Once more unto the breach: private class methods

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?

Hi –

On Fri, 24 Aug 2007, Zeekar wrote:

     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,

Not until you write a ‘var’ instance method :slight_smile:

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.

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

class C
def self.x # assume this is the kind of method you mean
end

 def self.y
   x           # This would fail?
 end

end

That’s a pretty tall order: having methods from which you can’t call
other singleton methods of the very same object. Or am I
misunderstanding?

David

On 8/23/07, Zeekar [email protected] wrote:

      end

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

On Aug 23, 5:02 pm, “David A. Black” [email protected] 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.

On Aug 23, 5:11 pm, “Robert D.” [email protected] wrote:

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…

Hi –

On Fri, 24 Aug 2007, Zeekar wrote:

On Aug 23, 5:02 pm, “David A. Black” [email protected] 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.

OK – I didn’t get that; there was something to the effect that you
wanted instance methods, but not singleton or class methods, to be
able to call these methods, which is what confused me.

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

The code I came up with is similar to Ara’s, except that I didn’t make
the instance method version private which I believe probably does make
sense.

David

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://drawohara.com/

On Aug 23, 2:20 pm, Zeekar [email protected] 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 W.

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