· f

F#: A day writing a Feedburner graph creator

I’ve spent a bit of the day writing a little application to take the xml from my Feedburner RSS feed and create a graph showing the daily & weekly average subscribers.

What did I learn?

  • I decided that I wanted to parameterise the feedburner url so that I would be able to run the code for different time periods and against different feeds. In C# we’d probably make use of 'string.Format()' which has an equivalent in F# called 'sprintf' My initial thought was that I would be able to do something like this: ~ocaml let ShowFeedBurnerStats feed = let statsUrl = "https://feedburner.google.com/api/awareness/1.0/GetFeedData?uri=%s&dates=2009-01-01,2009-07-11" sprintf statsUrl feed |> GetXml // more code ~ Which actually results in the following compilation error: ~text The type 'string' is not compatible with the type 'Printf.StringFormat ~ After a bit of searching I found a post by Robert Pickering where he explains that the format string needs to be next to the sprintf function to work as expected: ~ocaml let ShowFeedBurnerStats feed = let statsUrl = sprintf "https://feedburner.google.com/api/awareness/1.0/GetFeedData?uri=%s&dates=2009-01-01,2009-07-11" statsUrl feed |> GetXml // more code ~ 'statsUrl' therefore becomes a function taking in a 'string' and returning a 'string'.

  • I’m still trying to work out the best way to decompose the code I write into functions which make sense in terms of the domain I’m working in. I often found myself splitting up a function along the boundary of where any I/O interaction was happening so that I could execute the I/O function and save the data before using it in another function which I would execute a lot more frequently (using F# interactive) while I was tweaking it.

  • I still haven’t come up with a completely satisfactory approach to coding these little applications - right now I’m finding that the feedback cycle is significantly quicker if I just write functions and then run them in F# interactive and then tweak anything which isn’t working as expected. I didn’t write any unit tests while coding this although I did find myself writing shorter functions than I originally did when writing my little twitter application. The problem of not writing the tests is that I lose the protection against regression that I would otherwise get.

  • I still have a bit of a love hate relationship with tuples - I found myself making use of them early on when I was focused on getting the code to work and I could still understand the code easily. Originally I was only storing 'date' and 'circulation' in the tuple but once I added a third value to the tuple ('weeklyAverage') it became too confusing for me to understand so I decided to introduce the 'FeedBurnerStats' type to simplify things for myself.

  • I ended up writing a function called 'Join' which is quite similar to 'Seq.zip' because I wanted to join two sequences together but only join items which had the same date (the 'string' value in the tuple). Therefore, if I had some data like this: 'dailyStats' ~ocaml "2009-01,07", 200 "2009-01,08", 222 ~ 'weeklyAverages' ~ocaml "2009-01,07", 300 "2009-01,08", 322 ~ I wanted the join of the two sequences to look like this: ~ocaml "2009-01,07", 200, 300 "2009-01,08", 222, 322 ~ Which wasn’t working as expected when I used 'Seq.zip' - the items that were getting matched together seemed to be quite random to me. ~ocaml let Join (dailyStats:seq<decimal*string>) (weeklyAverages:seq<decimal*string>) = dailyStats |> Seq.map (fun d -> { Date = d |> snd; Circulation = d |> fst; WeeklyAverage = weeklyAverages |> Seq.find (fun w -> snd d = snd w) |> fst}) ~ </ul> I’ve included the code is at the end of the post - there are some areas where I don’t really like the way I’ve solved a problem but I’m not sure of a better way at the moment. In particular:

    • I wanted to make use of 'Seq.windowed' to find the rolling weekly average but I needed it to go back 7 days rather than forward 7 days which meant I needed to reverse the sequence. Right now I’ve done this by converting it to a list and using 'List.rev' to do so but this seems like a fairly inefficient way of doing this. The alternative seemed to be to write a function to change the order of the items in the sequence but again this doesn’t seem like a great approach.

    • What do you do with functions which are only used by one other areas of the code? For example 'ConvertToCommaSeparatedString' is only used by 'CreateGoogleGraphUri' so I defined it inside that function - I could then pull it to a function in its own right if other areas of the code need it. I did this to reduce the clutter of functions hanging around but it then makes 'CreateGoogleGraphUri' more difficult to read.

