Wide Awake Developers

Ruby Expressiveness and Repeating Yourself

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Just this week, I was reminded again of how Java forces you to repeat yourself. I had an object that contains a sequence of "things to be processed". The sequence has to be traversed twice, once before an extended process runs and once afterwards.

The usual Java idiom looks like this:

public void preProcess(ActionContext context) {
  for (Iterator iter = updates.iterator(); iter.hasNext(); ) {
    TwoPhaseUpdate update = (TwoPhaseUpdate) iter.next();
    update.preProcess(context);
  }
}

public void postProcess(ActionContext context) {
  for (Iterator iter = updates.iterator(); iter.hasNext(); ) {
    TwoPhaseUpdate update = (TwoPhaseUpdate) iter.next();
    update.preProcess(context);
  }
}

Notice that there are only two symbols different between these two methods, out of 20 semantically significant symbols. According to the Pragmatic Programmers, even iterating over the collection counts as a kind of repetition (and therefore a violation of DRY - don’t repeat yourself.)

The Ruby equivalent would be something like:

def preProcess(context)
   updates.each { |u| u.preProcess(context) }
end

def postProcess(context)
   updates.each { |u| u.postProcess(context) }
end

Now, there are two differening symbols out of 10 (20% variance instead of 10%). There’s been no loss of expressiveness, in fact, the main intention of the code is clearer in the Ruby version than in the Java version.

Can we make the variance higher? Perhaps.

def preProcess(context)
   each_update(:preProcess, context)
end

def postProcess(context)
   each_update(:postProcess, context)
end

def each_update(method, context)
   updates.each { |u| u.send(method, context) }
end 

Now the two primary methods have 2 symbols out of 7 different or nearly 28%. The expressiveness is damaged a little bit by the dynamic dispatch via "send". It would be unthinkable to use reflection in Java to make the code clearer. (Anyone who’s worked with reflection knows what I mean.) Here, it’s not unthinkable, but it might just not help clarity.

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