Fundle

• Summary
• What is Dependency Injection
• Classic Dependency Injection
• How Fundle Works
• Capabilities
• Quick Start
• Name Origin

Summary

Safe compile-time dependency injection for Rust.

Fundle is a dependency injection system for service libraries that provides compile-time safety and zero-cost abstractions for managing complex dependency graphs in large applications.

What is Dependency Injection?

Dependency injection is an implementation technique for the Inversion of Control design pattern, where objects receive their dependencies from external sources rather than creating them internally. This pattern is essential for:

• Testability: Replace real implementations with mocks during testing
• Modularity: Decouple components from their concrete dependencies
• Configuration: Switch implementations based on environment (dev/prod/test)
• Maintainability: Change implementations without modifying dependent code

In large enterprise applications, components often depend on many services like databases, loggers, configuration systems, external APIs, and other business logic components. Managing these dependencies manually becomes unwieldy as the application grows. Dependency injection is intended to help.

Classic Dependency Injection

Classic dependency injection frameworks (like those found in Java/.NET) suffer from several fundamental issues:

• Runtime Failures. Dependencies are resolved at runtime, meaning missing or misconfigured dependencies only surface when an application starts (or worse, when specific code paths execute).

• Virtual Dispatch Overhead. Traditional DI relies heavily on interfaces and virtual dispatch, introducing performance overhead for every method call.

• Complex Configuration. Setting up DI containers requires extensive boilerplate and configuration that's often error-prone and hard to maintain.

How Fundle Works

Fundle takes a fundamentally different approach from classic dependency injection frameworks by using Rust's type system and compile-time guarantees.

• Compile-Time Safety. All dependencies must be satisfied at compile time. Missing dependencies result in compilation errors, not runtime panics.

• Zero-Cost Abstraction. Fundle generates code that compiles down to simple struct field accesses with no virtual dispatch. Dependencies are resolved statically, resulting in the same performance as hand-written code. And monomorphization ensures no runtime overhead.

• Dependency Graph Validation. Fundle automatically validates that dependency graphs are acyclic and that all required dependencies are available when constructing each component: As applications grow to hundreds of components, Fundle's compile-time validation prevents the "integration hell" common in large codebases. New team members can't accidentally break the dependency graph.

Capabilities

• Type-safe builder pattern - Each field must be set exactly once before building
• Dependency injection - Fields can access previously set fields during construction
• Automatic AsRef implementations - Generated for unique field types
• Multiple setter variants - Regular, try (fallible), async, and async-try setters

Quick Start

#[fundle::bundle]
pub struct AppState {
    logger: Logger,
    database: Database,
    config: Config,
}

fn main() {
    let app = AppState::builder()
        .logger(|_| Logger::new())
        .config(|x| Config::new_with_logger(x))
        .database(|x| Database::connect("postgresql://localhost", x))
        .build();
}

Name Origin

The name fundle::bundle comes from the "Take Your Daughter to Work Day" episode of the American version of The Office.