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define-modify-macro

define-modify-macro Macro

Syntax:

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 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.

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 Eects:

A macro definition is assigned to name.

See Also:

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

Expanded Reference: define-modify-macro

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(define-modify-macro )