Can I tighten this up with block or eval?


#1

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.


#2

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 W.


#3

Gary W. removed_email_address@domain.invalid wrote:

Here is one possibility. Note, I changed ClassMaker to a module
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…
:slight_smile:

m.