define-modify-macro name lambda-list function [documentation] → name

name — a symbol.

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

function — a symbol.

documentation — a string; not evaluated.

define-modify-macro defines a macro named name to read and write a place.

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)

A macro definition is assigned to name.

defsetf, define-setf-expander, documentation, Section 3.4.11 (Syntactic Interaction of Documentation Strings and Declarations)