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, 1 insertions, 0 deletions

diff --git a/parser.h b/parser.h
index fdd45e44..9b336d70 100644
--- a/parser.h
+++ b/parser.h
@@ -46,6 +46,7 @@ typedef struct {
 extern const wchar_t *spec_file;
 extern val form_to_ln_hash;
 extern val parser_s;
+extern val unique_s;
 void yyerror(scanner_t *scanner, parser_t *, const char *s);
 void yyerr(scanner_t *scanner, const char *s);
 void yyerrorf(scanner_t *scanner, val s, ...);