SexpPath

Structural pattern matching against S-Expressions.

SexpPath allows you to define patterns that can be matched against S-Expressions. SexpPath draws inspiration from Regular Expressions, XPath, and CSS Selectors.

I’m still figuring out how SexpPath should work so either fork this, or send me some feedback.

  http://github.com/adamsanderson/sexp_path
  netghost@gmail.com

Installation

SexpPath is distributed as a ruby gem:

    gem install adamsanderson-sexp_path

Notation

In ruby you’re most likely to come across S-Expressions when dealing with ParseTree’s representation of the abstract syntax tree. An S-Expression is just a set of nested lists. The SexpProcessor library displays them like this:

    s(:a, :b,
      s(:c)
    )

Where that means that there is a list containing `:a`, `:b`, and then another list which contains `:c`. We will refer to `:a`,`:b`, and `:c` as atoms, while `s( something )` is an S-Expression or Sexp.

General Syntax

SexpPath is an internal ruby DSL, which means a SexpPath query is valid ruby code. SexpPath queries are built with the SexpQueryBuilder through the Q? convenience method:

    Q?{ s(:a, :b, :c)}    # Matches s(:a, :b, :c)

This will match the S-Expression `s(:a, :b, :c)`. If you want to match something more complicated, you will probably want to employ one of the many matchers built into SexpPath.

    _  => s(), :a, or s(:cat)

    atom => :a, :b, or :cat

    m('cat') => :cat
    m(/rat/) => :rat, :brat, or :rate
    m(/^test_/) => :test_sexp_path

    include(:a) => s(:a), s(:a, :b), or s(:cat, :a, :b)
    include( s(:cat) ) => s(:pet, s(:cat))

    child( s(:a) ) => s(:b, s(:a)) or even s(s(s(s(s( s(:a))))))

    s(:a) >> s(:c) => s( s(:a), s(:b), s(:c) )

    type(:a) => s(:a), s(:a, :b), or s(:a, :b, s(:c))

    any( s(:a), s(:b) ) => s(:a) or s(:b)
    any( s(:a), s(atom, :b) ) => s(:a), s(:a, :b), or s(:cat, :b)

    all( s(:a, atom), s(atom, :b) ) => s(:a,:b)

    -s(:a) => s(:a,:b), s(:b), but not s(:a)
    s(is_not :a) => s(:b), s(:c), but not s(:a) or s(:a, :b)

Searching

You may use any SexpPath to search an S-Expression. SexpPath defines the `/` operator as search, so to search `s( s(:a) )` for `s(:a)` you may just do:

    s( s(:a) ) / Q?{ s(:a) }

This will return a collection with just one result which is `s(:a)`. You could also do something more interesting:

    s( s(:a), s(:b) ) / Q?{ s(atom) }

This will return two matches which are `s(:a)` and `s(:b)`. You can also chain searches, so this works just fine as well:

    sexp = s(:class, :Calculator,
      s(:defn, :add),
      s(:defn, :sub)
    )

    sexp / Q?{ s(:class, atom, _) } / Q?{ s(:defn, _) }

In this case you would get back `s(:defn, :add)` and `s(:defn, :sub)`.

Capturing

It is useful to also capture results from your queries. So using the same Sexp from above we could modify our query to actually capture some names. Capturing is done by using `%` operator followed by the name you would like the value to be captured as.

    sexp / Q?{ s(:class, atom % 'class_name', _) } / Q?{ s(:defn, _ % 'method_name') }

The results will now capture `:Calculator` in `class_name`, and then `:add` and `:sub` in `method_name`.

Examples

Here is an example of using SexpPath to grab all the classes and their methods from a file:

    require 'rubygems'
    require 'sexp_path'
    require 'parse_tree'

    path = ARGV.shift
    code = File.read(path)
    sexp = Sexp.from_array(ParseTree.new.parse_tree_for_string(code, path))

    class_query = Q?{ s(:class, atom % 'class_name', _, _) }
    method_query = Q?{ s(:defn, atom % 'method_name', _ ) }

    results = sexp / class_query / method_query

    puts path
    puts "-" * 80

    results.each do |sexp_result|
      class_name = sexp_result['class_name']
      method_name = sexp_result['method_name']
      puts "#{class_name}##{method_name}"
    end

Neat huh? Check the `examples` folder for some more little apps.

Project Information

Hop in and fork it or add some issues over at GitHub:

  http://github.com/adamsanderson/sexp_path

Ideas for Hacking on SexpPath:

  * More examples
  * Add new matchers
  * Connivence matchers, for instance canned matchers for matching ruby classes, methods, etc

I’d love to see what people do with this library, let me know if you find it useful.

    Adam Sanderson, netghost@gmail.com