From: Gunnar Andersson <dff180@...>
Date: 2001-10-30T07:45:52+09:00
Subject: [ruby-talk:23768] RCR: Fun with attribute shortcuts solves RCR #3 and more

Hi everyone, remember this?

>I'm constantly creating small classes, and many of them start something
>like: 
>  class Msg
>    def initialize(name, type, txt)
>      @name, @type, @txt = name, type, txt
>    end
>    # ...
>  end

Yeah, it's a classic, RCR #3.  We all hate repetition and typing
stuff that is obvious, right?

Last time it was Joe McDonald who said:

> and if you have a sec, vote on RCR #3.  It's a pet peeve of mine. 

I think it's a pet peeve for a lot of us but I can see why 
RCR #3 was rejected as it stands now.
What would it mean for example, if you created another
function in the same way, like def func(@instance_var) - 
is @instance_var assigned also in that case?
If not, why should there be a special case for initialize?

So instead I propose to extend on the much loved shortcut 
methods that exist for accessors.

I.e. attr_accessor :a, :b, :c creates getters and setters for 
@a, @b and @c so why not:

class Mine
    attr_initializer :a, :b, :c
    ...
end

attr_initializer creates an initializer
for them:

def initialize(a, b, c)
  @a, @b, @c = a, b, c
end

So there's the idea - I thought it was quite nice but
now comes the fun part.  I love this - when you get a good idea, 
often it's easy to implement in Ruby without touching any 
interpreter or such!   For example, if you didn't get this yet, 
notice that "attr_accessor" and the like are described in docs as 
being just plain methods, in the class Module!

So in some spare time at RubyConf and OOPSLA I fooled around with this
and got really positive responses, for example at the BOF session
at OOPSLA.
Trying it out for real makes a much better impact.
Using the eval-family of functions and the open classes, for
example "Class" and "Module", you can do a lot of cool stuff
that looks a lot like extentions of the language syntax, so here 
is my implementation of attr_initializer:

# file: AttrInitializer
class Module
   def attr_initializer(*attrs)
      alist = plist = ""
      attrs.each do |v|
         alist += "@#{v},"     # >> @var1,@var2,@var3, ...
         plist += "#{v},"      # >> var1,var2,var3, ...
      end
      alist.chop!; plist.chop! # Remove last comma 
      class_eval "def initialize(#{plist}); #{alist} = #{plist}; end"
   end
end

Since the Class class inherits from Module, now you can just 
require this file and you are able to use the attr_initializer 
shortcut in all classes.


OK, now let's keep it going.  What else is common to define in many
classes?  Well, the default definition of equality is that the 
compared references must be refs to the same object.  However, it is 
more common to want to compare (part of) the inner state of objects 
and if they are equal, then the two different objects should be 
considered equal.

So you end up often defining == in your classes, like this, right?

class Mine
   attr_accessor :foo, :bar, :moo
   # @foo, @bar, and @moo are my
   # important inner state

   def ==(obj)
      @foo = obj.foo && 
      @bar = obj.bar &&
      @moo = obj.moo
   end
end

Nowadays I just do this:

class Mine
   attr_equality :foo, :moo, :bar
   ...
end

Another option is to leave out the names of objects to make
it even simpler.  This requires the equality function to introspect 
self and the compared object to find all their instance vars.

class Mine
   attr_equality   
   ...
end

I won't post the implementation of attr_equality because it's
longish, to support also the second option but you get the
idea from the attr_initializer example.

I'll make an RCR for this and we'll see if people like it.
If you want a full implementation of both shortcuts
with some tests to go with them Drop me an email directly.
Don't reply to the list by mistake...
(Yes, XP-enthusiasts, I wrote the tests first! Great isn't it?
Although I updated them to run with Lapidary later)

Have fun

/Gunnar

---
Gunnar Andersson
gand -at- acm.org


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