From: Mark Wooding <mdw@distorted.org.uk>
Date: Wed, 21 Oct 2015 23:46:28 +0000 (+0100)
Subject: mdw-base.lisp: Rewrite `setf-default' using `with-places/gensyms'.
X-Git-Url: https://git.distorted.org.uk/~mdw/lisp/commitdiff_plain/79ae1f5c8df5bfd2e4db967c82426e19763a400b

mdw-base.lisp: Rewrite `setf-default' using `with-places/gensyms'.

Now that we have `with-places' and friends, there's no reason to write a
`setf'-like macro out longhand ever again.
---

diff --git a/mdw-base.lisp b/mdw-base.lisp
index 4bed97d..2bad2c2 100644
--- a/mdw-base.lisp
+++ b/mdw-base.lisp
@@ -353,30 +353,21 @@
   (do-case2-like 'ecase vform clauses))
 
 (export 'setf-default)
-(defmacro setf-default (&rest specs &environment env)
+(defmacro setf-default (&rest specs)
   "Like setf, but only sets places which are currently nil.
 
    The arguments are an alternating list of PLACEs and DEFAULTs.  If a PLACE
    is nil, the DEFAULT is evaluated and stored in the PLACE; otherwise the
    default is /not/ stored.  The result is the (new) value of the last
    PLACE."
-  (labels ((doit (specs)
-	     (cond ((null specs) nil)
-		   ((null (cdr specs))
-		    (error "Odd number of arguments for SETF-DEFAULT."))
-		   (t
-		    (let ((place (car specs))
-			  (default (cadr specs))
-			  (rest (cddr specs)))
-		      (multiple-value-bind
-			  (vars vals store-vals writer reader)
-			  (get-setf-expansion place env)
-			`(let* ,(mapcar #'list vars vals)
-			   (or ,reader
-			       (multiple-value-bind ,store-vals ,default
-				 ,writer))
-			   ,@(and rest (list (doit rest))))))))))
-    (doit specs)))
+  `(progn ,@(do ((list nil)
+		 (specs specs (cddr specs)))
+		((endp specs) (nreverse list))
+	      (unless (cdr specs)
+		(error "Odd number of arguments for `setf-default'."))
+	      (push (with-places/gensyms ((place (car specs)))
+		      `(or ,place (setf ,place ,(cadr specs))))
+		    list))))
 
 ;;;--------------------------------------------------------------------------
 ;;; Update-in-place macros built using with-places.