ifa: take advantage of read-once.

* stdlib/ifa.tl (ifa): When the form bound to the it
anaphoric variable is a place, such that we use placelet,
wrap the place in (read-once ...) so that multiple
evaluations of it don't cause multiple accesses of the
place.

* txr.1: Documented.

2 files changed, 8 insertions, 2 deletions

diff --git a/stdlib/ifa.tl b/stdlib/ifa.tl
index eaab6f3a..317ef9e7 100644
--- a/stdlib/ifa.tl
+++ b/stdlib/ifa.tl
@@ -54,7 +54,7 @@
                (let* ((btemps (mapcar (ret (gensym)) before-it))
                       (atemps (mapcar (ret (gensym)) after-it)))
                  ^(let (,*(zip btemps before-it))
-                    (placelet ((it ,it-form))
+                    (placelet ((it (read-once ,it-form)))
                       (let (,*(zip atemps after-it))
                         (if (,sym ,*(if (eq 'dwim sym) ^(,(second test)))
                                   ,*btemps it ,*atemps)
diff --git a/txr.1 b/txr.1
index 66df2a59..e41b36ff 100644
--- a/txr.1
+++ b/txr.1
@@ -16915,7 +16915,13 @@ alias is used multiple times in the
 .meta then
 or
 .meta else
-expressions.  Otherwise, if the form is not a syntactic place
+expressions. Furthermore, the place form is implicitly surrounded with
+.code read-once
+so that the place's value is accessed only once, and multiple references to
+.code it
+refer to a copy of the value cached in a hidden variable, rather than
+generating multiple accesses to the place.
+Otherwise, if the form is not a syntactic place
 .code it
 is bound as an ordinary lexical variable to
 the form's value.