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comp.lang.ruby

can I tighten this up with block or eval?

matt

3/15/2009 5:16:00 PM

I have here a class called ClassMaker whose greeter method returns an
instance of anonymous class containing a hello method:

class ClassMaker
def initialize(s); @howdy = s; end
def greeter
c = Class.new do
def initialize(s); @hello = s; end
def hello
puts @hello # this is the nub of the matter
end
end
c.new(@howdy)
end
end

ClassMaker.new("gday").greeter.hello

I would like the anonymous class's hello method to access whatever the
value of my ClassMaker instance's @howdy ivar may be at the moment
greeter is called. I am presently doing this, as you can see, by giving
the anonymous class an initialize method and passing the @howdy ivar's
value into an ivar of the anonymous class's instance.

This works but seems heavy-handed. I feel I should be able to do it
without the anonymous class having any initialize method at all. The
whole situation has "closure" written all over it. So I tried to define
the hello method to accomplish the same thing using evals and blocks and
other tricky stuff, and became hopelessly confused. Could someone
straighten me out? Thx - m.

--
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2 Answers

Gary Wright

3/15/2009 5:46:00 PM

0


On Mar 15, 2009, at 1:17 PM, matt neuburg wrote:
> This works but seems heavy-handed. I feel I should be able to do it
> without the anonymous class having any initialize method at all. The
> whole situation has "closure" written all over it. So I tried to
> define
> the hello method to accomplish the same thing using evals and blocks
> and
> other tricky stuff, and became hopelessly confused. Could someone
> straighten me out? Thx - m.

Here is one possibility. Note, I changed ClassMaker to a module
since it is really just a factory for creating classes and doesn't
really need to create intermediate instances of ClassMaker to
accomplish your goal.

module ClassMaker
def self.greeter(greeting)
Class.new do
define_method :hello do
puts greeting # this is the nub of the matter
end
end
end
end

ClassMaker.greeter("gday").new.hello
ClassMaker.greeter("aloha").new.hello


Gary Wright

matt

3/15/2009 6:25:00 PM

0

Gary Wright <gwtmp01@mac.com> wrote:

> On Mar 15, 2009, at 1:17 PM, matt neuburg wrote:
> > This works but seems heavy-handed. I feel I should be able to do it
> > without the anonymous class having any initialize method at all. The
> > whole situation has "closure" written all over it. So I tried to
> > define
> > the hello method to accomplish the same thing using evals and blocks
> > and
> > other tricky stuff, and became hopelessly confused. Could someone
> > straighten me out? Thx - m.
>
> Here is one possibility. Note, I changed ClassMaker to a module
> since it is really just a factory for creating classes and doesn't
> really need to create intermediate instances of ClassMaker to
> accomplish your goal.
>
> module ClassMaker
> def self.greeter(greeting)
> Class.new do
> define_method :hello do
> puts greeting # this is the nub of the matter
> end
> end
> end
> end
> ClassMaker.greeter("gday").new.hello

Actually, I have to keep the architecture I've got - greeter is going to
be called without a parameter, that's just how it is. (The problem as I
posed it is just a schematic reduction of a vast existing architecture
that does something else entirely.)

However, your solution shows me exactly the trick I was missing: I need
to turn @howdy into a local so that a block can see it:

class ClassMaker
def initialize(s); @howdy = s; end
def greeter
greeting = @howdy # the magic step!
Class.new do
define_method :hello do
puts greeting
end
end.new
end
end
ClassMaker.new("gday").greeter.hello

Thanks! It's nice to stop banging myself over the head with a hammer...
:)

m.

--
matt neuburg, phd = matt@tidbits.com, http://www.tidbits...
Leopard - http://www.takecontrolbooks.com/leopard-custom...
AppleScript - http://www.amazon.com/gp/product/...
Read TidBITS! It's free and smart. http://www.t...