How does Bison deal with some optional part in BNF grammars?

Question

I'm a freshman in Bison and Compiler principles, now I'm trying to write an easy Verilog parser in Flex&Bison according to the IEEE Standard. Here is the question: when a grammar's body has optional parts, which are enclosed by square brackets, what should I do in Bison? The BNF grammar maybe like this:

input_declaration ::= input [ net_type ] [ signed ] [ range ]  list_of_port_identifiers 

1, Should I enumerate them one by one like the following?

input_declaration : INPUT list_of_port_identifiers
                             | INPUT net_type list_of_port_identifiers
                             | INPUT signed list_of_port_identifiers
                             | INPUT net_type signed list_of_port_identifiers
                             ....

This way can do solve the problem, but I feel it's so stupid.

2, Should I write a %empty directive in the optional parts' grammar like the following?

net_type:
         %empty
        | SUPPLY0 
        | SUPPLY1 
        | TRI     
        | TRIAND  
        | TRIOR   
        | WIRE    
        | WAND    
        | WOR     
        ;

But I think this will cause some conflicts. So what is the best idea for this?

Answer 1

The best solution is (2):

net_type: %empty | ...;
signed:   %empty | ...;
range:    %empty | ...;

You should not see any conflicts between them unless they share tokens.

If you attempted to write all combinations as (1) and it worked, I still believe that you probably left out one combination by mistake, as Bison would in any case detect the conflict as in (2) and warn you.