No Tags

Nate Daiger and Jason Sydes also talked about Foreman, the job scheduling framework they’ve been working on. Video of their talk will be coming a bit later this week.

No Tags

I’ve attached the slides I used, as well as the sample code we discussed. I don’t know how useful they’ll be on their own, but here they are.

You’ll need OpenOffice to read the slides. The interesting bit of the code is in the lib directory and the models, along with a couple of unit tests to demonstrate useage.

Meta-Programming Slides

Sample Codemeetings, Ruby

meetings, spiders, videos

Some ideas for future meetings included:

• Meta-Programming
• MIT Lectures (Structure and Interpretation of Computer Programs
• Fundamental OO
• Thinking in Terms of Lists and Collections
• Intro Ruby
• Intro Rails
• Rails Tips
• Higher Order Messaging
• DSLs
• Writing Rails Plugins
• Creating Gems
• BackupGem

We’ve planned our next meeting for May 15th, 6:30pm, same location. The topic of discussion will be “Rails Meta-Programming”. Steve and Jacob will be giving the main presentation but everyone is welcome and encouraged to bring their own ideas.

Stay tuned for a video of Matt’s Spider presentation.meetings, spiders

We will be meeting on Tuesday April 17th at 6:30pm. The location is posted on the mailing list.

Thanks goes out to Steve Holder for finding a place for us to hold this meeting!

Currently there isn’t much of an agenda. As this is our first meeting it is primarily a “get to know” time. However, if someone has something they want to present that would be great! (If you’re interested, be thinking about something you could share in case we have the opportunity)

Hope to see you guys there!

-Nateannouncements, meetings

Just recently I needed to store a rational number in a database. YAML is perfect for this sort of thing. Unfortunately there isn’t a built in to_yaml for the standard Rational class.

Notice that rat gets emitted as a vanilla ruby object with class Rational but then the emitter just converts rat into a string and we get "4/3" appended to the YAML output. Because the YAML parser doesn’t know what do to with the string "4/3" we get back a Rational object but it doesn’t have its numerator or denominator set. We want back to be set to Rational(4, 3), just like the original object.

Register Your Class

What we need to do is register our Rational class with YAML so that it knows how to emit and parse our specific type of object.

We can specify our yaml_type by defining a method to_yaml_type. We then register with YAML by calling YAML::add_domain_type and passing it a block. The YAML parser will then call this block when it tries to emit an object of this matching type.

Notice below that YAML::add_domain_type yields two variables type and val. type is the YAML type we specified with to_yaml_type and the val is the value that was stored during the YAML creation process.

Simple Parsing

Notice here that YAML.load returned the value returned by the block we passed add_domain_type. In this case it is val("4/3"). We are a little closer to our goal, but back is still not a Rational, it’s a String. What we need to do is improve on the block we are passing to add_domain_type.

We are getting the string "4/3" in val so we can derive the numerator and denominator from that string and then return a Rational number from that string.

Notice that back is Rational(4, 3), just as we originally wanted.

However this method is not as perfect as it could be. In this case we are able to derive the attributes we need pretty easily, but what if we had a more complicated object that did not store all of its attributes if you call #to_s on the object? What we need is more control over the YAML creation process. Thankfully that power is available by creating our own #to_yaml method.

Advanced Emitting

If you look at the #to_yaml method below you will see that we are iterating through the instance_variables and setting the key to be the instance variable name and the value is the instance variable value.

Then when we need to create the Rational number from that we just grab the hash keys from val.

Conclusion

As you can see YAML is a very powerful way to get complex objects into strings. There are a few other shortcuts to get custom objects into YAML such as defining #to_yaml_properties. If you are interested in doing something simple, I’d start by looking here.

• _why’s comment on the mailing list
• yaml4r documentation

Articles, Ruby, Yaml

encapsulate the execution context at some particular place in the code and retain this context for future use.

You can get a Binding for the current context by calling Kernel#binding.

The Binding stores information about the variables, methods, and self and you can access them by passing the Binding to eval.

You can see here that @title gets evaluated differently depending on the binding. The first eval returns "nice and shiny" because that is the value of @title for the first Product p.

Blocks and Procs

Blocks carry information about their Binding.

