menu_book Course Curriculum (Common Lisp) expand_more
defun
Learn defun in Common Lisp through a focused explanation, a dedicated construct-specific example, an engineering boundary, and a mastery check.
defun is an atomic Common Lisp topic inside Functions. This lesson focuses on how the defun construct is written, what it changes, and when it is the right tool.
Trace the example line by line. Identify the exact syntax for defun, 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
(defun square (n) (* n n))
(square 6)For defun, 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.