Programming user interfaces using F# workflows

Numerous Manning partners already published several exceprts from my Real-World Functional Programming book. You can find a list on the book's web page. However, the last excerpt published at DotNetSlackers is particularly interesting. It discusses how to use F# asynchronous workflows to write GUI applications. This is a very powerful programming pattern that is very difficult to do in any other .NET language. We first discussed it with Don Syme during my internship at Microsoft Research and I found it very elegant, so I made some space for it in the book. In fact, the entire Chapter 16 discusses various reactive programming techniques that can be used in F#.

When designing applications that don't react to external events, you have lots of control flow constructs available, such as if-then-else expressions, for loops and while loops in imperative languages, or recursion and higher-order functions in functional languages. Constructs like this make it easy to describe what the application does. The control flow is clearly visible in the source code, so drawing a flowchart to describe it is straightforward.

Understanding reactive applications is much more difficult. A typical C# application or GUI control that needs to react to multiple events usually involves mutable state. When an event occurs, it updates the state and may run more code in response to the event, depending on the current state. This architecture makes it quite difficult to understand the potential states of the application and the transitions between them. Using asynchronous workflows, we can write the code in a way that makes the control flow of the application visible even for reactive applications.

You can read the complete article here: Programming user interfaces using F# workflows [^]. It is also worth adding that Manning offers 30% discount to DotNetSlackers readers (see the article for details!)

Using custom grouping operators in LINQ

You can use LINQ to write queries that perform grouping of data using group by or ordering of data using orderby clause. LINQ provides the default (and the most common) implementation of both of the operations, but sometimes you may need a slightly different behavior when grouping or ordering data (this article is motivated by a question on StackOverflow which needs to do exactly that for grouping).

Let's look at a simple example, which shows when we may need a different behavior when grouping data. For example, we may have the following list of stock trades containing a name of a stock and the price of the trade (stored for example as a list of TradeInfo classes with properties Name and Price):

{ { Name = "MSFT", Price = 80.00 },
  { Name = "MSFT", Price = 70.00 },
  { Name = "GOOG", Price = 100.00 },
  { Name = "GOOG", Price = 200.00 },
  { Name = "GOOG", Price = 300.00 },
  { Name = "MSFT", Price = 30.00 },
  { Name = "MSFT", Price = 20.00 } }

Now, we may want to group adjacent trades into a single summary record which will contain the name of the stock (which is same for all trades in each group), the number of trades in the group and an average price in the group. The desired results are:

{ { Name = "MSFT", Count = 2, AvgPrice = 75.00 },
  { Name = "GOOG", Count = 3, AvgPrice = 200.00 },
  { Name = "MSFT", Count = 2, AvgPrice = 25.00 } }

The operation that we want to do is very similar to group by in LINQ, but it doesn't do quite the same thing! If we used group by, we would get only two groups as the result. However, as I wrote earlier, we want to group only adjacent trades. You could write your own extension method to do this, but then you need to leave the elegant LINQ query syntax. In this article, I'll show you how to get the desired results using a simple LINQ query with a group by clause...

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