What is %% for Happy?

Question

I create a parser with Happy and notice that this is an online documentation :

Like yacc, we include %% here, for no real reason.

%%

However, there must be a reason, even if it is trivial. Does anyone know what it is?

Answer 1

Separates sections in the Yacc source file. e.g. See http://dinosaur.compilertools.net/yacc/

Names refer to either tokens or nonterminal symbols. Yacc requires token names 
to be declared as such. In addition, for reasons discussed in Section 3, it is 
often desirable to include the lexical analyzer as part of the specification 
file; it may be useful to include other programs as well. Thus, every 
specification file consists of three sections: the declarations, (grammar) 
rules, and programs. The sections are separated by double percent ``%%'' marks. 
(The percent ``%'' is generally used in Yacc specifications as an escape 
character.)

In other words, a full specification file looks like

        declarations
        %%
        rules
        %%
        programs
The declaration section may be empty. Moreover, if the programs section is 
omitted, the second %% mark may be omitted also; thus, the smallest legal Yacc 
specification is

        %%
        rules