Typical/idiomatic examples of dynamic code generation with Ruby?

ruby-talk
1

Hi there,

I'm writing a paper on the "rediscovery of dynamic languages".
Definition issues aside (what *is* a "dynamic" language, anyway?), in
it I try to show how "old" (Smalltalk, Lisp) and "new" (Python, Ruby)
programming languages are vastly superior for the cost-effective
development of scalable and secure applications compared with static
languages like C/C++ and half-breeds like Java and C#. Nothing
original for sure, but then I again I'm trying to postulate some
convincing arguments for a non-geek, managerial audience.

In the paper I'm comparing "idiomatic" examples of how these languages
deal with common problems. Problem is, I'm quite new with Ruby and I'm
a bit at a loss for examples on how to show Ruby's strengths with
dynamic code generation (metaprogramming). That is often mentioned as one of Ruby's strengths. Don't get me wrong, I've found that Ruby has got the necesarry nuts and bolts but I couldn't really figure out what I could do with it, what I could not easily do with Python as well.

So my question: do you know of any examples on the subject of dynamic
code generation which would be typical for Ruby and could not be
easily implemented in languages like Python?

Regards,

Iwan

2

Iwan van der Kleyn wrote:

Hi there,

I'm writing a paper on the "rediscovery of dynamic languages".
Definition issues aside (what *is* a "dynamic" language, anyway?), in
it I try to show how "old" (Smalltalk, Lisp) and "new" (Python, Ruby)
programming languages are vastly superior for the cost-effective
development of scalable and secure applications compared with static
languages like C/C++ and half-breeds like Java and C#. Nothing
original for sure, but then I again I'm trying to postulate some
convincing arguments for a non-geek, managerial audience.

In the paper I'm comparing "idiomatic" examples of how these languages
deal with common problems. Problem is, I'm quite new with Ruby and I'm
a bit at a loss for examples on how to show Ruby's strengths with
dynamic code generation (metaprogramming). That is often mentioned as one of Ruby's strengths. Don't get me wrong, I've found that Ruby has got the necesarry nuts and bolts but I couldn't really figure out what I could do with it, what I could not easily do with Python as well.

So my question: do you know of any examples on the subject of dynamic
code generation which would be typical for Ruby and could not be
easily implemented in languages like Python?

I think stuff like "attr_accessor" is typical for Ruby. It could be implemented like this:

   class Module
     def attr_accessor(*attrs)
       attrs.each do |attribute|
         class_eval "def #{ attribute }() @#{ attribute } end"
         class_eval "def #{ attribute }=(val) @#{ attribute } = val end"
       end
     end
   end

   # and use it

   class MyClass
     attr_accessor :a
     def initialize
       @a = "test"
     end
   end

   m = MyClass.new
   p m.a # => "test"
   m.a = 2
   p m.a # => 2

You can do the same for defining abstract methods:

   class Module
     def abstract(*meths)
       meths.each do |meth|
         class_eval "def #{ meth }(*args, &block) raise 'abstract method' end"
       end
     end
   end

And for much more else. Have a look at ActiveRecords, which uses quite a lot of metaprogramming, I guess.

Regards,

   Michael

3

I don't know if this is the kind of thing you're after and I don't know Python, so I have no idea how it compares, but and interesting example did come up in my project today.

I'm building a server. When some event happens in the server, it notifies monitoring code with an Event object. My first crack at that class looked like:

  class Event
    CONNECT = "Connect"
    LOGIN = "Login"
    COMMAND = "Command"
    DISCONNECT = "Disconnect"
    
    def initialize( connection, identity,
            type, details = nil, time = Time.now )
      @connection = connection
      @identity = identity
      
      @type = type
      @details = details
      @time = time
    end
    
    attr_reader :connection, :identity, :type, :details, :time
    
    def connect?() @type == CONNECT end
    def login?() @type == LOGIN end
    def command?() @type == COMMAND end
    def disconnect?() @type == DISCONNECT end
  end

That worked, of course. However, one of my major goals with this server is to keep it very open to change. I know for a fact it will be modified after deployment, so I'm planning ahead.

The problem with the above version comes when I add a new event type. I provide those little helper methods like command?(), to keep you from having to write:

  if event.type == Event::COMMAND
    # ...

