3 files changed, 25 insertions, 0 deletions
diff --git a/parser.h b/parser.h
index 3e45e32d..9d4d857c 100644
--- a/parser.h
+++ b/parser.h
@@ -57,10 +57,12 @@ struct parser {
 
 enum prime_parser { prime_lisp, prime_regex };
 
+extern const int have_yydebug;
 extern const wchar_t *spec_file;
 extern val form_to_ln_hash;
 extern val parser_s;
 extern val unique_s;
+void yydebug_onoff(int);
 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, ...);
diff --git a/parser.y b/parser.y
index 54cc494b..983b05c9 100644
--- a/parser.y
+++ b/parser.y
@@ -1039,6 +1039,8 @@ not_a_clause : ALL              { $$ = mkexp(all_s, nil, num(parser->lineno)); }
 
 %%
 
+const int have_yydebug = YYDEBUG;
+
 int yylex(YYSTYPE *, yyscan_t scanner);
 
 /* C99 inline instantiations. */
@@ -1046,6 +1048,13 @@ int yylex(YYSTYPE *, yyscan_t scanner);
 val rlcp(val to, val from);
 #endif
 
+void yydebug_onoff(int val)
+{
+#if YYDEBUG
+  yydebug = val;
+#endif
+}
+
 static val sym_helper(parser_t *parser, wchar_t *lexeme, val meta_allowed)
 {
   scanner_t *scnr = parser->scanner;
diff --git a/txr.c b/txr.c
index 08c5ede7..559e8ec1 100644
--- a/txr.c
+++ b/txr.c
@@ -148,6 +148,7 @@ static void help(void)
 "--gc-delta=N           Invoke garbage collection when malloc activity\n"
 "                       increments by N megabytes since last collection.\n"
 "--debug-autoload       Allow debugger to step through library auto-loading.\n"
+"--yydebug              Debug Yacc parser, if compiled with YYDEBUG support.\n"
 "\n"
 "Options that take no argument can be combined. The -q and -v options\n"
 "are mutually exclusive; the right-most one dominates.\n"
@@ -525,6 +526,19 @@ int txr_main(int argc, char **argv)
         no_dbg_support(opt);
         return EXIT_FAILURE;
 #endif
+      } else if (equal(opt, lit("yydebug"))) {
+        if (have_yydebug) {
+          yydebug_onoff(1);
+          format(std_error,
+                 lit("~a: option --~a takes no argument, ~a given\n"),
+                 prog_string, opt, org, nao);
+          continue;
+        } else {
+          format(std_error,
+                 lit("~a: option ~a requires YYDEBUG support compiled in\n"),
+                 prog_string, arg, nao);
+          return EXIT_FAILURE;
+        }
       } else if (equal(opt, lit("noninteractive"))) {
         opt_noninteractive = 1;
         stream_set_prop(std_input, real_time_k, nil);