Diagnose bad consing dot syntax like (a . b . c).

* parser.y (r_exprs): Use unique object in the terminating cons to
indicate the empty spot where the dotted cdr item will go. Check for
misplaced consing dot.  (misplaced_consing_dot_check): New static
function. Checks for the terminator atom spot being taken already.
Thus, the spot may be taken only by the very last reduction, such that
the next reduction is r_exprs -> n_exprs where the terminating atom is
processed.

* parser.c (unique_s): New global variable.
(parse_init): Initialize unique_s.

* parser.h (unique_s): Declared.

* share/txr/stdlib/place.tl (sys:placelet-1): We have a misplaced
consing dot here! It was working correctly by "terminating atom
propagation" behavior, which allowed (a . b c d) to produce
(a c d . b).  If a single terminating atom occurred in the middle of a list, it
was promoted to the end.

1 files changed, 3 insertions, 1 deletions

diff --git a/parser.c b/parser.c
index b12a0a10..b79bf8ea 100644
--- a/parser.c
+++ b/parser.c
@@ -47,7 +47,7 @@
 #include "stream.h"
 #include "parser.h"
 
-val parser_s;
+val parser_s, unique_s;
 
 static val stream_parser_hash;
 
@@ -322,7 +322,9 @@ val parser_errors(val parser)
 void parse_init(void)
 {
   parser_s = intern(lit("parser"), user_package);
+  unique_s = gensym(nil);
   prot1(&stream_parser_hash);
+  prot1(&unique_s);
   stream_parser_hash = make_hash(t, t, nil);
   parser_l_init();
 }