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Common Lisp / Objects & Conditions

defmethod

Learn defmethod in Common Lisp through a focused explanation, a dedicated construct-specific example, an engineering boundary, and a mastery check.

10 min read • Advanced

defmethod is an atomic Common Lisp topic inside Objects & Conditions. This lesson focuses on how the defmethod construct is written, what it changes, and when it is the right tool.

Trace the example line by line. Identify the exact syntax for defmethod, the input state before it runs, and the resulting value, control flow, database state, hardware behavior, or runtime effect afterward. Neighboring lessons use different constructs rather than renamed copies of this one.

Topic-specific example

common-lisp-07-04-defmethod.lisp
common-lisp Try It Live 
(defclass circle () ((r :initarg :r :reader r)))
(defgeneric area (x))
(defmethod area ((c circle)) (* pi (r c) (r c)))
Edge case / engineering note

For defmethod, test the boundary that belongs to this exact feature rather than a generic failure case. Check invalid or empty input, NULL/unknown state, numeric limits, transaction or exception behavior, synthesis/runtime constraints, ownership/permission rules, or backtracking/control-flow effects where they apply.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

defmethod Mastery Check →
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defgeneric
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method specialization