defset: bind new-val-sym to temporary variable.

Users of defset no longer have to ensure that in the store
form, the symbol which gives the new value to be stored is
inserted only once.

* share/txr/stdlib/defset.tl (defset-expander): Transform the
store form by inserting a temporary variable using alet.
(sub-list, sub-vec, sub-str): These place forms no longer
require a local gensym.

* txr.1: Updated doc.

* tests/012/defset.tl: The expected output for the inc case
now incorporates a gensym that comes from the compiled
defset macro. Since we can't control that by means of the
gensym counter, we resort to extracting it from the expansion
itself, then check the test case against a template which
incorporates that gensym. We check that the extracted item
really is a gensym: it's a symbol with no home package whose
name starts with "g".

1 files changed, 12 insertions, 2 deletions

diff --git a/tests/012/defset.tl b/tests/012/defset.tl
index 601be292..110f3c64 100644
--- a/tests/012/defset.tl
+++ b/tests/012/defset.tl
@@ -2,10 +2,20 @@
 
 (defset foo (:key x y -- a b c (d 4)) n ^(bar ,x ,y, a, b, c ,d ,n))
 
+;; obtain identity of new-val gensym: this is baked into defset
+(defvarl %new-val-sym% (caar (last (cadr (expand '(inc (foo 1 2)))))))
+
+(test
+  (and (symbolp %new-val-sym%)
+       (null (symbol-package %new-val-sym%))
+       (starts-with "g" (symbol-name %new-val-sym%)))
+  t)
+
 (test
   (expand '(set (foo 1 2 :a 3 :b 4) 5))
   (bar 1 2 3 4 nil 4 5))
 
-(test
+(vtest
   (expand '(inc (foo 1 2 :a 3 :b 4) 5))
-  (bar 1 2 3 4 nil 4 (+ (foo 1 2 :a 3 :b 4) 5)))
+  ^(let ((,%new-val-sym% (+ (foo 1 2 :a 3 :b 4) 5)))
+    (bar 1 2 3 4 () 4 ,%new-val-sym%)))