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comp.lang.ruby

Once more unto the breach: private class methods

Zeekar

8/23/2007 8:08:00 PM

Found a lot of discussion of this, but it didn't seem to suggest a
solution to a common pattern. The below completely useless example is
modeled after a longer and more useful one in the Agile Web
Development with Rails book (2nd ed):

class Foo
class << self
private
def staticHelperFunction
12345
end
end
public
def initialize
@var = self.class.staticHelperFunction
end
end
Foo.new.var

Now, the above won't work, obviously. If I replace the direct call to
staticHelperFunction with instance_eval or send, it will work, but
that feels hackish. Is there an idiomatic Ruby way to handle this
sort of situation?

In case the goal isn't clear, I want instance (not singleton/class)
methods of a given class to have access to what are essentially helper
functions/subroutines (not other instance methods of the same class)
which are not generally accessible outside of that class.

Doable?
7 Answers

David A. Black

8/23/2007 9:02:00 PM

0

Robert Dober

8/23/2007 9:11:00 PM

0

On 8/23/07, Zeekar <markjreed@gmail.com> wrote:
> Found a lot of discussion of this, but it didn't seem to suggest a
> solution to a common pattern. The below completely useless example is
> modeled after a longer and more useful one in the Agile Web
> Development with Rails book (2nd ed):
>
> class Foo
> class << self
> private
> def staticHelperFunction
> 12345
> end
> end
> public
> def initialize
> @var = self.class.staticHelperFunction
> end
> end
> Foo.new.var
>
> Now, the above won't work, obviously. If I replace the direct call to
> staticHelperFunction with instance_eval or send, it will work, but
> that feels hackish. Is there an idiomatic Ruby way to handle this
> sort of situation?
>
> In case the goal isn't clear, I want instance (not singleton/class)
> methods of a given class to have access to what are essentially helper
> functions/subroutines (not other instance methods of the same class)
> which are not generally accessible outside of that class.
>
> Doable?
Sure, but clumsy, I do not know if it is worth it

class Foo
class << self
private
def x; 60 end
end
attr_reader :x
helper = method(:x)
define_method :initialize do @x = helper.call end
end

puts Foo.new.x

HTH
Robert
>
>
>


--
I'm an atheist and that's it. I believe there's nothing we can know
except that we should be kind to each other and do what we can for
other people.
-- Katharine Hepburn

Zeekar

8/23/2007 9:20:00 PM

0

On Aug 23, 5:02 pm, "David A. Black" <dbl...@rubypal.com> wrote:

> It seems like you're threading a very narrow needle. Do you mean that
> you would not want class methods to see each other?

Not at all. I want to define a method C.foo() that other classes
outside of C can't call directly. Other class methods of C can call
it, fine. But the goal is to have *instances* of C to be able to call
it.

I guess I could make it a private instance method that just happens
not to care about self., but that seems like misleading design.

Zeekar

8/23/2007 9:22:00 PM

0

On Aug 23, 5:11 pm, "Robert Dober" <robert.do...@gmail.com> wrote:

> Sure, but clumsy, I do not know if it is worth it
>
> class Foo
> class << self
> private
> def x; 60 end
> end
> attr_reader :x
> helper = method(:x)
> define_method :initialize do @x = helper.call end
> end

Well, that is an interesting approach. Thanks! It might be in the
same category as using instance_eval or send, but at first blush it
feels somewhat cleaner to me...

ara.t.howard

8/23/2007 9:35:00 PM

0


On Aug 23, 2007, at 2:10 PM, Zeekar wrote:

> In case the goal isn't clear, I want instance (not singleton/class)
> methods of a given class to have access to what are essentially helper
> functions/subroutines (not other instance methods of the same class)
> which are not generally accessible outside of that class.

easy:

cfp:~ > cat a.rb
class C
singleton_class =
class << self
self
end

helper = Object.new.instance_eval do
def foo() 42 end
self
end

[self, singleton_class].each do |c|
c.module_eval{
define_method(:helper){ helper }
private :helper
}
end

def initialize
@foo = helper.foo
end
end

p C.new
C.helper


cfp:~ > ruby a.rb
#<C:0x24b94 @foo=42>
a.rb:25: private method `helper' called for C:Class (NoMethodError)


but in the presence of both instance_eval, eval, and send - why bother?

a @ http://draw...
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama

David A. Black

8/23/2007 9:56:00 PM

0

John Wilger

8/23/2007 10:19:00 PM

0

On Aug 23, 2:20 pm, Zeekar <markjr...@gmail.com> wrote:
> I want to define a method C.foo() that other classes
> outside of C can't call directly. Other class methods of C can call
> it, fine. But the goal is to have *instances* of C to be able to call
> it.
>
> I guess I could make it a private instance method that just happens
> not to care about self., but that seems like misleading design.

Without seeing the code you're actually working with, it seems to me
that I'd much rather see the method implemented as a private instance
method than as a class method (assuming that the purpose is to serve
as a utility method for use inside instances of the class).

I'm not sure why that would be considered misleading, and it would be /
much/ easier to follow the code than if you try to use some of the
other solutions offered. Not that the code suggested is /bad/, it's
just overkill for what you seem to be trying to accomplish, IMO.

--
Regards,

John Wilger