  # can be written as...

  if event.command?
    # ...

However, when I decide I need a new event type, I have to add another constant and a new helper method. That goes against DRY philosophy. So, I changed the class to:

  class Event
    CONNECT = "Connect"
    LOGIN = "Login"
    COMMAND = "Command"
    DISCONNECT = "Disconnect"
    
    def initialize( connection, identity,
            type, details = nil, time = Time.now )
      @connection = connection
      @identity = identity
      
      @type = type
      @details = details
      @time = time
    end
    
    attr_reader :connection, :identity, :type, :details, :time
    
    constants.each do |e|
      value = const_get e
      define_method((e.downcase + "?").to_sym) do
        @type == value
      end
    end
  end

Problem solved. Helper methods are now auto-generated from class constants. Add a new constant, get a new helper method.

Hopefully, that's the kind of thing you are looking for. Good luck with your paper.

James Edward Gray II

···

On Nov 21, 2004, at 3:28 PM, Iwan van der Kleyn wrote:

So my question: do you know of any examples on the subject of dynamic
code generation which would be typical for Ruby and could not be
easily implemented in languages like Python?

4

Michael Neumann ha scritto:

In the paper I'm comparing "idiomatic" examples of how these languages
deal with common problems. Problem is, I'm quite new with Ruby and I'm
a bit at a loss for examples on how to show Ruby's strengths with
dynamic code generation (metaprogramming). That is often mentioned as one of Ruby's strengths. Don't get me wrong, I've found that Ruby has got the necesarry nuts and bolts but I couldn't really figure out what I could do with it, what I could not easily do with Python as well.

So my question: do you know of any examples on the subject of dynamic
code generation which would be typical for Ruby and could not be
easily implemented in languages like Python?

I'm not sure samples in ruby would be hard in python and viceversa, but there are different idioms. Maybe ruby has some more runtime hooks like Class#inherited or Module#included. I /think/ that a good example could be the Interface pkg (java style check for interfaces) http://raa.ruby-lang.org/project/interface/

Toghether with the just named keyword-like class methods there is the class generation at runtime idiom, a built-in example is the Struct class, and a really nice one is Test::Unit::Mock, see a sample on this page: Test::Unit::Mock: Mock objects for Test::Unit tests

Another thing is the usual relying on method_missing tricks, for wich I guess the best example is the Criteria library (wich happen to generate sql instead of ruby, so not really metaprogramming I think :slight_smile: look at:
http://mephle.org/Criteria/

It seem that your paper will be very interesting.. I even wonder how static languages handle this situations.

just my 0.02 euro

5

Thanks for the tips. Today I got the Pickaxe (2004 edition) and I found the relevant references. I'll be playing around with it for a while. In the paper I'm comparing SQLObject (Python object-relational mapper, kind like ActiveRecords I guess) which uses lots of metaclass wizzardry. Although you would be able to something similar in Java, it would be far less natural or easy to the enduser (programmer) and it would be absolutely hell to implement. I'm hoping to come with some nice examples.

Look in Google for some nice references when searching for "Java dynamic code generation". The way to do this in Java would be through "regular" code generation though, which would be generated by a special tool like XDoclet at compile time.

> It seem that your paper will be very interesting.. I even wonder how
> static languages handle this situations.

With luck it will be released under an Open Conent license in January.

Thanks again,

Iwan

6

ERb and REXML:

Iwan van der Kleyn <none@none.net> wrote in message news:<41A237AF.8070105@none.net>...

···

Thanks for the tips. Today I got the Pickaxe (2004 edition) and I found
the relevant references. I'll be playing around with it for a while. In
the paper I'm comparing SQLObject (Python object-relational mapper,
kind like ActiveRecords I guess) which uses lots of metaclass wizzardry.
Although you would be able to something similar in Java, it would be far
less natural or easy to the enduser (programmer) and it would be
absolutely hell to implement. I'm hoping to come with some nice examples.

Look in Google for some nice references when searching for "Java dynamic
code generation". The way to do this in Java would be through "regular"
code generation though, which would be generated by a special tool like
XDoclet at compile time.

> It seem that your paper will be very interesting.. I even wonder how
> static languages handle this situations.

With luck it will be released under an Open Conent license in January.

Thanks again,

Iwan