Useful tips

Build performance

To get an optimized build performance when using parol's Builder API in your script you can insert the following overrides into your Cargo.toml file:

# Optimized build performance
opt-level = 3

opt-level = 3


Thanks to dalance for reporting #49 ( performance)

Performance of parser generation

First you need to understand that the necessity to frequently generate the parser from a given grammar is drastically diminished in parol because of its design. That means parol generates besides the data structures for your grammar only an interface and the plumping to call its methods. This cuts the dependencies for parser generation from any code you write to process your grammar, i.e. the interface's implementation.

By the way, this property enables ad hoc generation of acceptors for any valid grammar, which I like to call rapid prototyping for your grammar.

So, you only need to generate the parser if you change anything in your grammar description, i.e. in your *.par file. If parser generation is expensive for your grammar, what indeed can be the case, I advice you to put the generated parser and the user trait under source control.

The next thing you should understand is that you should design your grammar to be LL(k) with k as minimal as possible. I know, this can be hard but will pay out in the end.

Also try to optimize your grammar for the goal "Minimal number of productions". This can be often broken down to these constraints:

  • Avoid productions that only rename a non-terminal, i.e. the ones in the form
    A: B;
  • Try to disambiguate your productions, i.e. avoid duplications that have the following form
    A: X Y Z;
    B: X Y Z;
    Pin down why you need productions with identical right-hand-sides. Aren't they actually the same and shouldn't they rather be unified?

If you have a historical grammar definition that is left recursive, which in deed is possible for instance because of the ubiquity of Yacc/Bison grammar descriptions, you should allow for extra time and effort to convert it to a working right recursive one.

Or you use the opportunity to use LALR(1) grammars without having to forgo the comfort of parol. Have a look at the specification of a grammar type.

parol currently provides no special support for this phase except that it is able to detect left recursions in your grammar.

I may provide support for parts of this problem in the future, for instance to remove direct left recursions somehow.