From 1cafc3e511b19f22ad6eb8c49c3f4cd7f87ece6a Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Thu, 7 Apr 2016 23:25:13 -0700
Subject: Allow symbol macro in function call dot position.

* eval.c (expand_forms): If the forms list is an atom, then
don't just return it. Try to expand it as a macro.
If the macro produces a compound form, diagnose with
an exception, otherwise return the expansion.

* txr.1: Document the treatment of symbol macros in
function call dot position.
---
 eval.c |  6 +++++-
 txr.1  | 14 ++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/eval.c b/eval.c
index 28a7a921..0133caea 100644
--- a/eval.c
+++ b/eval.c
@@ -2459,7 +2459,11 @@ static val me_quasilist(val form, val menv)
 val expand_forms(val form, val menv)
 {
   if (atom(form)) {
-    return form;
+    val ex_f = expand(form, menv);
+    if (consp(ex_f))
+      uw_throwf(error_s, lit("symbol macro ~s in dot position must produce "
+                             "atom form, not compound"), form, nao);
+    return ex_f;
   } else {
     val f = car(form);
     val r = cdr(form);
diff --git a/txr.1 b/txr.1
index b030b9b7..62ffe92c 100644
--- a/txr.1
+++ b/txr.1
@@ -10079,6 +10079,20 @@ Examples:
   (call (op list 1 . @1) 2) ;; yields 2
 .cble
 
+A special consideration is given to symbol macros which occur in the
+dot position. If a symbol macro occurs in this position, then it must not
+expand to a compound form, otherwise a macro-expansion-time error results.
+If it expands to an atom, then it is substituted by that atom, and
+the semantics of the function call proceeds as described above.
+
+.cblk
+  (symacrolet ((x (list 1 2)))
+    (list 1 . x))  ;; error: x expands to form (list 1 2)
+
+  (symacrolet ((x 2))
+    (list 1 . x))  ;; yields (1 . 2)
+.cble
+
 Dialect Note:
 
 In some other Lisp dialects like ANSI Common Lisp, the improper list syntax may
-- 
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