Notice here that a is "inside a" and not "resetting a". This is beacuse a block stores the variables as they were originally defined. The a in try_to_set_a does not interfere with the a in a_block.

An interesting note is that you can redefine variable within a Binding.

This is because the Binding in this case is the top-level binding which happens to be the same binding in which a was defined in originally.

Practical Use

Bindings are often used when evaluating ERB. (For those of you who don’t know, ERB is a template system that is included in the Ruby Standard Library.)

ERB#result takes a Binding object as its argument and the variables in the ERB template are evaluated in this context.

Going back to our Product example from earlier, lets see how we can use the Product’s bindings in this fashon:

Conclusion

As you can see Binding is a very handy object but this article serves as only an introduction to the subject. Here are a couple articles that deal with binding a little more in-depth.

Jim Weirich’s Variable Bindings in Ruby

Pick Axe page on BindingArticles, Bindings, Ruby

Exec

Kernel#exec (or simply exec) replaces the current process by running the given command For example:

  $ irb
  >> exec 'echo "hello $HOSTNAME"'
  hello nate.local
  $

Notice how exec replaces the irb process is with the echo command which then exits. Because the Ruby effectively ends this method has only limited use. The major drawback is that you have no knowledge of the success or failure of the command from your Ruby script.

System

The system command operates similarly but the system command runs in a subshell instead of replacing the current process. system gives us a little more information than exec in that it returns true if the command ran successfully and false otherwise.

  $ irb             
  >> system 'echo "hello $HOSTNAME"'
  hello nate.local
  => true
  >> system 'false' 
  => false
  >> puts $?
  256
  => nil
  >> 

system sets the global variable $? to the exit status of the process. Notice that we have the exit status of the false command (which always exits with a non-zero code). Checking the exit code gives us the opportunity to raise an exception or retry our command.

System is great if all we want to know is “Was my command successful or not?” However, often times we want to capture the output of the command and then use that value in our program.

Backticks (`)

Backticks (also called “backquotes”) runs the command in a subshell and returns the standard output from that command.

  $ irb
  >> today = `date`
  => "Mon Mar 12 18:15:35 PDT 2007n" 
  >> $?
  => #<Process::Status: pid=25827,exited(0)>
  >> $?.to_i
  => 0

This is probably the most commonly used and widely known method to run commands in a subshell. As you can see, this is very useful in that it returns the output of the command and then we can use it like any other string.

Notice that $? is not simply an integer of the return status but actually a Process::Status object. We have not only the exit status but also the process id. Process::Status#to_i gives us the exit status as an integer (and #to_s gives us the exit status as a string).

One consequence of using backticks is that we only get the standard output (stdout) of this command but we do not get the standard error (stderr). In this example we run a Perl script which outputs a string to stderr.

  $ irb
  >> warning = `perl -e "warn 'dust in the wind'"`
  dust in the wind at -e line 1.
  => "" 
  >> puts warning

  => nil

Notice that the variable warning doesn’t get set! When we warn in Perl this is output on stderr which is not captured by backticks.

IO#popen

IO#popen is another way to run a command in a subprocess. popen gives you a bit more control in that the subprocess standard input and standard output are both connected to the IO object.

  $ irb
  >> IO.popen("date") { |f| puts f.gets }
  Mon Mar 12 18:58:56 PDT 2007
  => nil

While IO#popen is nice, I typically use Open3#popen3 when I need this level of granularity.

Open3#popen3

The Ruby standard library includes the class Open3. It’s easy to use and returns stdin, stdout and stderr. In this example, lets use the interactive command dc. dc is reverse-polish calculator that reads from stdin. In this example we will push two numbers and an operator onto the stack. Then we use p to print out the result of the operator operating on the two numbers. Below we push on 5, 10 and + and get a response of 15\n to stdout.

  $ irb
  >> stdin, stdout, stderr = Open3.popen3('dc') 
  => [#<IO:0x6e5474>, #<IO:0x6e5438>, #<IO:0x6e53d4>]
  >> stdin.puts(5)
  => nil
  >> stdin.puts(10)
  => nil
  >> stdin.puts("+")
  => nil
  >> stdin.puts("p")
  => nil
  >> stdout.gets
  => "15n" 

Notice that with this command we not only read the output of the command but we also write to the stdin of the command. This allows us a great deal of flexibility in that we can interact with the command if needed.

popen3 will also give us the stderr if we need it.

