Re: Redefining constants for a given instance only

On Sun, Aug 5, 2012 at 12:48 PM, Andrea Dallera <[e-mail]>wrote:

>
> Unfortunately, I'd like to avoid storing the class in a variable and then
> referring to it because then the code inside what I called Evaluator would
> have to be aware of the mechanics. The method Evaluator#evaluate might also
> refer to more than one constant and, ideally, shouldn't be at all aware
> that one of the constant it's using is not what it originally was defined:
>
>
I don't really understand why you can't pass it in when you initialize. Are
you saying that the implementation of #evaluate isn't known? Can't you just
scroll down a bit and see it? Or are you overriding it in subclasses or
something?

If so, it sounds like you need to pull out an object that wraps the
evaluate method, then you can initialize the evaluator (which would perhaps
be misnamed if you do this) with that object, and your evaluate method
could just delegate to it:


class MoreThanN
  def initialize(n)
    @n = n
  end

  def call(value)
    value > @n
  end
end


class IsEven
  def call(value)
    value.even?
  end
end


# in this contrived example, this class is irrelevant,
# but presumably in your more complex domain, it does something useful
class Evaluator
  def initialize(strategy)
    @strategy = strategy
  end

  def evaluate(value)
    @strategy.call value
  end
end


Evaluator.new(MoreThanN.new 3).evaluate 2 # => false
Evaluator.new(MoreThanN.new 3).evaluate 4 # => true
Evaluator.new(IsEven.new).evaluate      3 # => false
Evaluator.new(IsEven.new).evaluate      4 # => true