Software design often wrestles with the challenge of reusing behavior without the downsides of deep inheritance trees or brittle architecture. Ruby and Rust—two very different languages—each provide elegant tools to solve this: mixins in Ruby and traits in Rust.

Though they differ in philosophy (dynamic vs. static, object-oriented vs. systems-level), both languages offer behavior composition that avoids the pitfalls of traditional multiple inheritance.

This post explores how Ruby and Rust achieve this, using clear examples and a side-by-side comparison.

Part 1: Mixins in Ruby

Reusable Behavior Without Inheritance Hell

Ruby is dynamically typed and deeply object-oriented. Rather than supporting multiple inheritance (which often leads to diamond problems), Ruby lets you include modules (mixins) into your classes.

Let’s see this in action.

Example: Basic Inheritance

class MyParent
  def woof
    puts "woof!"
  end
end

class MyClass < MyParent
end

object = MyClass.new
object.woof

Simple. MyClass inherits the woof method from MyParent.

Now, Add a mixins

module MyMixin
  def woof
    puts "hijacked the woof method!"
  end
end

And include it:

class MyBetterClass < MyClass
  include MyMixin
end

newobject = MyBetterClass.new
newobject.woof

Ruby inserts MyMixin into the method lookup chain between the class and its superclass. When woof is called, Ruby finds MyMixin#woof first.

Lookup chain

MyBetterClass
→ MyMixin
→ MyClass
→ MyParent
→ Object
→ Kernel
→ BasicObject

You can confirm it with: MyBetterClass.ancestors

Why Ruby Mixins Shine

  • Behavior Injection: Extend any class with functionality without inheritance or monkey-patching.
  • Linear, Predictable Lookup: Avoids ambiguity of multiple inheritance.
  • Minimal Boilerplate: No need for interfaces or abstract base classes.
  • Dynamic and Flexible: Mixins can be conditionally included or extended at runtime.

Part 2: Traits in Rust

Rust is statically typed and prioritizes safety, zero-cost abstractions, and explicitness. It doesn’t have classes or inheritance. Instead, it uses traits to define shared behavior.

trait Woof {
    fn woof(&self) {
        println!("woof!");
    }
}

struct Dog;
impl Woof for Dog {}

Here, Dog gets the default woof behavior from the Woof trait.

struct BetterDog;
impl Woof for BetterDog {
    fn woof(&self) {
        println!("hijacked the woof method!");
    }
}

Now, BetterDog overrides the behavior.

fn main() {
    let dog = Dog;
    dog.woof(); // "woof!"

    let better = BetterDog;
    better.woof(); // "hijacked the woof method!"
}

Why Rust Traits Are Powerful

  • Static Dispatch: No runtime penalty. Method calls are resolved at compile time.
  • Modular Design: Traits define behavior contracts cleanly, similar to interfaces.
  • Overridable Defaults: Like Ruby mixins, you can override default methods.
  • Safe and Explicit: No accidental overrides or surprising inheritance chains.

Ruby Mixins vs Rust Traits

Feature Ruby Mixins Rust Traits
Typing Dynamic Static
Inheritance Single inheritance + mixins No inheritance
Behavior Reuse include Module impl Trait for Type
Override Defaults Yes Yes
Method Lookup At runtime At compile time
Composition Philosophy Flexible and implicit Safe and explicit
Multiple Inclusion Order Linear, introspectable Controlled via trait bounds

Conclusion

Ruby and Rust both offer elegant ways to compose behavior:

  • Ruby’s mixins give you runtime flexibility, ideal for rapid prototyping and DSLs.
  • Rust’s traits give you compile-time safety and performance, great for scalable, maintainable systems.

Both avoid the trap of deep or multiple inheritance by encouraging modular, reusable, and overrideable behavior containers.

Choose the tool that fits your domain—but appreciate how both languages have solved this old OO problem with modern clarity.