#### 5.3.60 define-modify-macro [Macro] ¶

`define-modify-macro` name lambda-list function [documentation] ⇒ name

#### Arguments and Values:: ¶

name—a symbol.

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

function—a symbol.

documentation—a string; not evaluated.

#### Description:: ¶

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 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.

#### Examples:: ¶

     (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)

#### Side Effects:: ¶

A macro definition is assigned to name.

#### See Also:: ¶

defsetf [Macro] ,

define-setf-expander [Macro] ,

documentation, (setf documentation) [Standard Generic Function] , Syntactic Interaction of Documentation Strings and Declarations