From: Phrogz <phrogz@...>
Date: 2010-01-13T04:25:07+09:00
Subject: instance_eval with local variable

I have an object with an instance variable:
  obj = Object.new
  obj.instance_variable_set :@who, "foo"

I have a proc with arity 1:
  @who= "main"
  holla = lambda{ |str| p @who, str }

I want to evaluate the lambda inside the scope of obj, but also pass
in a local variable. I can't figure out how to do this (or if it's
even possible).

I can't instance_eval the proc in the context of obj, because the proc
has arity:
  obj.instance_eval( &holla )
  #=> ArgumentError: wrong number of arguments (0 for 1)

And just invoking the proc within the context of obj doesn't set the
scope:
  obj.instance_eval{ holla[ "hoorah" ] }
  #=> "main"
  #=> "hoorah"

Any sweet meta trickery you can think of to accomplish what I want?

For those interested in second-guessing my approach and coming up with
a better way to achieve the same end goal, the application is a
documentation system which converts markup (markdown, textile, haml,
etc.) into CHM. I have a system for registering an arbitrary number of
HTML post-processors with the application. The post-processors
associate a regexp with a proc that converts the matched code:

  module DocuBot
    @snippets = {}
    def self.handle_snippet( regexp, &handler )
      @snippets[ regexp ] = handler
    end
    def self.process_snippets( html )
      @snippets.inject(html){ |h,(regexp,handler)| h.gsub( regexp,
&handler ) }
    end
  end

  ...

  # Represents an entire documentation site
  class DocuBot::Bundle
    def write_html
      @pages.each do |page|
        html = DocuBot.process_snippets( page.to_html )
        ...
      end
    end
  end

  ...

  # Mark $$foo$$ as glossary items.
  DocuBot.handle_snippet /\$\$\w[^$]+\$\$/ do |match|
    # TODO: look up the glossary term in the bundle's glossary
    # and insert appropriate text, or flag a new glossary term
    "<span class='glossary'>#{match[2..-3]}</span>"
  end

This works fine, except for the TODO above. I want to be able to
instance_eval that block within the scope of a particular bundle so I
can look at @glossary before returning the correct final HTML for gsub
to use.