placelet: add comment.

* share/txr/stdlib/place.tl (sys:placelet-1): Adding
comment explaining why the explicit expansion and temporary
proxy place are necessary, even though in most cases
it can work fine without them.

1 files changed, 13 insertions, 1 deletions

diff --git a/share/txr/stdlib/place.tl b/share/txr/stdlib/place.tl
index 26818ce4..985c06fb 100644
--- a/share/txr/stdlib/place.tl
+++ b/share/txr/stdlib/place.tl
@@ -867,7 +867,19 @@
   (with-gensyms (tmp-place pl-getter pl-setter)
     (unwind-protect
       (progn
-        (sethash *place-update-expander* tmp-place
+        ;; This temporary proxy place installed into the
+        ;; *place-update-expander* hash, and the forced expansion
+        ;; of the symacrolet form are necessary for correctness.
+        ;; If we don't perform that expand, then the temporary proxy
+        ;; place is not used, and sym ends up being an alias
+        ;; for the getter form (,',pl-getter) of the original place.
+        ;; Then, placelet will only work for places whose getter forms
+        ;; themselves places. This is not required in general. A (foo ...)
+        ;; place can, for instance, use (get-foo ...) and (set-foo ...)
+        ;; getters and setters, where (get-foo ...) is not a place.
+        ;; If sym turns into a symbol macro for a (get-foo ...) form,
+        ;; uses of sym as a place will fail due to get-foo not being a place.
+       (sethash *place-update-expander* tmp-place
                  (lambda (tmp-getter tmp-setter tmp-place tmp-body)
                    ^(macrolet ((,tmp-getter () ^(,',pl-getter))
                                (,tmp-setter (val) ^(,',pl-setter ,val)))