defmacro

(defmacro name lambda-list body...)

Defines a user macro named name and returns the name symbol. A macro call receives its argument forms unevaluated: the body runs at expansion time with the parameters bound to the raw forms, and the form it returns (the expansion) is evaluated in place of the call. The lambda list is a macro lambda list, destructured over the argument forms like destructuring-bind: patterns nest in required positions, and &optional (with defaults), &rest/&body, &key, and &aux are supported; &whole and &environment are not. With such an extended lambda list, matching is lenient (a missing position binds to nil, surplus forms are ignored); a plain lambda list (required parameters plus one trailing &rest/&body) keeps the strict argument-count check. A standard operator (when, setf, ...) cannot be redefined, and a macro has no function value (#'name is an error).

Macro bodies usually build the expansion with the backquote template syntax, which is also available anywhere else in a program:

• `form quotes form except where a comma unquotes it
• ,expr inserts the value of expr
• ,@expr splices the value of expr (a list) into the surrounding list

Nested backquote (a backquote template inside another) is supported and fully expanded at read time, so classic macro-writing macros such as once-only work. Use gensym to generate capture-safe temporaries in macro bodies, and macroexpand-1/macroexpand to inspect an expansion.

The interpreter expands macro calls at evaluation time (so defmacro also works in the REPL and via load/eval). On the compilation path the CLI fully expands every macro call before the JVM/WASM compilers run and removes the definitions, so compiled output contains only ordinary forms; consequently the runtime eval/read of a compiled program does not know defmacro or the backquote character, and a macro must be defined before its first use.