Structs and CLOS Basics

Once a program needs more structure than a bare list or hash table naturally provides — a record with named, typed fields — Common Lisp offers two tools at different levels of sophistication. defstruct gives you a lightweight, fast record type with auto-generated accessors and constructors, ideal for straightforward data aggregates. The Common Lisp Object System, CLOS, builds a full object-oriented layer on top with classes, inheritance, and generic functions whose behavior can be extended after the fact, without touching the original class definition.

(defstruct person name age email) generates a constructor make-person, accessors person-name, person-age, and person-email, a predicate person-p, and a printer, all automatically. You create an instance with (make-person :name "Ada" :age 30 :email "[email protected]") and read or write fields with (person-name p) or (setf (person-age p) 31). Structs are stored efficiently (typically as a vector-like block with a type tag) and dispatch on their slot accessors is direct and fast, but a struct's shape is essentially fixed once defined, and structs support only single inheritance via :include with no runtime redefinition flexibility.

CLOS classes are defined with defclass, where each slot can specify :initarg (the keyword used at construction), :initform (a default value, evaluated lazily per-instance if needed), and :accessor (which generates both a reader and a setf-able writer in one declaration). Unlike a struct, a class definition can be redefined at runtime and existing instances updated to match via update-instance-for-redefined-class, which is invaluable during interactive development — you can fix a class definition without restarting your program and losing all your live objects.

defmethod defines a method specialized on the class of one or more of its arguments — (defmethod describe-account ((a account)) ...) only applies when the first argument is (or inherits from) the account class. Calling describe-account triggers CLOS's dispatch mechanism, which selects the most specific applicable method for the actual runtime classes of the arguments; this is multiple dispatch, since specialization can occur on more than one argument at once, unlike single-dispatch OOP languages where only the receiving object's class matters.

A class can inherit from one or more superclasses by listing them where account () had an empty list above — (defclass savings-account (account) (...)) inherits account's slots and is eligible for account's methods too, and CLOS supports true multiple inheritance, resolving slot and method conflicts via a well-defined class precedence list. Inside a method, (call-next-method) explicitly invokes the next-most-general applicable method up the inheritance chain, letting a subclass method extend rather than fully replace its parent's behavior — for instance, a savings-account's withdraw method might check a minimum-balance rule and then call-next-method to perform the actual balance deduction defined on account.