问题描述
我正在使用JavaScript和语法分析语法为Haskell编写词法分析器，我使用的实现是 PEG.js .
我在使它适用于保留字方面遇到问题，如此处的简化形式所示:
I am writing a lexer for Haskell using JavaScript and Parsing Expression Grammar, the implementation I use being PEG.js.
I have a problem with making it work for reserved words, as demonstrated in a simplified form here:
program = ( word / " " )+
word = ( reserved / id )
id = ( "a" / "b" )+
reserved = ( "aa" )
这里的重点是获得一系列令牌，这些令牌可以是a:s和/或b:s的任意序列，也可以是序列"aa"，并用空格分隔.
我真正得到的是不是每个空格的每个令牌都被识别为id，或者应该被识别为id的令牌已将所有初始的一对a:s吞噬为reserved，例如
"aab"被识别为reserved "aa"，后跟id "b".
The point here is to get a series of tokens that are either arbitrary sequences of a:s and/or b:s or the sequence "aa", and they are separated by spaces.
What I really get is either that every token that is not a space is recognized as id or that a token that should be recognised as id has all initial pairs of a:s eaten up as reserved, e.g.
"aab" gets recognized as reserved "aa" followed by id "b".
Haskell词汇规范解决这种歧义的方式是指定id，如下所示:
The way the Haskell lexical specification solves this ambiguity is to specify id like this:
id = ( "a" / "b" )+[BUT NOT reserved]
我尝试使用PEG的各种组合来复制它！和& -运算符可达到相同的效果，但还没有找到使它正常工作的方法.
解决方案:
I have tried replicating this using various combinations of the PEG ! and & -operators to acheive the same effect, but have not found a way to get this to work properly.
The solution:
id = !reserved ( "a" / "b" )+
我在几个地方看到的建议
不起作用.
这是对特定的PEG实施的限制吗，是PEG本身还是我的方法(希望如此)?
that I've seen suggested in several places does not work.
Is this a limitation in the particular PEG-implementation, PEG in itself or (hopefully) my methods?
提前谢谢！
推荐答案
!reserved ident在任何PEG实现中都是完全可以接受的技术，并且PEG.js也支持该技术.顺便说一句，您应该在reserved的定义之后添加!id.
!reserved ident is a perfectly acceptable technique in any PEG implementation, and PEG.js seems to support it as well. Btw, you should add !id after the definition of reserved.
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