lambda-list - a define-modify-macro lambda list

function - a symbol.

documentation - a string; not evaluated.

The arguments to the new macro are a place, followed by the arguments that are supplied in lambda-list. Macros defined with define-modify-macro correctly pass the environment parameter to get-setf-expansion.

When the macro is invoked, function is applied to the old contents of the place and the lambda-list arguments to obtain the new value, and the place is updated to contain the result.

Except for the issue of avoiding multiple evaluation (see below), the expansion of a define-modify-macro is equivalent to the following:

 (defmacro name (reference . lambda-list)
   documentation
   `(setf ,reference
          (function ,reference ,arg1 ,arg2 ...)))

where arg1, arg2, ..., are the parameters appearing in lambda-list; appropriate provision is made for a rest parameter.

The subforms of the macro calls defined by define-modify-macro are evaluated as specified in Section 5.1.1.1 Evaluation of Subforms to Places.

Documentation is attached as a documentation string to name (as kind function) and to the macro function.

If a define-modify-macro form appears as a top level form, the compiler must store the macro definition at compile time, so that occurrences of the macro later on in the file can be expanded correctly.

 (define-modify-macro appendf (&rest args) 
    append "Append onto list")   APPENDF
 (setq x '(a b c) y x)  (A B C)
 (appendf x '(d e f) '(1 2 3))  (A B C D E F 1 2 3)
 x  (A B C D E F 1 2 3)
 y  (A B C)
 (define-modify-macro new-incf (&optional (delta 1)) +)
 (define-modify-macro unionf (other-set &rest keywords) union)