The Universe of Disco

Don't do this

[ This article has undergone major revisions since it was first published yesterday. ]

Here is a line of Perl code:

  if ($self->fidget && blessed $self->fidget eq 'Widget::Fidget') {

This looks to see if $self has anything in its fidget slot, and if so it checks to see if the value there is an instance of the class Widget::Fidget. If both are true, it runs the following block.

That blessed check is bad practice for several reasons.

1. It duplicates the declaration of the fidget member data:

   has fidget => (
     is  => 'rw',
     isa => 'Widget::Fidget',
     init_arg => undef,
   );
   

   So the fidget slot can't contain anything other than a Widget::Fidget, because the OOP system is already enforcing that. That means that the blessed … eq test is not doing anything — unless someone comes along later and changes the declared type, in which case the test will then be checking the wrong condition.

2. Actually, that has already happened! The declaration, as written, allows fidget to be an instance not just of Widget::Fidget but of any class derived from it. But the blessed … eq check prevents this. This reneges on a major promise of OOP, that if a class doesn't have the behavior you need, you can subclass it and modify or extend it, and then use objects from the subclass instead. But if you try that here, the blessed … eq check will foil you.

   So this is a prime example of “… in which case the test will be checking the wrong condition” above. The test does not match the declaration, so it is checking the wrong condition. The blessed … eq check breaks the ability of the class to work with derived classes of Widget::Fidget.

3. Similarly, the check prevents someone from changing the declared type to something more permissive, such as

   “either Widget::Fidget or Gidget::Fidget”

   or

   “any object that supports wiggle and waggle methods”

   or

   “any object that adheres to the specification of Widget::Interface”

   and then inserting a different object that supports the same interface. But the whole point of object-oriented programming is that as long as an object conforms to the required interface, you shouldn't care about its internal implementation.

4. In particular, the check above prevents someone from creating a mock Widget::Fidget object and injecting it for testing purposes.

5. We have traded away many of the modularity and interoperability guarantees that OOP was trying to preserve for us. What did we get in return? What are the purported advantages of the blessed … eq check? I suppose it is intended to detect an anomalous situation in which some completely wrong object is somehow stored into the self.fidget member. The member declaration will prevent this (that is what it is for), but let's imagine that it has happened anyway. This could be a very serious problem. What will happen next?

   With the check in place, the bug will go unnoticed because the function will simply continue as if it had no fidget. This could cause a much more subtle failure much farther down the road. Someone trying to debug this will be mystified: At best “it's behaving as though it had no fidget, but I know that one was set earlier”, and at worst “why is there two years of inconsistent data in the database?” This could take a very long time to track down. Even worse, it might never be noticed, and the method might quietly do the wrong thing every time it was used.

   Without the extra check, the situation is much better: the function will throw an exception as soon as it tries to call a fidget method on the non-fidget object. The exception will point a big fat finger right at the problem: “hey, on line 2389 you tried to call the rotate method on a Skunk::Stinky object, but that class has no such method`. Someone trying to debug this will immediately ask the right question: “Who put a skunk in there instead of a widget?”

It's easy to get this right. Instead of

  if ($self->fidget && blessed $self->fidget eq 'Widget::Fidget') {

one can simply use:

  if ($self->fidget) {

Moral of the story: programmers write too much code.

I am reminded of something chess master Aron Nimzovitch once said, maybe in Chess Praxis, that amateur chess players are always trying to be Doing Something.

[Other articles in category /prog/perl] permanent link

In which, to my surprise, I find myself belonging to a group

My employer ZipRecruiter had a giant crisis at last month, of a scale that I have never seen at this company, and indeed, have never seen at any well-run company before. A great many of us, all the way up to the CTO, made a heroic effort for a month and got it sorted out.

It reminded me a bit of when Toph was three days old and I got a call from the hospital to bring her into the emergency room immediately. She had jaundice, which is not unusual in newborn babies. It is easy to treat, but if untreated it can cause permanent brain damage. So Toph and I went to the hospital, where she underwent the treatment, which was to have very bright lights shined directly on her skin for thirty-six hours. (Strange but true!)

