matcher: @(as) must always bind fresh variable.

It is documented that as binds a new variable. Furthermore, it
used to be called let. Yet, it back-references. Let's fix it.

* share/txr/stdlib/match.tl (compile-new-var-match): New
function: like compile-var-match but binds new variable, as if
back-referencing didn't exist.
(compile-as-match): Use compile-new-var-match.

* txr.1: Improve as documentation. Clarify that it binds a
fresh variable, and that pattern is processed in its scope.
Include an example with circular structure.

1 files changed, 16 insertions, 1 deletions

diff --git a/share/txr/stdlib/match.tl b/share/txr/stdlib/match.tl
index ab12ef7d..ea0134fc 100644
--- a/share/txr/stdlib/match.tl
+++ b/share/txr/stdlib/match.tl
@@ -205,6 +205,21 @@
             guard-chain (list (new match-guard
                                    guard-expr ^(equal ,obj-var ,sym)))))))
 
+(defun compile-new-var-match (sym obj-var var-list)
+  (cond
+    ((null sym)
+       (new compiled-match
+            obj-var obj-var))
+    ((not (bindable sym))
+       (compile-error *match-form* "~s is not a bindable symbol" sym))
+    (t var-list.(record sym)
+       (new compiled-match
+            pattern sym
+            obj-var obj-var
+            guard-chain (if sym (list (new match-guard
+                                           vars (list sym)
+                                           var-exprs (list obj-var))))))))
+
 (defun compile-vec-match (vec-pat obj-var var-list)
   (let* ((elem-gensyms (mapcar (op gensym `elem-@1-`) (range* 0 (len vec-pat))))
          (elem-exprs (mapcar (ret ^[,obj-var ,@1]) (range* 0 (len vec-pat))))
@@ -327,7 +342,7 @@
 
 (defun compile-as-match (exp obj-var var-list)
   (mac-param-bind *match-form* (op sym pat) exp
-    (let ((var-match (compile-var-match sym obj-var var-list))
+    (let ((var-match (compile-new-var-match sym obj-var var-list))
           (pat-match (compile-match pat obj-var var-list)))
       (new compiled-match
            pattern exp