The Buckblog

Mind explaining in a couple of quick examples, why you don’t need DI in ruby?

Anko

More, please . . . would be nice to have some concrete code examples that show how di tripped you up and how you (un)refactored your code.

I’d like some more details too. I’m new to Ruby and actually just learned the DI pattern and thought it was a good idea ;-)

The example I was shown was when a class needs a “logger” object. That you’d use DI to “inject” a logger object into the class from the larger environment, rather than having the object know how the create/configure the logger.

How would you do it without DI?

Thanks.

Take a look at the code in here: http://s3.amazonaws.com/harrisj-share/oscon2007.pdf

One thing it explains is how Rails Observers work, which should be architecturally the same as Loggers. It’s actually amazingly simple and beautiful

Good ol’ metaprogramming. We need a book on the subject – or is there already one? Metaprograming ‘techniques’ I’ve learnt over the last 12 odd months is from tinkering with esoteric Ruby… I’ve tried to write a talk on it and found that there is really no concise way of teaching it without it coming across as a bunch weird ‘hacks’ that allow you to write really, really nice client code. Cheers for the code examples and explanation.

I would like to see Net::Rsync. There is a python version already.

Thanks for the follow-up, Jamis. That makes a lot of sense.

Does anyone sort of freak out at the notion of redefining methods in arbitrary locations? Doesn’t that hint of “action at a distance” and raise the specter of horrible-to-find bugs?

And as an old-time Perl programmer, is this any different than how you redefine methods in Perl? Seems equivalent.

Thanks.

i see that you indent some of the modules differently, so that they aren’t that deeply nested

module Net; module SSH; module Transport; module HMAC

but have you thought about putting end’s at the end of the terminating block. here is an example from capistrano:

    def load(*args, &block)
      options = args.last.is_a?(Hash) ? args.pop : {}
      args.each { |arg| load options.merge(:file => arg) }
      return unless args.empty?

      if block
        raise "loading a block requires 0 parameters" unless args.empty?
        load(options.merge(:proc => block))

      elsif options[:file]
        file = options[:file]
        unless file[0] == ?/
          load_paths.each do |path|
            if File.file?(File.join(path, file))
              file = File.join(path, file)
              break
            elsif File.file?(File.join(path, file) + ".rb")
              file = File.join(path, file + ".rb")
              break
            end
          end
        end

        load :string => File.read(file), :name => options[:name] || file

      elsif options[:string]
        instance_eval(options[:string], options[:name] || "<eval>")

      elsif options[:proc]
        instance_eval(&options[:proc])

      else
        raise ArgumentError, "don't know how to load #{options.inspect}" 
      end
    end

    def load(*args, &block)
      options = args.last.is_a?(Hash) ? args.pop : {}
      args.each { |arg| load options.merge(:file => arg) }
      return unless args.empty?

      if block
        raise "loading a block requires 0 parameters" unless args.empty?
        load(options.merge(:proc => block))

      elsif options[:file]
        file = options[:file]
        unless file[0] == ?/
          load_paths.each do |path|
            if File.file?(File.join(path, file))
              file = File.join(path, file)
              break
            elsif File.file?(File.join(path, file) + ".rb")
              file = File.join(path, file + ".rb")
              break end end end

        load :string => File.read(file), :name => options[:name] || file

      elsif options[:string]
        instance_eval(options[:string], options[:name] || "<eval>")

      elsif options[:proc]
        instance_eval(&options[:proc])

      else
        raise ArgumentError, "don't know how to load #{options.inspect}" end end

what do you think? i feel it much more readable.

woohoo Net::SCP happy days, many thanks :)

Nice article, mainly as it’s good to see that a programming wizard such as your good self, needs to backtrack from time to time like the rest of us mortals :) P

@Jamis thanks you for your comment. just wanted a long-time Ruby user’s opinion.

So i’m pretty new to ruby but i am in need of this scp script, how would you ‘install’ it to make it usable? For instance, i have scp.rb and download.rb and upload.rb in the same folder as my main.rb. in my main.rb i have require ‘scp’ and in my scp.rb i have require ‘download’ and require ‘upload’ but i still get an error.

Any help? Thanks!

Excellent. I’m glad to see that the bloat caused by Java is finally being stripped back.

There is absolutely nothing wrong with being specific when you are using a dynamic language.

I thought in Ruby that the ‘initialize’ method was the factory for the class – since you can decide to call ‘new’ to create a new object, or ‘initialize’ directly to reinitialize an existing one.

I’ve been thinking similar thoughts about dependency injection frameworks in Ruby. They look good on paper but are overkill with this language.

Here’s a Ruby code snippet (with unit tests!) for enhancing attr_reader and attr_accessor. The upshot is a “just enough” solution for DI: http://snippets.dzone.com/posts/show/4382 .

If you have a global logger why would you want to have to reopen and declare a method for each class that needed access to a logger?

To simplify things you would most likely use modules as mixins which would achieve the same effect as using a declarative style DI approach (much like http://injection.rubyforge.org uses) where you say “inject :logger” or “constructor :logger”, but even then you are still applying the same technique to your code just using a different mechanism.

Going the non-DI route do you see it making your tests harder to read or potentially more unmanageable or error prone? going non-DI you would end up with either subclassing in your tests, overriding methods on your actual object or partial mocking.

If I am testing an instance of MyClass it seems to be a cleaner approach to not have to touch it’s implementation in my test. And partial mocking seems to be something easy to abuse as you generally don’t want to mock out the object you’re testing.

Thoughts?

Does anyone have any clue what could be causing this error? /var/lib/gems/1.8/gems/net-ssh-1.1.2/lib/net/ssh/transport/session.rb:274:in `wait_for_message’: disconnected: Protocol error: expected packet type 30, got 34 (2) (Net::SSH::Transport::Disconnect)

Im trying to log into a Mikrotik router and without fail it gives me this error. Ive looked everywhere and cant find a thing about this error. Any ideas? Please e-mail

Calvin

Hello Jamis, That was the fix, Net::SSH.start(host, user, pass, :kex => ‘diffie-hellman-group1-sha1’) Logged right in! Thanks!