allow h_bind's function argument to return NULL as a shortcut for h_nothing_p()

src/hammer.h

@@ -675,7 +675,7 @@ HAMMER_FN_DECL(HParser*, h_get_value, const char* name);
  * Sequencing where later parsers may depend on the result(s) of earlier ones.
  *
  * Run p and call the result x. Then run k(env,x).  Fail if p fails or if
- * k(env,x) fails.
+ * k(env,x) fails or if k(env,x) is NULL.
  *
  * Result: the result of k(x,env).
  */

src/parsers/bind.c

@@ -14,6 +14,9 @@ static HParseResult *parse_bind(void *be_, HParseState *state) {
         return NULL;
 
     HParser *kx = be->k(res->ast, be->env);
+    if(!kx)
+        return NULL;
+
     return h_do_parse(kx, state);
 }
 

src/t_parser.c

@@ -580,6 +580,11 @@ static HParser *f_test_bind(const HParsedToken *p, void *env) {
     v = v*10 + p->seq->elements[i]->uint - '0';
   }
 
+  if(v > 26)
+    return h_nothing_p();	// fail
+  else if(v > 127)
+    return NULL;		// equivalent to the above
+  else
     return h_ch(one - 1 + v);
 }
 static void test_bind(gconstpointer backend) {
@@ -594,6 +599,8 @@ static void test_bind(gconstpointer backend) {
   g_check_parse_failed(p, be, "1x", 2);
   g_check_parse_failed(p, be, "29y", 3);
   g_check_parse_failed(p, be, "@", 1);
+  g_check_parse_failed(p, be, "27{", 3);
+  g_check_parse_failed(p, be, "272{", 4);
 }
 
 void register_parser_tests(void) {