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计算FOLLOW（）还是传播？

[英]Calculating FOLLOW() or propagate?

原文 2014-02-17 05:45:49 0 1 parsing/ grammar/ lalr

I am a little confused regarding the lookaheads computation for LALR(1) parsers. 对于LALR（1）解析器的超前计算，我有些困惑。

In the book "Parsing Techniques - A Practical Guide" they state that propagating lookaheads (+ spontaneous generated lookaheads) is equivalent to calculating the FOLLOW() for variables. 在《解析技术-实践指南》一书中，他们指出传播先行（+自发生成的先行）等同于计算变量的FOLLOW（）。 So why LALR(1) parser (according to the Dragon book) uses the propagate/spontaneous technique at all, if it could do with FIRST() and FOLLOW(), which are simple and straightforward to compute? 那么，为什么LALR（1）解析器（根据《龙书》）完全使用传播/自发技术，如果它可以与FIRST（）和FOLLOW（）一起使用的话，它们就很容易计算了呢？

If this is not the case, what is the difference between the two techniques? 如果不是这种情况，两种技术之间有什么区别？

1 个解决方案

Found a very good explanation at Design and Construction of Compilers 在编译器的设计和构建中找到了很好的解释
In a nutshell, the lookahead set in LALR(1) is only a subset of FOLLOW(). 简而言之，在LALR（1）中设置的超前行为只是FOLLOW（）的子集。 And it is computed by the propagate technique. 它是通过传播技术计算的。