haskell - Happy 中的 %% 是什么？

Question

我正在用 Happy 构建一个解析器，并注意到这是在线文档：

像 yacc 一样，我们在此处包含 %%，没有任何实际原因。

%%

一定是有原因的，即使是微不足道的。有谁知道它是什么？

Answer 1

它分隔 Yacc 源文件中的各个部分。例如参见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