defmethod

(defmethod name [qualifier] (param... ) body...)

Adds a method to the generic function name (creating it when no defgeneric preceded it) and returns the name symbol. Only the first parameter may carry a specializer, written (var specializer):

• (var (eql literal)) — matches when the first argument is the literal (a keyword, quoted symbol, number, or character)
• (var class-name) — matches instances of a defclass class and its subclasses
• (var type-name) — matches a built-in type (integer, float, number, string, symbol, keyword, character, cons, list, null, hash-table, function, ...)
• (var t) or a plain var — the default method

A call runs the most specific matching method: eql methods first, then class methods (subclass before superclass), then built-in types (subtypes such as integer before their supertypes such as number), then the default method; with no match the call signals an error. Defining the same specializer again replaces the previous method. The body may start with a docstring and (declare ...) (both are ignored).

Method qualifiers and call-next-method

An optional :before, :after, or :around qualifier before the lambda list adds an auxiliary method (standard method combination). For one call:

• every applicable :around method runs, most specific first, each wrapping the rest;
• then every :before method runs for effect, most specific first;
• then the most specific applicable primary (unqualified) method runs — its value is the result;
• then every :after method runs for effect, least specific first.

Inside a primary or :around method, (call-next-method) invokes the next less specific method (passing the current arguments, or new ones if given as (call-next-method arg...)), and (next-method-p) returns whether such a method exists. Calling call-next-method with no next method signals an error.

Lite subset: required parameters only, specializers on later parameters are errors, and standard method combination is supported for class and default methods (an :around/:before/:after with an eql or built-in-type specializer combines only with primaries of the same specializer plus the default method). On the compilation path defmethod is only supported as a top-level form; the dispatched method set of a compiled program is fixed at compile time.