Parsing Roman Numerals in C# and Haskell

This is a follow-up from my last post Functional Monadic Parsers ported to C# where I showed the implementation of basic parsers from the book Programming in Haskell by Graham Hutton in C#.

When these primitives are used to compose a parser for Roman Numerals the result yet again demonstrates the amazing capabilities and elegance of functional programming. The problem of parsing Roman Numerals is not a very difficult one. But still, I find the simplicity of constructing a solution by combining primitive parsers fascinating.

Here is the implementation. It is super easy, I was able to write this in less than 15 minutes without tests first, worked the first time.
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Functional Monadic Parsers ported to C#

While taking the MOOC Introduction to Functional Programming by Erik Meijer on edX the current lecture greatly increased my interest in functional parsers. Functional parsers are about the essence of (functional) programming because they are about the concept of composition which is one of the core concepts of programming whatsoever.

The content of the lecture is closely related to the chapter 8 Functional Parsers of the book Programming in Haskell by Graham Hutton. It starts out with the definition of a type for a parser and a few very basic parsers.

I’m totally amazed by the simplicity, the elegance and the compositional aspect of these examples. I find it impressive how primitive but yet powerful these simple parsers are because they can easily be combined to form more complex and very capable parsers.

As an exercise, out of curiosity, and because of old habits I implemented the examples from the book in C#.

At the end of this post there will be an ultimate uber-cool parser example of a parser for arithmetic expressions.

The complete code from this post can be found here on GitHub.

Here is what I got:

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XPlot ‘hello world’ – Using Google Charts in Web Applications

In this post I will describe the very basics, in other words the ‘hello world example’, of how to create cool looking Google Charts using XPlot and integrate them into Web Applications.

The F# library XPlot is part of the FsLab data science package, which you can do very neat things with. But it is also possible to reference an XPlot stand-alone package to just use the Google Charts library.

What is XPlot?

XPlot is a cross-platform data visualization package for the F# programming language powered by popular JavaScript charting libraries Google Charts and Plotly. The library provides a complete mapping for the configuration options of the underlying libraries and so you get a nice F# interface that gives you access to the full power of Google Charts and Plotly. The XPlot library can be used interactively from F# Interactive, but charts can equally easy be embedded in F# applications and in HTML reports. —From XPlot Home Page

Creating the chart

There are only a few easy steps necessary to generate the HTML for let’s say a simple bar chart.

First we need an F# library project that references the XPlot.GoogleCharts package via NuGet.

Here is a function that will use XPlot to create the HTML and JavaScript code for a simple bar chart:

namespace Charts

module BarCharts =
    open XPlot.GoogleCharts

    let statusCount () =

        let series = [("Open",23); ("In Progress",5); ("Resolved", 58); ("Closed",5)]

        let options =
            Options(
                title = "Status", 
                orientation = "horizontal")

        if series |> Seq.isEmpty then "no data" else
            let chart = 
                series
                |> Chart.Bar
                |> Chart.WithOptions options
                |> Chart.WithLabels ["Count"]

            chart.InlineHtml

The result can be used in any web application if the Google Charts libraries are loaded.

series can be any list of tuples of type (string * int). Off course, in a real project this list would be either passed to the function as an argument, or the function would have a data access dependency injected.

Here is the output of the statusCount() function:

<script type="text/javascript">
  google.charts.setOnLoadCallback(drawChart);
  function drawChart() {
    var data = new google.visualization.DataTable({"cols": [{"type": "string" ,"id": "Column 1" ,"label": "Column 1" }, {"type": "number" ,"id": "Count" ,"label": "Count" }], "rows" : [{"c" : [{"v": "Open"}, {"v": 23}]}, {"c" : [{"v": "In Progress"}, {"v": 5}]}, {"c" : [{"v": "Resolved"}, {"v": 58}]}, {"c" : [{"v": "Closed"}, {"v": 5}]}]});

    var options = {"legend":{"position":"right"},"orientation":"horizontal","title":"Status"} 

    var chart = new google.visualization.BarChart(document.getElementById('9af3e989-c37b-4257-b71f-5053b52f3b48'));
    chart.draw(data, options);
        }
</script>
<div id="9af3e989-c37b-4257-b71f-5053b52f3b48" style="width: 900px; height: 500px;"></div>

Displaying the chart in an ASP.NET MVC application

Now, as an example, we will see how to integrate this into an ASP.NET MVC application.

Loading the Google Charts Library

Load the Google Charts library by adding the following lines to the <head> section of the page e.g. in _Layout.cshtml. (Also see
Load the Libraries
)

<script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>
<script type="text/javascript">
  google.charts.load('current', {packages: ['corechart']});
  google.charts.setOnLoadCallback(drawChart);
</script>

Initializing the View Model

Add a reference to the F# project that was created earlier and build the F# project.

Next add a reference to FSharp.Core.

Then initialize a View Model with the chart and pass it to the View:

public ActionResult Index()
{
    var vm = new BarChartVm
    {
        Chart = Charts.BarCharts.statusCount()
    };

    return View(vm);
}

Add the following code to the page at the position where the chart should be displayed:

<div>
    @Html.Raw(@Model.Chart)
</div>

When running the application, the chart should be displayed like this:

Here is the source code from this post.