Charting with F# does not take in Seq?

2
votes

Following this from the Real-World Functional Programming blog about drawing bar and column charts, I was trying to draw a histogram for my data which is stored at a set of tuples (data_value, frequency) in a lazy sequence.

It does not work unless I convert the sequence into a List, the error message being that in case of sequence "the IEnumerable 'T does not support the Reset function". Is there any way to generate a histogram/chart etc. using the .NET library from a lazily-evaluated sequence?

Also (ok newbie query alert), is there any way to make the chart persist when the program is run from the console? The usual System.Console.ReadKey() |> ignore makes the chart window hang, and otherwise it disappears in an instant. I've been using "Send to Interactive" to see results til now.

2
f#chartsf#-interactive
I'm wondering why it should be lazy. If you're drawing a chart, then by definition you must calculate the output to get the chart, and therefore the computation can be eager. - Robert Harvey
the values are meant to be read in from a file that takes instrument readings at 2-5 nanosecond intervals. the sizes can safely be assumed to be greater than can fit in main memory. - AruniRC
But don't you still need "all" of the data to render the chart? Or are you going to partially render the chart while new data comes in? - Robert Harvey
no, entire dataset doesn't contribute to the histogram. I'd just like to know if some variant of the function can accept lazily-evaluated collections (or if the reason for the error is the laziness, or something else entirely that I am missing) - AruniRC
For a chart window you do Application.Run(new Form()) - John Palmer

2 Answers

4
votes

The problem is that sequences (of type seq<T>, which is just an alias for IEnumerable<T>) generated using the F# sequence expression notation do not support the Reset method. The method is required by the charting library (because it needs to obtain the data each time it redraws the screen).

This means that, for example, the following will not work:

seq { for x in data -> x } |> FSharpChart.Line

Many of the standard library functions from the Seq module are implemented using sequence expressions, so the result will not support Reset. You can fix that by converting the data to a list (using List.ofSeq) or to an array (using Array.ofSeq) or by writing the code using lists:

[ for x in data -> x ] |> FSharpChart.Line

... and if you're using some function, you can take the one from List (not all of the Seq functions are available for List, so sometimes you will need to use conversion):

[ for x in data -> x ] |> List.choose op |> FSharpChart.Line
3
votes

No, it does not accept sequences.

That said, there is a good reason for not supporting seq. it is about the structure itself : A seq is just that, a seq, and therefore does not and should not, support the kind of operations needed in drawing a graph. That said, I really wish this stack was more advanced and supported more use style.

So the answer is to 

|> Seq.toArray

or

|> Seq.toList

before sending to the chart library