Strangely Consistent

Theory, practice, and languages, braided together

July 10 2012 — things which can be opened

Another day of relatively little coding. Well, that suits me fine; no sense in burning out early in the month. And I am kind of on vacation this week.

So, we make a thing openable, and then we open it. And it works, fantastic!

But then there are two sad paths, one where the thing we try to open doesn't exist, and the other where the thing is already open.

Again, note how basically all the interesting logic goes in with the sad paths, just like yesterday. This is a pretty new style of coding for me too. I reflected over this via privmsg to jnthn today:

<masak> I think the single most important idea in BDD is "black box testing".
        the idea that if we have to resort to getters in the 'then' part, we've 
<masak> we've failed *because* we need to break encapsulation to get important
        result information out of the object.
<masak> it's the testing equivalent of the Internet meme "pics or it didn't
<masak> black box testing naturally leads to a wider focus on sad-path testing,
        because every sad path corresponds to one way to validate, and is what 
        necessitates keeping state between calls.

Maybe the right way to think about this is to engage in a kind of dialogue with the client (or yourself, as it were):

<dev> *why* do you want to use a getter afterwards to check that the car
      is open?
<client> because otherwise how will I know it actually is open?
<dev> by testing whether it behaves as if it were open. what's particular about
      an open car? are there exceptions thrown now that weren't before?
<client> well, heh, I guess if I try to open it again, it should throw...
<dev> and that's all we need.

The trick is realizing that this is a general principle, not just something that works here. We don't install windows into the black box, because that kind of ruins its black-box-ness. Instead we make it behave sensibly through return values and exceptions on its behaviors.

I have a feeling the next two days will be a bit more challenging and may make some decisions jump around a bit.