  # (irb continued...)
  >> stdin.puts("asdfasdfasdfasdf")
  => nil
  >> stderr.gets
  => "dc: stack emptyn" 

However, there is a shortcoming with popen3 in ruby 1.8.5 in that it doesn’t return the proper exit status in $?.

  $ irb
  >> require "open3" 
  => true
  >> stdin, stdout, stderr = Open3.popen3('false')
  => [#<IO:0x6f39c0>, #<IO:0x6f3984>, #<IO:0x6f3920>]
  >> $?
  => #<Process::Status: pid=26285,exited(0)>
  >> $?.to_i
  => 0

0? false is supposed to return a non-zero exit status! It is this shortcoming that brings us to Open4.

Open4#popen4

Open4#popen4 is a Ruby Gem put together by Ara Howard. It operates similarly to open3 except that we can get the exit status from the program. popen4 returns a process id for the subshell and we can get the exit status from that waiting on that process. (You will need to do a gem instal open4 to use this.)

  $ irb
  >> require "open4" 
  => true
  >> pid, stdin, stdout, stderr = Open4::popen4 "false" 
  => [26327, #<IO:0x6dff24>, #<IO:0x6dfee8>, #<IO:0x6dfe84>]
  >> $?
  => nil
  >> pid
  => 26327
  >> ignored, status = Process::waitpid2 pid
  => [26327, #<Process::Status: pid=26327,exited(1)>]
  >> status.to_i
  => 256

A nice feature is that you can call popen4 as a block and it will automatically wait for the return status.

  $ irb
  >> require "open4" 
  => true
  >> status = Open4::popen4("false") do |pid, stdin, stdout, stderr|
  ?>            puts "PID #{pid}" 
  >>          end
  PID 26598
  => #<Process::Status: pid=26598,exited(1)>
  >> puts status
  256
  => nil

Please send comments and revision suggestions to Nate Murray

$Id$ Tue Mar 13 07:45:42 PDT 2007

Articles, Shell

def test_private_method
  product = products(:first) # grab our fixture
  product.class.class_eval do
    public :some_private_method
  end

  assert product.some_private_method
end

Beginner, Ruby, Snippets

The folders:

       content/policy
       content/policy/privacy_policy.txt
       content/policy/about_us.txt
       content/policy/mean_policy
       content/policy/mean_policy/nice_people.txt
       content/policy/mean_policy/mean_people.txt
       content/index.txt
       content/content
       content/content/misc

Creates the structure:

      {"content"=>
         [{"content"=>[{"misc"=>[]}]},
           "index.txt",
          {"policy"=>
            ["about_us.txt",
            {"mean_policy"=>["mean_people.txt", "nice_people.txt"]},
             "privacy_policy.txt"]}]}

The recursive code snippet is posted below:

    def content_files_in_dir(dir, results = {}, opts = {})
      return nil unless File.exist?(dir)
      entries = Dir.entries(dir).delete_if { |f| f =~ /^./ }

      key = File.basename(dir)
      values = []

      entries.each do |entry|
        full_entry = File.join(dir, entry)
        values << ( File.directory?(full_entry) ?
          content_files_in_dir(full_entry, results, opts) :
          entry )
      end
      { key => values }
    end

Recursion, Ruby, Snippets

[nathan@nate ~]$ irb
>> def foo(arg1, arg2 = [arg1])
>>   puts arg1.inspect
>>   puts arg2.inspect
>> end
=> nil
>> foo 3
3
[3]
=> nil

Interesting, Ruby, Snippets

Example 1

For our breadcrumbs the first thing one might think to do is create a helper to perform this function.

For those of you who are new to Rails, helpers are often designed to return a string and this string is added to the output.

Below is our first attempt at a helper for this problem. This code returns a string with our anchor tags separated by an image.

