Wednesday, November 28, 2007

What's this?

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An HTML list that came out of GeekWeaver, when I called this recursive function :



::rec
.<ul>
:for x,, #__
.<li>
${x/=}
:hasChild x
:rec ++ #x__


on a chunk of OPML.

That's pretty much how it's going to look, folks.

::rec defines the recursive block (or function)

.<ul> creates the UL tag

:for x,, #__

is creating a loop through all the anonymous (unlabelled) children of the tree which is being passed to the call. (This list is represented by the symbol __)

Note that I'm trying to keep GeekWeaver as functional as possible so there's no real "assignment". x is scoped so that it only exists inside the block of the for-loop, it's bound once before the block is evaluated, but can't be updated after that.

${x/=}

This is the new way of accessing variables. x is actually bound to a sub-tree (Almost everything in GeekWeaver is a sub-tree, except a couple of weird cases like __ which is a list.)

Although a simple $x still works if you want the whole of a tree flattened into a string, ${} expressions give you a way to pull data from a single path in the tree. In this case we're just getting the text content of the root node of x.

But it's also possible to write stuff like this : ${company/employees/__3/name} which means from the symbol "company" get the labelled child "employees" from which get the non-labelled 3rd child, from which get the child labelled "name".

:hasChild, like :for, is a build-in function. If the tree in x has any children, it evaluates the body of the :hasChild tree. If not, it returns nothing. This tests for our "base-case" when we've hit a leaf of the tree we're recursing through.

The final line :

:rec ++ #x__

is the one which I've been struggling with for the last three months, ever since I started trying to figure out how to support recursion. I'm still not 100% happy with the solution I've come up with, but it's getting there.

:rec is calling the recursive block again. And obviously I need to pass the children of x as arguments. However the way the function is written, it is working on the anonymous children (inside the default variable __)

How am I going to get the children of x into the __ variable inside the next call of :rec? That's what ++ does. Not sure what I'm going to call this, I may call it a "pivot" although that may confuse as much as help. It captures something of the idea that I need to swing the anonymous children of x (#x__) around so that they can go into the next call of :rec as though they were children of the calling node

Like I say, don't know if I like this name or the ++ symbol being used for it. So consider both as provisional for now.

OK, I'm too tired to make the build with this stuff working tonight. I'll try to get a build together in the next couple of days, but meanwhile, if this looks interesting to you and you can't contain yourself, send me an email (interstarATgmail.com), say hello and I'll see what I can do.
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