I'm teaching myself F#. I've chosen to port an existing program I wrote. It's based on my RSS aggregator, SharpReader. This creates a set of .xml files, one for each RSS feed. When it displays the feeds to me, it lets me set a optional Flag value from 0 to 9, indicating an interest level. If a Flag level is set, this is then saved in the .xml file, on a per-item basis.

My program runs through the .xml files, creates a Feed type (with Title and Link), one for each file/feed, and within that, a list of FeedItem types (with ItemTitle, Link, PubDate and FlagValue, and a reference to the Feed it belongs to). I then sort these in various ways, and produce an HTML file that shows the data sorted by Date, by Priority (Flag value), and by Feed. Fine, I've got this working (and with only one minor instance of mutabilty left, which I think I can get around).

To be able to have a Feed have a list of FeedItems, which in turn refers back to its parent Feed, I finally found the "and" keyword, and that works.

But now I want to expand the program, and this is where I'm running into difficulty.

Each FeedItem can have 0 or more <Subject> tags, such as F#, C Sharp, etc. So I thought it would be nice to add a new report which shows the FeedItems sorted by Subject.

Now I can think of several algorithms to do this. The first is to make a pass over all FeedItems, gathering up the Subjects into a Set. Once I have a unique Set of Subjects, I can iterate through each Subject and find the FeedItems that specify that Subject, and (after a sort by-PubDate) write out their info to my .html file.

That should work, but it's an n ** 2 algorithm, which is clearly poor.

I can improve on that by making the original scan, then running through the FeedItems and adding them to a Dictionary<string (*Subject*), FeedItem>. That should also be do-able, and with Dictionary's being tree-based, this should be an n * log n algorithm.

But suppose I wanted to construct the Dictionary inside the FeedItem constructor. I could have a "global variable" as follows

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let SubjectDict = new Dictionary<string, FeedItem)()

But where do I put this? I can't put it before my "type FeedItem", since at that point the compiler hasn't heard about FeedItem, so I get a compile time error.

And I can't put it after the "type FeedItem" since at that point the FeedItem constructor will get a compile time error trying to reference SubjectDict.

So if, in the interest of algorithm efficiency, I want to create the Dictionary on-the-fly, how do I do this?

P.S. I tried -- type Feed ... and type FeedItem ... and let SubjectDict = new Dictionary<string, FeedItem>() -- but the compiler didn't like that.