      I decided to run it against some blogs I follow to see what the graphs, created using Google’s Charts API, would look like: ~ocaml ShowFeedBurnerStats "scotthanselman" "2009-03-01" "2009-07-11"

      ShowFeedBurnerStats "youdthinkwithallmy" "2009-03-01" "2009-07-11";; ShowFeedBurnerStats "codinghorror" "2009-03-01" "2009-07-11";; ~ hanselman.png jasonyip.png codinghorror.png Interestingly you can actually see the points where feedburner for some reason counted a particular days circulation as being 0. And here’s the code: ~ocaml open System.IO open System.Net open Microsoft.FSharp.Control open System.Xml.Linq open System let downloadUrl (url:string) = async{ let request = HttpWebRequest.Create(url) let! response = request.AsyncGetResponse() let stream = response.GetResponseStream() use reader = new StreamReader(stream) return! reader.AsyncReadToEnd() } let xName value = XName.Get value let GetDescendants element (xDocument:XDocument) = xDocument.Descendants(xName element) let GetAttribute element (xElement:XElement) = xElement.Attribute(xName element) let GetXml = downloadUrl >> Async.Run >> XDocument.Parse let GetDateAndCirculation (document:XDocument) = document |> GetDescendants "entry" |> Seq.map (fun element -> GetAttribute "circulation" element, GetAttribute "date" element) |> Seq.map (fun attribute -> Decimal.Parsefst attribute).Value), (snd attribute).Value) let CalculateAverage days (feedStats:seq<decimal * string>) = let ReverseSequence (sequence:seq<_>) = sequence |> Seq.to_list |> List.rev |> List.to_seq feedStats |> ReverseSequence |> Seq.windowed days |> Seq.map (fun x -> x |> Array.map (fun y -> y |> fst) |> Array.average, x.[0] |> snd) |> ReverseSequence let CalculateWeeklyAverage (feedStats:seq<decimal * string>) = CalculateAverage 7 feedStats type FeedBurnerStats = { Date : string; Circulation: decimal; WeeklyAverage: decimal } let Join (dailyStats:seq<decimal*string>) (weeklyAverages:seq<decimal*string>) = dailyStats |> Seq.map (fun d -> { Date = d |> snd; Circulation = d |> fst; WeeklyAverage = weeklyAverages |> Seq.find (fun w -> snd d = snd w) |> fst}) let GetFeedBurnerStats feed startDate endDate = let statsUrl = sprintf "https://feedburner.google.com/api/awareness/1.0/GetFeedData?uri=%s&dates=%s,%s" let allStats = GetDateAndCirculation (statsUrl feed startDate endDate |> GetXml) let weeklyAverages = allStats |> CalculateWeeklyAverage let dailyStats = allStats |> Seq.filter (fun x -> weeklyAverages |> Seq.exists (fun y -> snd y = snd x Join dailyStats weeklyAverages let CreateGoogleGraphUri feed (stats:seq) = let ConvertToCommaSeparatedString (value:seq) = let rec convert (innerVal:List) acc = match innerVal with | [] -> acc | hd::[] -> convert [] (acc + hd) | hd::tl -> convert tl (acc + hd + ",") convert (Seq.to_list value) "" let graphUrl = sprintf "http://chart.apis.google.com/chart?cht=lc&chtt=%s&&chco=000000,FF0000&chdl=WeeklyAverage|Daily&chs=600x240&chds=%s,%s&chd=t:%s|%s" let weeklyAverages = stats |> Seq.map (fun f -> f.WeeklyAverage.ToString("f0")) |> ConvertToCommaSeparatedString let circulation = stats |> Seq.map (fun f -> f.Circulation.ToString("f0")) |> ConvertToCommaSeparatedString let maximum = stats |> Seq.map (fun f -> f.Circulation) |> Seq.max let minimum = stats |> Seq.map (fun f -> f.Circulation) |> Seq.min new System.Uri(graphUrl feed (minimum.ToString("f0")) (maximum.ToString("f0")) weeklyAverages circulation) let ShowFeedBurnerStats feed startDate endDate = CreateGoogleGraphUri feed (GetFeedBurnerStats feed startDate endDate) ~

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