haskell - Happy 中的 %% 是什么意思？

Question

我正在用 Happy 构建一个解析器，并注意到这是 online documentation :

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

%%

一定有原因，即使它是微不足道的。有谁知道它是什么？

Answer 1

它将 Yacc 源文件中的部分分开。例如见 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