Making object methods available externally

Hi all,

I'm having trouble figuring out a way of creating a method within a
class that's available not only to the other object methods (object
instances), but also externally, as if I were to use 'self.method'.
Below is an (admittedly, lame) example of what I'm referring to:

#############################

class CheesePhrase
  def initialize(phrase)
    @phrase = phrase
    say_it
  end

  def say_it(phrase=@phrase)
    puts "#{phrase}"
  end
end

#############################

The above code works fine if I create an object instance, and (if I
choose to do so) call the method 'say_it', like so:

some_cheese_phrase = CheesePhrase.new("I like cheese.")
some_cheese_phrase.say_it

However, what if I'd also like to call the method directly, without
creating a new instance? For example, simply with:

CheesePhrase.say_it("I like cheese, too.")

I can't do this without using 'self.say_it' in the method declaration.
Though, if I do that, I am no longer able to call the method in
'initialize' since 'self.say_it' is now a class method. I get the
following error:

NoMethodError: undefined method `say_it' for #<CheesePhrase:0x284372f4
@phrase="I like cheese">
        from (irb):4:in `initialize'
        from (irb):11:in `new'

I'm sure I must be doing something wrong, or my concept of how to
implement this is totally wrong. The only other thought I've had is to
create duplicate methods, but this seems to violate the famed DRY
principle. Any advice would be much appreciated.

Thanks in advance!

-David
-- 
Posted via  http://www.**--****.com/ 

class CheesePhrase
   def initialize(phrase)
     @phrase = phrase
     say_it
   end

   def self.say_it(phrase)
     puts phrase
   end
   def say_it
     self.class.say_it(@phrase)
   end
end

irb> some_cheese_phrase = CheesePhrase.new("I like cheese.")
I like cheese.
=> #<CheesePhrase:0x629d0 @phrase="I like cheese.">
irb> some_cheese_phrase.say_it
I like cheese.
=> nil
irb> CheesePhrase.say_it("I like cheese, too.")
I like cheese, too.
=> nil

-Rob

Rob Biedenharn		 http://www.**--****.com/ 
 XXXX@XXXXX.COM 

Thanks a lot, Rob. Is this really the standard way of implementing 
something like this? It just seems a little redundant, no?

Thanks!

-David
-- 
Posted via  http://www.**--****.com/ 

Hi David, 
i don't understand well what you need. Is this that you are looking for:
class CheesePhrase 
  def initialize(phrase) 
    @phrase = phrase 
    CheesePhrase.say_it(phrase) 
  end
  def CheesePhrase.say_it(phrase=@phrase) 
    puts "#{phrase}"
  end 
end 

a = CheesePhrase.new('I like cheese')
a = CheesePhrase.say_it('I like too much cheese);

On Thu, 5 Feb 2009 03:37:30 +0900, David Stanford < XXXX@XXXXX.COM >

When you need it, yes. However, it isn't typical that you'd do  
something like this.  Either the method belongs on the instance or it  
belongs on the class itself.  Occasionally, for convenience, you want  
a class method to be called on an instance and then you'd likely do it  
this way. Your example is simple (and, being an example, somewhat  
contrived), but illustrates a point.  You can call #say_it with no  
parameter because the instance variable is there so an instance  
*knows* what to say, but to call .say_it on the class, you need to  
pass the parameter.

-Rob

Rob Biedenharn		 http://www.**--****.com/ 
 XXXX@XXXXX.COM 

[Note:  parts of this message were removed to make it a legal post.]

On Wed, Feb 4, 2009 at 3:06 PM, Rob Biedenharn




Ruby does have a method Module#module_function which can be used in cases
similar to this. For a rather silly example:

module Test
  def foo
    puts "Foo"
  end

  module_function :foo
end

Test.foo

begin
  foo
rescue Exception => ex
  puts "oops! #{ex}"
end

self.extend Test
foo

when run produces:

Foo
oops! undefined local variable or method `foo' for main:Object
Foo

What module_function does is to copy one or more instance methods and make
them singleton methods of the module.  It only works for modules not
classes.

The canonical use case is in the Math module so that you can either write:

Math.sin(.2)

or

class X
   include Math

   def compute(val)
        2 + sin(val)
   end
end
-- 
Rick DeNatale

Blog:  http://www.**--****.com/ 
Twitter:  http://www.**--****.com/ 

Put "self." in front of the method name in it's definition. This is  
called a class method. If you need both, have both.

Blog:  http://www.**--****.com/ 
Learn rails:  http://www.**--****.com/ 

Thanks for all the responses, guys.

Judging from everyone's response, it sounds like what I'm trying to do 
isn't typically done. My mindset was, first, to simply create a standard 
class. While creating one of the object methods for that class, I 
realized that the same method code would be useful to call directly, 
without having to create an object instance; so, as I said, I was 
thinking there must be some way to do this, without repeating the same 
(or a similar piece) of code somewhere else.

So, thanks again!
-- 
Posted via  http://www.**--****.com/ 

To make a minor point, I see this use of module_function often, even in 
the standard library.  I don't like it.

In addition to the module_function method, there's the module_function 
scoping state, like public or private or protected, which is likewise 
enabled by calling module_function without arguments.  Therefore we can 
make a D.R.Y. equivalent:

module Test
  module_function

  def foo
    puts "Foo"
  end
end
-- 
Posted via  http://www.**--****.com/ 

2009/2/5 David Stanford < XXXX@XXXXX.COM >:


Agreed.


In that case I would have the method definition *only* at class level
since apparently it is independent of instance state.  I wouldn't even
have it as instance method at all.  That avoids confusion.  If you
need to invoke the method from instance methods the
self.class.a_method() idiom works well IMHO.

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end

you could do something like
module X
 def shared_method
 end
end
class Y
 include X
 extend X
end
?
-=r
-- 
Posted via  http://www.**--****.com/ 

Probably s bit extreme for one method, but say you had 10 you wanted
to do this with:

module Foo
  def self.included(base)
     base.extend(self)
  end

  def method1; end
  def method2; end
  #...
end

class A
  include Foo # no need for manual extend call
end

Not tested or necessarily recommended. :)

-greg

-- 
Technical Blaag at:  http://www.**--****.com/ 
 Non-tech stuff at:  http://www.**--****.com/ 
"Ruby Best Practices"  Book now in O'Reilly Roughcuts:
 http://www.**--****.com/ 

On Fri, Feb 6, 2009 at 6:58 AM, Robert Klemme




I think that depends on why the method is useful at both places; one
thing I've seen occasionally is the use case where it is useful at the
class level because there are lots of times you'd like to create an
instance, call the method, and then be done with the instance, so it
makes sense to have something like:

class Foo
  def initialize(init_arg1, ..., init_argn)
    # setup
  end

  def action(action_arg1, ..., action_argn)
    # do the action
  end

  def self.action(init_arg1, ..., init_argn, action_arg1, ..., action_argn)
    new(init_arg1, ..., init_argn).action(action_arg1, ..., action_argn)
  end
end

If that's the reason, using the CheesePhrase example:

class CheesePhrase
  # all the stuff in the original example
  def self.say_it(phrase)
    new(phrase).say_it
  end
end

* David Stanford < XXXX@XXXXX.COM > (2009-02-04) schrieb:







Where do you see the redundancy? Nothing is repeated.

mfg,                   simon .... l