== helpers/sites_helper.rb

  def make_breadcrumbs(breadcrumbs)
    crumbs = []  
    spacer = "<img src=’/images/separator.gif’ width=5 height=5
        border=0>"
    breadcrumbs.each do |crumb|
      crumbs <<  "<a href=’#{url_for_category(crumb)}‘>
       #{crumb.name}</a>"
    end
    return crumbs.join(spacer)
  end

We can call this helper easily from our view:

== views/sites/_breadcrumbs.rhtml

 <%= make_breadcrumbs(@breadcrumbs) %>

This works just fine, but there are a couple consequences:

• its very specific to our problem.
• All the HTML is in the code instead of in our views.

Example 2

“But wait a minute,” you ask, “Doesn’t Rails come with built-in collection-rendering methods?” You’re right! In fact we can use Rails render method to achieve the same effect.

== views/sites/_breadcrumbs.rhtml

 <%= render :partial => "sites/breadcrumb", 
               :collection => @breadcrumbs,
               :spacer_template => ‘sites/breadcrumb_spacer‘ %>

In our view we call the render method and we pace the options :collection and :spacer_template. This renders the partial sites/breadcrumb and creates the local variable breadcrumb with each element of @breadcrumbs.

The two templates are below:

== views/sites/_breadcrumb.rhtml
<a href=‘<%= url_for_category(breadcrumb)%>‘><%=breadcrumb.name%></a>

== views/sites/_breadcrumb_spacer.rhtml
<img src=‘/images/separator.gif‘ width=5 height=5 border=0>

A benefit of this is that all of our HTML is in our views.However, a consequence that we have to create two new partials that contain only one line each.

What we would really want is a way to keep all of our html in our view while not having to create any extra templates.

Example 3

Here is an example of the syntax we want:

== views/sites/_breadcrumbs.rhtml

 <% spacer = capture do %>
   <img src="/images/separator.gif" width=5 height=5 border=0>
 <% end %>

 <% with_collection @breadcrumbs, :spacer_template => spacer do |crumb| %> 
   <a href=‘<%= url_for_category(crumb) %>‘><%= crumb.name %></a>
 <% end %>

The Rails helper capture takes the block and puts it in the variable spacer.

Then we put this in helpers/sites_helper.rb

== helpers/sites_helper.rb

  def with_collection(collection, opts={}, &proc)
    collection.each_with_index do |element, i|
      yield element
      if spacer = opts[:spacer_template]
        concat(spacer, proc.binding) unless i == collection.size - 1
      end
    end
  end

This helper takes the collection and yields each element. As each element is yielded the block in the view is added to the output. The trick is that we add the spacer to the output by using the concat method. The concat method outputs the string to the view from the helper. This is how we can output something to the template without using <%= (This is also what the helpers such as form_for use).

Articles, Helpers, Rails, Ruby

Gang of Four outlines a pattern for modifying an object depending on its state. This is called the State pattern. I’m not going to go into all the details here, so if you are unfamiliar with the State pattern I’d recommend looking here and here.

This post addresses how to implement the typical State pattern in Ruby and explores its consequences and alternatives.

Problem: An object’s behavior needs to be modified depending on what state it is in.

Solution: Create a State object and delegate the functionality to that State object.

Traditional State Pattern

Gang of Four outlines a pattern for modifying an object depending on its state. This is typically done be creating an Abstract State and then subclassing it to create a Concrete State. The originating object, referred to as the Context then delegates the specific method to an instance of the Concrete State along with any information needed.

For example, say we have a Product and we want the inventory levels to vary depending on where we are selling the Product. While the actual inventory we have doesn’t change, we may want to tell, say, eBay or Amazon, that the inventory is lower than what we actually have to prevent overselling.

Figure 1. is a traditional implementation of this.

A simple implementation would delegate the method to the state 100% of the time. However, in our system, assume that we only want to use the State if it exists and if it contains the method we are interested in. This allows us to have default behaviors in our Product object. This may not always be the case, but in our example it is.

In Ruby, this looks something like the following:

class Product
  attr_accessor :state

  def inventory
    if state && state.respond_to?(:inventory)
      return state.send(:inventory, self)
    else
      return @inventory
    end
  end

end # end Product

class AmazonProductState < ProductState

  # take the inventory from the Context object (the Product) and divide it by 2
  def inventory(context)
    context.inventory / 2
  end

end

Consequences

• A consequence of this approach is that you have to write the lines if state && state.respond_to?(:inventory) return state.send(:inventory, self) every time. Ideally we wouldn’t have to write this over and over for every method we want to delegate.

