Treetop is one of the most underrated, yet powerful, Ruby libraries out there. If you want to write a parser, it kicks ass. The only problem is unless you're into reading up about and playing with parsers, it's not always obvious how to get going with them, or Treetop in particular. Luckily Aaron Gough, Toronto-based Ruby developer, comes to our rescue with some great blog posts.Aaron, who has a passion for messing around with parsers and language implementations, recently released Koi - a pure Ruby implementation of a language parser, compiler, and virtual machine. If you're ready to dive in at the deep end, the code for Koi makes for good reading.
Starting more simply, though, is Aaron's latest blog post: A quick intro to writing a parser with Treetop. In the post, he covers building a 'parsing expression grammar' (PEG) for a basic Lisp-like language from start to finish - from installing the gem, through to building up a desired set of results. It's a great walkthrough and unless you're already au fait with parsers, you'll pick something up.
If thinking of 'grammars' and Treetop is enough to make your ears itch, though, check out Aaron's sister article: Writing an S-Expression parser in Ruby. On the surface, this sounds like the same thing as the other one, except that this is written in pure Ruby with no Treetop involvement. But while pure Ruby is always nice to see, it's a stark reminder of how much a library like Treetop offers us.
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