From 5ef6b01420fe68f40c266cec62c5b4a7931420f1 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Mon, 17 Jun 2019 06:54:47 -0700
Subject: carray-sub: improved accessor.

* share/txr/stdlib/ffi.tl (carray-sub): We use defset to
define a carray-sub place that updates via carray-replace,
rather than using a place macro to defer to the
generic sub accessor. This results in a more efficient
implementation. This new accessor doesn't require the carray
argument to be a place; unlike sub, it doesn't have to capture
the return value of carray-replace and store it back into
the carray variable.

* txr.1: Updated doc.
---
 share/txr/stdlib/ffi.tl |  4 ++--
 txr.1                   | 15 ++++++++++++---
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/share/txr/stdlib/ffi.tl b/share/txr/stdlib/ffi.tl
index ed8ad06d..6bbd43a9 100644
--- a/share/txr/stdlib/ffi.tl
+++ b/share/txr/stdlib/ffi.tl
@@ -145,8 +145,8 @@
 
 (define-accessor carray-ref carray-refset)
 
-(define-place-macro carray-sub (carray : (from 0) (to t))
-  ^(sub ,carray ,from ,to))
+(defset carray-sub (carray : (from 0) (to t)) items
+  ^(progn (carray-replace ,carray ,items ,from ,to) ,items))
 
 (defmacro znew (type . pairs)
   (if (oddp (length pairs))
diff --git a/txr.1 b/txr.1
index 4ecbc011..9de125f1 100644
--- a/txr.1
+++ b/txr.1
@@ -65108,9 +65108,18 @@ and modifications in the original are not reflected in the subrange.
 
 If
 .code carray-sub
-is used as a syntactic place, behaves exactly like the
-.code sub
-accessor, via a place macro substitution to that form.
+is used as a syntactic place, the argument expressions
+.metn carray ,
+.metn from ,
+.meta to
+and
+.meta new-val
+are evaluated just once. The prior value, if required, is accessed by calling
+.code carray-sub
+and
+.meta new-val
+is then stored via
+.codn carray-replace .
 
 .coNP Function @ carray-replace
 .synb
-- 
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