[Gcl-devel] Re: setf

[Paul F. Dietz]

Camm Maguire wrote:
> Greetings!  Paul, my investigations into the defgeneric and methods
> issues is leading me back to the issue of the proper treatment of
> '(setf foo) in 'function, 'flet, 'fboundp, etc.  Could you please
> briefly summarize for me what is supposed to happen, or what sections
> of the spec govern this?  It should be easy to get '(setf foo)
> recognized as a function name, but I don't know how it should react
> with defsetf -- my feeling is that in the absence of any (defun (setf
> foo) ...),
>
> (function '(setf foo)) -> (lambda (x) (setf foo x)),
>
> and to hope that the macroexpansion if any handles the rest.  How far
> off is this?


I don't believe defsetf causes a (setf ...) function to be defined
at all.  Instead, it causes a setf *expander* to be defined.  Macros
like SETF would call GET-SETF-EXPANSION to get this expansion.

Example from SBCL:

* (defun foo (x) (car x))

FOO
* (defsetf foo rplaca)

FOO
* #'(setf foo)

debugger invoked on condition of type UNDEFINED-FUNCTION:
  The function (SETF FOO) is undefined.

Within the debugger, you can type HELP for help. At any command prompt (within
the debugger or not) you can type (SB-EXT:QUIT) to terminate the SBCL
executable. The condition which caused the debugger to be entered is bound to
*DEBUG-CONDITION*. You can suppress this message by clearing
*DEBUG-BEGINNER-HELP-P*.

restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT   ] Reduce debugger level (leaving debugger, returning to toplevel).
  1: [TOPLEVEL] Restart at toplevel READ/EVAL/PRINT loop.
(FDEFINITION 1 (SETF FOO))[:EXTERNAL]
0] 0

* (macroexpand '(setf (foo x) y))

(RPLACA X Y)
T
*

        Paul