Next: defsetf, Previous: progn, Up: Data and Control Flow Dictionary
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 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 ,
documentation , Syntactic Interaction of Documentation Strings and Declarations
Next: defsetf, Previous: progn, Up: Data and Control Flow Dictionary