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5.8.1 LR Table Construction

For historical reasons, Bison constructs LALR(1) parser tables by default. However, LALR does not possess the full language-recognition power of LR. As a result, the behavior of parsers employing LALR parser tables is often mysterious. We presented a simple example of this effect in Mysterious Conflicts.

As we also demonstrated in that example, the traditional approach to eliminating such mysterious behavior is to restructure the grammar. Unfortunately, doing so correctly is often difficult. Moreover, merely discovering that LALR causes mysterious behavior in your parser can be difficult as well.

Fortunately, Bison provides an easy way to eliminate the possibility of such mysterious behavior altogether. You simply need to activate a more powerful parser table construction algorithm by using the %define lr.type directive.

— Directive: %define lr.type type

Specify the type of parser tables within the LR(1) family. The accepted values for type are:

(This feature is experimental. More user feedback will help to stabilize it.)

For example, to activate IELR, you might add the following directive to you grammar file:

     %define lr.type ielr

For the example in Mysterious Conflicts, the mysterious conflict is then eliminated, so there is no need to invest time in comprehending the conflict or restructuring the grammar to fix it. If, during future development, the grammar evolves such that all mysterious behavior would have disappeared using just LALR, you need not fear that continuing to use IELR will result in unnecessarily large parser tables. That is, IELR generates LALR tables when LALR (using a deterministic parsing algorithm) is sufficient to support the full language-recognition power of LR. Thus, by enabling IELR at the start of grammar development, you can safely and completely eliminate the need to consider LALR's shortcomings.

While IELR is almost always preferable, there are circumstances where LALR or the canonical LR parser tables described by Knuth (see Knuth 1965) can be useful. Here we summarize the relative advantages of each parser table construction algorithm within Bison:

For a more detailed exposition of the mysterious behavior in LALR parsers and the benefits of IELR, see Denny 2008 March, and Denny 2010 November.