arg_list:
     | arg ',' arg_list {
          int func_arg_list_count(){ int argCount = 0; return argCount+1; }

%union {
int val;
int count;
char *funcName;
}

%token <funcName> ID

%type <val> exp prop
%type <count> arg_list

error: $1 of ‘arg_list’ has no declared type
 | arg ',' arg_list { $$ = $1 + 1; }

%union {
   int count;
   /* other types */
}

%type <count> arg_list non_empty_arg_list

%%

arg_list: /* EMPTY */        { $$ = 0; /* An empty list has no arguments */ }
        | non_empty_arg_list { $$ = $1; /* This is the default action. */ }

non_empty_arg_list:
          arg                        { $$ = 1; /* Starts with one argument */ }
        | non_empty_arg_list ',' arg { $$ = $1 + 1; /* Now it has one more argument */ }
        ;