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#### 11.15.2 Defining new `setf` forms

This section describes how to define new forms that `setf` can operate on.

Macro: gv-define-simple-setter name setter &optional fix-return

  This macro enables you to easily define `setf` methods for simple cases. name is the name of a function, macro, or special form. You can use this macro whenever name has a directly corresponding setter function that updates it, e.g., `(gv-define-simple-setter car setcar)`.

  This macro translates a call of the form

      (setf (name args…) value)

  into

      (setter args… value)

  Such a `setf` call is documented to return value. This is no problem with, e.g., `car` and `setcar`, because `setcar` returns the value that it set. If your setter function does not return value, use a non-`nil` value for the fix-return argument of `gv-define-simple-setter`. This expands into something equivalent to

      (let ((temp value))
        (setter args… temp)
        temp)

  so ensuring that it returns the correct result.

Macro: gv-define-setter name arglist &rest body

  This macro allows for more complex `setf` expansions than the previous form. You may need to use this form, for example, if there is no simple setter function to call, or if there is one but it requires different arguments to the place form.

  This macro expands the form `(setf (name args…) value)` by first binding the `setf` argument forms `(value args…)` according to arglist, and then executing body. body should return a Lisp form that does the assignment, and finally returns the value that was set. An example of using this macro is:

      (gv-define-setter caar (val x) `(setcar (car ,x) ,val))

For more control over the expansion, see the macro `gv-define-expander`. The macro `gv-letplace` can be useful in defining macros that perform similarly to `setf`; for example, the `incf` macro of Common Lisp. Consult the source file `gv.el` for more details.

> Common Lisp note: Common Lisp defines another way to specify the `setf` behavior of a function, namely `setf` functions, whose names are lists `(setf name)` rather than symbols. For example, `(defun (setf foo) …)` defines the function that is used when `setf` is applied to `foo`. Emacs does not support this. It is a compile-time error to use `setf` on a form that has not already had an appropriate expansion defined. In Common Lisp, this is not an error since the function `(setf func)` might be defined later.

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