# ripper-plus

Ripper is the Ruby parser library packaged with Ruby 1.9. While quite complete, it has some quirks that can make use frustrating. This gem intends to correct them. The most important is bareword resolution in Ruby ASTs.

## Bareword Resolution

In Ruby, a local variable is created when it is assigned to. If Ruby finds a bareword `foo` before an assignment `foo = 5`, then Ruby will call `self.foo()`.

Kind of.

What happens when you run this code?

    def label
      'hello'
    end
    def print_label
      label = 'Label: ' + label
      puts label
    end
    print_label

If one takes the intuitive approach, one would assume it prints "Label: hello", right? Afraid not: it errors out. The intuitive approach says that `'Label: ' + label` is run before the assignment to the local variable `label`, so the `label` method will be called and appended to `"Label: "`. In reality, it won't, because local variables aren't created on assignment. Local variables are created *immediately when the Ruby parser parses a left-hand-side local variable*. Before the right-hand-side is even parsed, the local variable has been created. This also results in an infamous anomaly:

    def say_what?
      x = 'hello' unless defined?(x)
      puts x
    end
    say_what?

`say_what?` prints a blank line! This is because as soon as the `x` on the LHS is parsed, `x` is a local variable with value `nil`. By the time `defined?(x)` is executed, `x` has long since been a local variable!

## Ripper's Mistake

Ripper doesn't carry scope information as it parses, and as such, parses any lone identifier as an AST node of type `:var_ref`. It is up to consumers of the AST to figure out the meaning of the `:var_ref` node. It can be reasonably argued that the semantics of the `:var_ref` node should not be part of the AST, as my thesis adviser pointed out when I complained about this, as the two are syntactically identical. Unfortunately, the meaning of the `var_ref` node comes from the parser itself; any attempt to determine semantics based solely on the AST is simply re-creating the work the parser ordinarily does! Indeed, when Ruby parses the code for execution, it *does* create different internal node types upon seeing a method-call bareword and a local variable bareword!

I'd like to see this behavior rolled into Ripper proper. Until then, `ripper-plus` is a reasonable replacement.

## ripper-plus

By using our knowledge of Ruby's parser, we can recreate the scope-tracking behavior by walking the AST generated by Ripper. We simply must do normal scope tracking, observe the creation of local variables, and ensure that we walk each node of the tree in the order in which those nodes would have been parsed. Most subtrees generated by Ripper are already in this order, with the exception of the modifier nodes (`foo.bar if baz`, `x = bar() while x != nil`). Most importantly, since everything is an expression in Ruby and local variable assignments can occur just about anywhere, exact scoping semantics must be recreated. Corner cases such as this:

    def foo(x, y = y)
    end

Need to be properly resolved. Did you know that, unlike the `label = label` example above, the above code is equivalent to:

    def foo(x, y = y())
    end

Anyway, ripper-plus turns all method-call `:var_ref` nodes into `zcall` nodes; the node structure is otherwise unchanged.

The truth is, everybody who is using Ripper right now *should* be doing *all* of this. Anything short, and you have bugs. [Laser](https://github.com/michaeledgar/laser/) has bugs as a result. It's a pain in the ass to get it all right. So hopefully, in Ruby 1.9.x, this will be the default. For now, you *should* use ripper-plus.

## Contributing to ripper-plus
 
* Check out the latest master to make sure the feature hasn't been implemented or the bug hasn't been fixed yet
* Check out the issue tracker to make sure someone already hasn't requested it and/or contributed it
* Fork the project
* Start a feature/bugfix branch
* Commit and push until you are happy with your contribution
* Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.
* Please try not to mess with the Rakefile, version, or history. If you want to have your own version, or is otherwise necessary, that is fine, but please isolate to its own commit so I can cherry-pick around it.

## Copyright

Copyright (c) 2011 Michael Edgar. See LICENSE.txt for
further details.