cl-clos-patterns

CLOS design patterns organize Common Lisp Object System class definitions through inheritance strategies, method combination (:before/:after/:around), and slot semantics. Apply patterns like protocols, mixins, and visitor to achieve consistent, extensible OO designs with explicit defgeneric usage and proper slot options.

defclass defines class structure and slots; defgeneric declares method signatures and dispatch points; defmethod implements behavior for specific class combinations. Together they enable CLOS method combination, allowing multiple implementations to coexist and be selected at runtime based on argument types.

initialize-instance and make-instance protocols let you hook custom setup logic into object creation. Override initialize-instance via :after methods to run post-construction steps, or use make-instance wrapper patterns to enforce invariants and control slot initialization order.

:before executes pre-checks or side effects before primary methods; :after handles cleanup or post-conditions; :around wraps primary execution for logging, caching, or control flow. Choose based on whether your logic must run first, last, or wrap the entire call.

Yes. Visitor pattern uses defgeneric with methods dispatching on object type to separate logic from structure. Singleton pattern leverages CLOS slot allocation (:class slots) and initialization protocols to enforce single-instance constraints across the object hierarchy.

CPL determines the order in which parent classes are consulted during method lookup and slot inheritance. Understanding CPL prevents ambiguous method resolution in multiple inheritance, ensures predictable method combination semantics, and clarifies which slot definitions take precedence.