Skip to main content

Recursion Bites with Complex Nevow Pages

There is a recently discovered issue with Nevow's Athena LivePages. LiveFragment nesting fails if the nesting gets too deep, due to repeated cloning of the contexts, including the full chain of parents back to the root of the document. This was hitting the call depth limits in some tests idnar was working on. I found this to be a little worrysome, as my designs for the current project included some relatively deep nesting of LiveFragments; at least, as deep as the tests that found the error. Eventually, contexts are to be removed from nevow entirely, as I understand it, but this is far down the road. A temporary solution was needed, besides just not using so much nesting. I decided that instead of redesigning the system I was building, I would fix the bug, and I have. I posted Trac Ticket #602 along with a patch that fixes it.

Recursion is a very useful software construct, but sometimes it can bite you in the end when you don't even expect it to. There is usually a way around it, but I do wish there was a more explicit way in some language to express a construct meant to avoid recursion. All I had to do was traverse up the parent attributes and create a list of all the parents, in order. I took each of these, and if the clone method was the same as our own, so we know for sure how it works, I just manually clone it and make it the parent of the clone of its original child. This way, no recursion is ever needed between parents being cloned, unless a different context class is being used, which also redefines the clone method. If that becomes a problem, it should be fixable as easily.

This may seem a little hackish, but its a common enough issue that I am really surprised there has never been a really good recursion alternative in any language. I'm not sure what form this would take, but in places it would be nice to say "this works like recursion, but without recursing".

Comments

Popular posts from this blog

Respect and Code Reviews

Code Reviews in a development team only function best, or possible at all, when everyone approaches them with respect. That’s something I’ve usually taken for granted because I’ve had the opportunity to work with amazing developers who shine not just in their technical skills but in their interpersonal skills on a team. That isn’t always the case, so I’m going to put into words something that often exists just in assumptions.
You have to respect your code. This is first only because the nature and intent of code reviews are to safeguard the quality of your code, so even having code reviews demonstrates a baseline of respect for that code. But, maybe not everyone on the team has the same level of respect or entered a team with existing review traditions that they aren’t acquainted with.
There can be culture shock when you enter a team that’s really heavy on code reviews, but also if you enter a team or interact with a colleague who doesn’t share that level of respect for the process or…

On Pruning Your Passions

We live in a hobby-rich world. There is no shortage of pastimes to grow a passion for. There is a shortage of one thing: time to indulge those passions. If you're someone who pours your heart into that one thing that makes your life worthwhile, that's a great deal. But, what if you've got no shortage of interests that draw your attention and you realize you will never have the time for all of them?

If I look at all the things I'd love to do with my life as a rose bush I'm tending, I realize that careful pruning is essential for the best outcome. This is a hard lesson to learn, because it can mean cutting beautiful flowers and watching the petals fall to the ground to wither. It has to be done.

I have a full time job that takes a lot of my mental energy. I have a wife and a son and family time is very important in my house. I try to read more, and I want to keep up with new developments in my career, and I'm trying to make time for simple, intentional relaxing t…

CARDIAC: The Cardboard Computer

I am just so excited about this.


CARDIAC. The Cardboard Computer. How cool is that? This piece of history is amazing and better than that: it is extremely accessible. This fantastic design was built in 1969 by David Hagelbarger at Bell Labs to explain what computers were to those who would otherwise have no exposure to them. Miraculously, the CARDIAC (CARDboard Interactive Aid to Computation) was able to actually function as a slow and rudimentary computer. 
One of the most fascinating aspects of this gem is that at the time of its publication the scope it was able to demonstrate was actually useful in explaining what a computer was. Could you imagine trying to explain computers today with anything close to the CARDIAC?

It had 100 memory locations and only ten instructions. The memory held signed 3-digit numbers (-999 through 999) and instructions could be encoded such that the first digit was the instruction and the second two digits were the address of memory to operate on. The only re…