }

// Applications: anyhow
use anyhow::Result;
fn operation() -> Result<T> { ... }

Pattern 7: Layered Error Conversion

What to Look For:

• Multi-layer architecture (domain, infra, app)
• Need for error boundary between layers

Pattern:

use thiserror::Error;

// Infrastructure layer
#[derive(Error, Debug)]
pub enum InfraError {
    #[error("Database error")]
    Database(#[from] sqlx::Error),

    #[error("HTTP error")]
    Http(#[from] reqwest::Error),
}

// Domain layer (doesn't know about infrastructure)
#[derive(Error, Debug)]
pub enum DomainError {
    #[error("User not found: {0}")]
    UserNotFound(String),

    #[error("Invalid data: {0}")]
    InvalidData(String),
}

// Application layer unifies both
#[derive(Error, Debug)]
pub enum AppError {
    #[error("Domain error: {0}")]
    Domain(#[from] DomainError),

    #[error("Infrastructure error: {0}")]
    Infra(#[from] InfraError),
}

// Infrastructure to Domain conversion (at boundary)
impl From<InfraError> for DomainError {
    fn from(err: InfraError) -> Self {
        match err {
            InfraError::Database(e) if e.to_string().contains("not found") => {
                DomainError::UserNotFound("User not found in database".to_string())
            }
            _ => DomainError::InvalidData("Data access failed".to_string()),
        }
    }
}

Suggestion Template:

For layered architectures, convert errors at layer boundaries:

// Infrastructure → Domain conversion
impl From<InfraError> for DomainError {
    fn from(err: InfraError) -> Self {
        // Convert infrastructure concepts to domain concepts
        match err {
            InfraError::NotFound => DomainError::EntityNotFound,
            _ => DomainError::InfrastructureFailed,
        }
    }
}

Advanced Patterns

Pattern 8: Multiple Error Sources with Custom Logic

use thiserror::Error;

#[derive(Error, Debug)]
pub enum ProcessError {
    #[error("Stage 1 failed: {0}")]
    Stage1(String),

    #[error("Stage 2 failed: {0}")]
    Stage2(String),
}

// Custom conversion with different variants
impl From<Stage1Error> for ProcessError {
    fn from(err: Stage1Error) -> Self {
        ProcessError::Stage1(err.to_string())
    }
}

impl From<Stage2Error> for ProcessError {
    fn from(err: Stage2Error) -> Self {
        ProcessError::Stage2(err.to_string())
    }
}

fn process() -> Result<(), ProcessError> {
    stage1()?;  // Converts to ProcessError::Stage1
    stage2()?;  // Converts to ProcessError::Stage2
    Ok(())
}

Pattern 9: Fallible Conversion

use thiserror::Error;

#[derive(Error, Debug)]
pub enum ConversionError {
    #[error("Incompatible error type: {0}")]
    Incompatible(String),
}

// TryFrom for fallible conversion
impl TryFrom<ExternalError> for MyError {
    type Error = ConversionError;

    fn try_from(err: ExternalError) -> Result<Self, Self::Error> {
        match err.code() {
            404 => Ok(MyError::NotFound),
            500 => Ok(MyError::Internal),
            _ => Err(ConversionError::Incompatible(
                format!("Unknown error code: {}", err.code())
            )),
        }
    }
}

Common Mistakes

Mistake 1: Multiple #[from] for Same Type

// ❌ BAD: Can't have two #[from] for same type
#[derive(Error, Debug)]
pub enum MyError {
    #[error("First")]
    First(#[from] std::io::Error),

    #[error("Second")]
    Second(#[from] std::io::Error),  // ❌ Conflict!
}

// ✅ GOOD: Use #[source] and manual construction
#[derive(Error, Debug)]
pub enum MyError {
    #[error("Read failed")]
    ReadFailed(#[source] std::io::Error),

    #[error("Write failed")]
    WriteFailed(#[source] std::io::Error),
}

// Construct manually with context
let err = MyError::ReadFailed(io_err);

Mistake 2: Losing Error Information

// ❌ BAD: Converts to String, loses error chain
operation().map_err(|e| MyError::Failed(e.to_string()))?;

// ✅ GOOD: Preserves error chain
operation().map_err(|e| MyError::Failed(e))?;
// Or use #[from]

Mistake 3: Not Using ? When Available

// ❌ BAD: Manual error handling
let result = match operation() {
    Ok(val) => val,
    Err(e) => return Err(e.into()),
};

// ✅ GOOD: Use ?
let result = operation()?;

Decision Guide

Use #[from] when:

• Single error source type
• No need for additional context
• Want automatic conversion

Use #[source] when:

• Multiple variants for same source type
• Need to add context (like field names)
• Want manual construction

Use map_err when:

• One-off conversion
• Adding context to specific call
• Converting to types without From impl

Use anyhow when:

• Application-level code
• Need flexibility
• Want easy context addition

Use thiserror when:

• Library code
• Want specific error types
• Consumers need to match on errors

Your Approach

1. Detect: Identify error conversion needs or type mismatches
2. Analyze: Determine the best conversion pattern
3. Suggest: Provide specific implementation
4. Explain: Why this pattern is appropriate

Communication Style

• Explain the trade-offs between different approaches
• Suggest #[from] as the default, map_err as fallback
• Point out when error information is being lost
• Recommend type aliases for cleaner code

When you detect error conversion issues, immediately suggest the most appropriate pattern and show how to implement it.