Behavioral Design Patterns in Clojure

Behavioral patterns are about how work moves, how decisions are delegated, and how systems react over time. In Clojure, that usually means rethinking classic object-oriented patterns through functions, immutable data, channels, multimethods, protocols, and explicit state transitions.

That translation matters. Some patterns collapse into a small higher-order function. Others still matter, but their implementation shape changes because Clojure does not push you toward mutable object graphs as the default coordination mechanism.

Use this chapter to evaluate behavior patterns by runtime shape rather than by textbook name alone: who owns state, who decides next steps, how events flow, and where coupling accumulates if the pattern is applied lazily.

In this section

• Strategy Pattern with Higher-Order Functions
  Learn how the Strategy pattern maps naturally to higher-order functions in Clojure, when plain function arguments are enough, and when data-driven or protocol-based dispatch is a better fit.
• Observer Pattern Using core.async Channels
  Learn how the Observer pattern maps to `core.async` channels, pub/sub, and fan-out workflows in Clojure, and where channel-based observation is stronger or weaker than direct callback relationships.
• Command Pattern in Clojure: Functions and Maps for Flexible Execution
  Explore the Command Pattern in Clojure using functions and maps to encapsulate actions and parameters for flexible execution, including queuing, executing, and undoing commands.
• Chain of Responsibility with Composed Functions
  Learn how to model Chain of Responsibility in Clojure with composed handlers, early exit rules, and clear request contracts instead of class-based handler hierarchies.
• Template Method Pattern Using Protocols
  Learn how to model Template Method in Clojure with a shared orchestration function and protocol-defined variation points, and when plain data-plus-functions is simpler than type-based templates.
• State Pattern with Atoms and State Machines
  Learn how to model the State pattern in Clojure with explicit states, transition functions, and atoms for coordinated current-state ownership instead of scattered conditional logic.
• Visitor Pattern via Multimethods in Clojure
  Explore how Clojure's multimethods enable the visitor pattern, allowing new operations over existing data structures without modification.
• Mediator Pattern with core.async and Pub/Sub
  Learn how the Mediator pattern maps to central coordination channels, topic routing, and message hubs in Clojure, and how to avoid turning the mediator into an opaque god object.
• Memento Pattern for State Preservation
  Learn how the Memento pattern fits immutable Clojure data, including undo stacks, snapshots, and state history, and how to manage snapshot size without confusing mementos with event logs.
• Interpreter Pattern Using Evaluated Data Structures
  Explore the Interpreter Pattern in Clojure by evaluating data structures that represent code, leveraging Clojure's homoiconicity.
• Iterator Pattern with Sequences
  Learn how Clojure sequences, lazy evaluation, and transducers cover most Iterator needs, and when a custom sequence or reducible shape is a better fit than emulating object-style iterators.
• Mastering the Null Object Pattern in Clojure with `nil` and Default Behaviors
  Explore the Null Object Pattern in Clojure, leveraging `nil` and default behaviors to eliminate null checks and prevent errors.
• Specification Pattern for Complex Criteria
  Learn how to model Specification in Clojure with composable predicates, rule combinators, and data-driven criteria instead of hard-coded nested conditionals.
• Event Sourcing in Clojure
  Model state as domain events, build projections with reducers, and understand the versioning, replay, and snapshot trade-offs of event-sourced Clojure systems.
• Reactive Programming Concepts in Clojure: A Comprehensive Guide
  Explore the fundamentals of reactive programming in Clojure, focusing on data flow and change propagation. Learn how to model data flows, leverage core abstractions, and embrace the paradigm shift for enhanced responsiveness and scalability.
• Functional Reactive Programming with RxClojure
  Explore Functional Reactive Programming with RxClojure, a powerful library for composing asynchronous and event-based programs in Clojure. Learn about Observables, Subscribers, and real-time data processing.
• Service Locator Pattern
  Learn how a Service Locator can be modeled in Clojure, why it sometimes works for plugin-style registries, and why explicit dependency injection is usually clearer for ordinary application design.
• Blackboard Pattern: Harnessing Collaborative Problem Solving in Clojure
  Explore the Blackboard Pattern in Clojure, a design where components collaborate by reading and writing to a shared knowledge base, ideal for complex problem-solving and AI applications.