Sunday, January 29, 2006

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".

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I write here about programming, how to program better, things I think are neat and are related to programming. I might write other things at my personal website.

I am happily employed by the excellent Caktus Group, located in beautiful and friendly Carrboro, NC, where I work with Python, Django, and Javascript.

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