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 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.
section defsetf [Macro] ,
section define-setf-expander [Macro] ,
@xref{documentation; (setf documentation)} , section Syntactic Interaction of Documentation Strings and Declarations
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