The nurses in the hospital told me they had everything under control, and they would take care of Toph while I went home, but I did not go. I wanted to be sure that Toph was fed immediately and that her diapers were changed timely. The nurses have other people to take care of, and there was no reason to make her wait to eat and sleep when I could be there tending to her. It was not as if I had something else to do that I felt was more important. So I stayed in the room with Toph until it was time for us to go home, feeding her and taking care of her and just being with her.

It could have been a very stressful time, but I don't remember it that way. I remember it as a calm and happy time. Toph was in no real danger. The path forward was clear. I had my job, to help Toph get better, and I was able to do it undistracted. The hospital (Children's Hospital of Philadelphia) was wonderful, and gave me all the support I needed to do my job. When I got there they showed me the closet where the bedding was and the other closet where the snacks were and told me to help myself. They gave me the number to call at mealtimes to order meals to be sent up to my room. They had wi-fi so I could work quietly when Toph was asleep. Everything went smoothly, Toph got better, and we went home.

This was something like that. It wasn't calm; it was alarming and disquieting. But not in an entirely bad way; it was also exciting and engaging. It was hard work, but it was work I enjoyed and that I felt was worth doing. I love working and programming and thinking about things, and doing that extra-intensely for a few weeks was fun. Stressful, but fun.

And I was not alone. So many of the people I work with are so good at their jobs. I had all the support I needed. I could focus on my part of the work and feel confident that the other parts I was ignoring were being handled by competent and reasonable people who were at least as dedicated as I was. The higher-up management was coordinating things from the top, connecting technical and business concerns, and I felt secure that the overall design of the new system would make sense even if I couldn't always understand why. I didn't want to think about business concerns, I wanted someone else to do it for me and tell me what to do, and they did. Other teams working on different components that my components would interface with would deliver what they promised and it would work.

And the other programmers in my group were outstanding. We were scattered all over the globe, but handed off tasks to one another without any mishaps. I would come into work in the morning and the guys in Europe would be getting ready to go to bed and would tell me what they were up to and the other east-coasters and I could help pick up where they left off. The earth turned and the west-coasters appeared and as the end of the day came I would tell them what I had done and they could continue with it.

I am almost pathologically averse to belonging to groups. It makes me uncomfortable and even in groups that I have been associated with for years I feel out of place and like my membership is only provisional and temporary. I always want to go my own way and if everyone around me is going the same way I am suspicious and contrarian. When other people feel group loyalty I wonder what is wrong with them.

The up-side of this is that I am more willing than most people to cross group boundaries. People in a close-knit community often read all the same books and know all the same techniques for solving problems. This means that when a problem comes along that one of them can't solve, none of the rest can solve it either. I am sometimes the person who can find the solution because I have spent time in a different tribe and I know different things. This is a role I enjoy.

Higher-Order Perl exemplifies this. To write Higher-Order Perl I visited functional programming communities and tried to learn techniques that those communities understood that people outside those communities could use. Then I came back to the Perl community with the loot I had gathered.

But it's not all good. I have sometimes been able to make my non-belonging work out well. But it is not a choice; it's the way I am made, and I can't control it. When I am asked to be part of a team, I immediately become wary and wonder what the scam is. I can be loyal to people personally, but I have hardly any group loyalty. Sometimes this can lead to ugly situations.

But in fixing this crisis I felt proud to be part of the team. It is a really good team and I think it says something good about me that I can work well with the rest of them. And I felt proud to be part of this company, which is so functional, so well-run, so full of kind and talented people. Have I ever had this feeling before? If I have it was a long, long time ago.

G.H. Hardy once wrote that when he found himself forced to listen to pompous people, he would console himself by thinking:

Well, I have done one thing you could never have done, and that is to have collaborated with Littlewood and Ramanujan on something like equal terms.

Well, I was at ZipRecruiter during the great crisis of June 2018 and I was able to do my part and to collaborate with those people on equal terms, and that is something to be proud of.

[Other articles in category /brain] permanent link