Specializers

Specializers are a concept in the Common Lisp Object System (CLOS) that governs how methods are selected for a given set of arguments. A specializer is a designator used in a method definition to specify the criteria that an argument must meet for the method to be considered in the generic function’s dispatch.

In CLOS, a method can specialize on more than just a general class. While a class (or type) annotation on a parameter provides a broad type-based dispatch, specializers allow finer control. For example, a method can be specialized on a particular class, or on a specific value using an equality specializer, such as (eql 42). This enables value-based dispatch in addition to type-based dispatch. More complex combinations across multiple parameters are also possible, enabling methods that apply only when several conditions hold.

Example:

(defgeneric process (x y))

(defmethod process ((x integer) (y (eql 3))) ...)

(defmethod process ((x (eql 'start)) (y integer)) ...)

(defmethod process ((x number) (y number)) ...)

When a function is called, the system collects all applicable methods whose specializers match the arguments and selects the most specific one according to a defined priority scheme. If two equally specific methods apply, an ambiguity error is signaled.

Specializers thus enable both type-based and value-based dispatch within the same generic function, providing greater precision in method selection. They are a distinctive feature of CLOS and are less common in other object systems.