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SexpPath: A Ruby DSL for Pattern Matching S-Expressions

By Ari Brown / July 14, 2009

With people occasionally talking about "Code vs. Data", it only makes sense that you should be able process over code as you would a string. Sexp Path is a code processing tool that allows you to search over and process Ruby code in the form of S-Expressions.

For those who don't know, an S-Expression (or simply, a "sexp") is an iterable way of representing code or data. Using Ryan Davis' Parse Tree, you can parse Ruby files and process over them using Sexp Path. It's a bit like like XPath or regular expressions for your code.

The foundation of Sexp Path is the query, formed with Q?{ ... }, which is applied to the sexp via the / method. These methods can be chained, and the results processed via the each method. Using this as an example, Sexp Path also supports named captures like Q?{ s(:class, atom % 'class_name', _, _) } in line 16 so that the second atom is accessible via the class_name attribute in line 25.

The code is stored on GitHub. Unclear of where the project is headed, Adam Sanderson, the creator, encourages forking and feedback.

Comments

  1. Magnus Holn says:

    SexpPath is really sweet! One of the coolest DSLs out there, and definitely worth checking out if you're going to process S-expressions.

  2. Stephen Stillwater says:

    Does anyone have a good link explaining what this is most useful for?

    I've heard a lot of about Sexps from Lisp folk, but I haven't had a moment of grokking yet.

  3. Aslak Helles√ły says:

    ParseTree will probably never work on Ruby 1.9 (http://blog.zenspider.com/2009/04/parsetree-eol.html) so I find it a little puzzling that new projects based on ParseTree keep popping up.

    Another sign that some people don't want to move to 1.9?

  4. Magnus Holn says:

    Aslak: RubyParser also speak Sexps and runs nicely on 1.9 :-)

  5. Adam Sanderson says:

    Stephan: I have no idea what it's good for yet, but it's still pretty neat. I intend on using it to scan ruby code and using it for some interesting meta programming.

    Aslak: Magnus is right, you can use RubyParser on 1.9, the dependency here is on SexpProessor, though I do use ParseTree for a few examples.

  6. roger says:

    There are a few related libs:

    http://github.com/coatl/rubymacros/tree/master
    http://github.com/raganwald/rewrite_rails/tree/master
    http://allgems.ruby-forum.com/gems?search=macro

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