A slight improvement

What we could do is write a method that writes the delegation code for the method for us.

def define_state_method(name, &block)
  # … 
end

# Then just call that method fo r
define_state_method :quantity_on_hand do
  return @inventory
end # writes #1 in effect

Consequences

• Eliminates the repetition of #1
• However, you still have to plan ahead to make this method be delegated to the State object.

Ideally, what we want to do is have any method overwritten for a particular instance when the state is set. We want to keep the same class, and we don’t want to change the methods of other instances of that class.

Just extend it

We can achive the affect we are looking for by simply extending the class on our Product object. See below:

class AmazonProductState < ProductState
  def inventory
    @inventory / 2
  end

  def new_method
    "7 llamas"
  end
end

class Product
  …
  def set_state(klass)
    self.instance_eval do
      extend klass
    end
  end
  …
end
>> p = Product.find(1) # => #<Product:@id=1…>
   q = Product.find(1) # => #<Product:@id=1…>

   q.inventory # => 10

   p.set_state(AmazonProductState) # => nil
   p.inventory # => 5

   p.respond_to(:new_method) # => true
   q.respond_to(:new_method) # => false

Consequences

• Allows us to override any method without having to plan ahead when designing the Product object
• Allows us to add new methods that only exist in the state
• The new state does not change the class of the object
• The new state only affects particular instances of the object, not the whole class

Articles, Design Patterns, Gof, Mixins, Ruby

I have created a small presentation that tries to boil down these powerful concepts and converted the Lisp code to Ruby. The copy is taken almost directly from SICP with a few paraphrases and additional examples added.

You can view the presentation with notes at Slide-Share here. Or you can download the presentation as:

• PDF (preferred)
• syntax-highlighted text
• power-point-generated html(very ugly)

Articles, Lambda, Ruby

s = "hello world"
a = ["foo", "bar", s]

other_s = s            # not a copy!
other_s.upcase!

puts a[2]              # => "HELLO WORLD"

Notice that the contents of a were changed. This is because the variable other_s is simply a reference to the same object that is in a. It’s such a simple thing to overlook when things are more complicated. It’s subtle and that can make it difficult to track down.

The lesson? When you edit an object in place make sure that is what you want!

Ruby

One of the most annoying things was that there seemed to be a catchpoint on every single exception that was raised. I had to keep hitting c just to get to my first actual breakpoint.

I found the solution to this problem over at comp.lang.ruby. Basically, there is a default catchpoint on StandardError so you are prompted at all raise exceptions. What you need to do to fix this is just set a catchpoint for your own type of error. So when you start up simply:

  cat MyOtherError

It will remove the default catchpoint and catch only on MyOtherError.

Another helpful Ruby Debugger tip is to have the debugger load a debug.rc file on startup. Matthias Georgi posted this hack on comp.lang.ruby that could be helpful.

A short hack.
Make a copy of debug.rb and put these lines right after the Context
class.

class Context
  alias original_readline readline
  def readline(prompt, hist)
    @rc_file ||= File.readlines("debug.rc")
    if @rc_file.empty?
      original_readline(prompt, hist)
    else
      @rc_file.shift
    end
  end
end

Now create a file debug.rc with your desired breakpoint:
b 100
b 200

Start the debugger:
ruby -r./debug myscript.rb 

Of course, this isn’t a totally clean solution in that you are editing a standard file. Also, I don’t think this code would work if the debug.rc file did not exist. Nonetheless, its the easiest method I’ve seen so far.

Ruby

BackupGem aims to be the easiest and most flexible backup, archive and rotate tool. It’s a beginning-to-end solution for scheduled backups in a clean ruby package that is simple use and powerful when customized.

A configuration as simple as:

  set :backup_path, "/var/local/backups/my_old_logs"
  action :content,  :is_contents_of => "/var/my_logs"

Will compress, encrypt, deliver, and rotate the contents of /var/my_logs. But this is just a taste of the power this gem has.

If you are interested the gem is available via:

  gem install backupgem

Or view the manual at: http://tech.natemurray.com/backup/

RubyForge Project page: http://rubyforge.org/projects/backupgem/

Backupgem, Ruby, Scripts