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D.3 Obsolete Ways to Customize Setf

Common Lisp defines three macros, define-modify-macro, defsetf, and define-setf-method, that allow the user to extend generalized variables in various ways. In Emacs, these are obsolete, replaced by various features of gv.el in Emacs 24.3. See Adding Generalized Variables in GNU Emacs Lisp Reference Manual.

Macro: define-modify-macro name arglist function [doc-string]

This macro defines a “read-modify-write” macro similar to cl-incf and cl-decf. You can replace this macro with gv-letplace.

The macro name is defined to take a place argument followed by additional arguments described by arglist. The call

(name place args…)

will be expanded to

(cl-callf func place args…)

which in turn is roughly equivalent to

(setf place (func place args…))

For example:

(define-modify-macro incf (&optional (n 1)) +)
(define-modify-macro concatf (&rest args) concat)

Note that &key is not allowed in arglist, but &rest is sufficient to pass keywords on to the function.

Most of the modify macros defined by Common Lisp do not exactly follow the pattern of define-modify-macro. For example, push takes its arguments in the wrong order, and pop is completely irregular.

The above incf example could be written using gv-letplace as:

(defmacro incf (place &optional n)
  (gv-letplace (getter setter) place
    (cl-once-only ((v (or n 1)))
      (funcall setter `(+ ,v ,getter)))))
Macro: defsetf access-fn update-fn

This is the simpler of two defsetf forms, and is replaced by gv-define-simple-setter.

With access-fn the name of a function that accesses a place, this declares update-fn to be the corresponding store function. From now on,

(setf (access-fn arg1 arg2 arg3) value)

will be expanded to

(update-fn arg1 arg2 arg3 value)

The update-fn is required to be either a true function, or a macro that evaluates its arguments in a function-like way. Also, the update-fn is expected to return value as its result. Otherwise, the above expansion would not obey the rules for the way setf is supposed to behave.

As a special (non-Common-Lisp) extension, a third argument of t to defsetf says that the return value of update-fn is not suitable, so that the above setf should be expanded to something more like

(let ((temp value))
  (update-fn arg1 arg2 arg3 temp)

Some examples are:

(defsetf car setcar)
(defsetf buffer-name rename-buffer t)

These translate directly to gv-define-simple-setter:

(gv-define-simple-setter car setcar)
(gv-define-simple-setter buffer-name rename-buffer t)
Macro: defsetf access-fn arglist (store-var) forms…

This is the second, more complex, form of defsetf. It can be replaced by gv-define-setter.

This form of defsetf is rather like defmacro except for the additional store-var argument. The forms should return a Lisp form that stores the value of store-var into the generalized variable formed by a call to access-fn with arguments described by arglist. The forms may begin with a string which documents the setf method (analogous to the doc string that appears at the front of a function).

For example, the simple form of defsetf is shorthand for

(defsetf access-fn (&rest args) (store)
  (append '(update-fn) args (list store)))

The Lisp form that is returned can access the arguments from arglist and store-var in an unrestricted fashion; macros like cl-incf that invoke this setf-method will insert temporary variables as needed to make sure the apparent order of evaluation is preserved.

Another standard example:

(defsetf nth (n x) (store)
  `(setcar (nthcdr ,n ,x) ,store))

You could write this using gv-define-setter as:

(gv-define-setter nth (store n x)
  `(setcar (nthcdr ,n ,x) ,store))
Macro: define-setf-method access-fn arglist forms…

This is the most general way to create new place forms. You can replace this by gv-define-setter or gv-define-expander.

When a setf to access-fn with arguments described by arglist is expanded, the forms are evaluated and must return a list of five items:

  1. A list of temporary variables.
  2. A list of value forms corresponding to the temporary variables above. The temporary variables will be bound to these value forms as the first step of any operation on the generalized variable.
  3. A list of exactly one store variable (generally obtained from a call to gensym).
  4. A Lisp form that stores the contents of the store variable into the generalized variable, assuming the temporaries have been bound as described above.
  5. A Lisp form that accesses the contents of the generalized variable, assuming the temporaries have been bound.

This is exactly like the Common Lisp macro of the same name, except that the method returns a list of five values rather than the five values themselves, since Emacs Lisp does not support Common Lisp’s notion of multiple return values. (Note that the setf implementation provided by gv.el does not use this five item format. Its use here is only for backwards compatibility.)

Once again, the forms may begin with a documentation string.

A setf-method should be maximally conservative with regard to temporary variables. In the setf-methods generated by defsetf, the second return value is simply the list of arguments in the place form, and the first return value is a list of a corresponding number of temporary variables generated by cl-gensym. Macros like cl-incf that use this setf-method will optimize away most temporaries that turn out to be unnecessary, so there is little reason for the setf-method itself to optimize.

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