The Duality of Object and Event references

Mathematical duality [2] is a very useful and elegant concept that gives us a nice way of speaking about objects or structures that behave in some way exactly conversely. There is no clear definition of what duality is. However, the idea is that when we have two structures that behave conversely and we know that something is true about the first one, then we get the opposite statement about the other structure "for free" (if you know how to translate from one structure to the other).

In this article, I'll talk about an interesting example of duality that (to my best knowledge) wasn't described by anyone before. The two dual structures are references between objects in a normal program and references between events in a reactive application. The statement that is going to become obvious thanks to the duality principle is describing which of the objects (or events) are garbage and can be safely collected by garbage collector.

This topic is in much more details discussed in a paper [4] I wrote with Don Syme and that I presented at the ISMM Workshop (see also my trip report from PLDI). In this article, I'll try to give an accessible description of the most interesting idea from the paper...

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Monday, July 19, 2010

Dynamic in F#: Reading data from SQL database

When reading data from a SQL database in F#, you have a couple of options. You can use F# implementation of LINQ, which allows you to write queries directly in the F# language or you can use standard ADO.NET classes such as SqlCommand. The second option is often good enough - when you just need to call a simple query, you probably don't want to setup the whole LINQ infrastructure (generate classes, reference F# PowerPack, etc.) Unfortunately, ADO.NET classes are a bit ugly to use. When calling an SQL stored procedure, you need to specify the name of the procedure as a string, you need to add parameters one-by-one by creating instances of SqlParameter, you need to dynamically cast results from the obj type and so on...

In this article, we'll look how to use the dynamic operator to make the experience of using ADO.NET from F# dramatically better. Dynamic operator (there are actually two of them) are a simple way of supporting dynamic invoke in F#. We can use it to write code that looks almost like an ordinary method call or property access, but is resolved dynamically at runtime (using the name of the method or property). The following example shows what we'll be able to write at the end of this article:

// Call 'GetProducts' procedure with 'CategoryID' set to 1
use conn = new DynamicSqlConnection(connectionString)
use cmd = conn?GetProducts
cmd?CategoryID <- 1
conn.Open()

// Read all products and print their names
use reader = cmd.ExecuteReader()
while reader.Read() do
  printfn "Product: %s" reader?ProductName

If you ever tried to call a SQL stored procedure directly using the SqlCommand, then you can surely appreciate the elegance of this code snippet. Let's now take a look at a larger example and some of the neat tricks that make this possible...

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Friday, July 09, 2010

PLDI 2010 Trip Report

In June, I attended my first "big" academic conference Programming Languages Design and Implementation (PLDI 2010) in Toronto. I attended the conference, because I presented a paper that I wrote with Don Syme as a result of my internship at Microsoft Research, but I'll write more about that shortly (and thanks to MSR for partly supporting my attendance at the conference!)

As far as I understand it, the focus of the conference is more on implementation. Many people believe that the current programming languages are good enough and we need to make sure that they run well (e.g. compilers optimize code to run better on multi-core) and that we need better tools for working with them (e.g. automatic verification of the code we write), so these areas were the main focus of the conference. However, there were some very interesting talks on the design of programming languages, which is an area that I personally find more interesting...

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Monday, July 05, 2010

Recording and samples from my Reactive F# talk

Almost a week ago, I posted an invitation to my F# talk at the F#unctional Londoners user group. The theme of my talk was reactive programming, which was also a topic of my Master's thesis (defended earlier in June), so I really enjoyed talking about it. In the talk, I discussed the two approaches that you can use for developing reactive applications in F# (using examples in Silverlight):

  • Declarative (or data-flow oriented) allows you to describe "what" should be done with data that your component receives from various events (such as mouse position etc.) This can be written using F# event combinators such as Event.map and Event.scan.
  • Imperative (or control-flow oriented) - in this style, we describe various states of the component (e.g. semaphore with green, orange and red) and describe transitions between the states. This can be written using F# asynchronous workflows and the AwaitObservable primitive (which you can get as part of the source code).

Thanks to the folks from SkillsMatter who provided place for the meetup and helped with the organization, the talk was also recorded and is already available online! Below, you can also get all the nice Silverlight demos that I used during the talk...

Thanks again to Carolyn Miller and Phil Trelford for organizing the talk and also to Don Syme for taking me to the airport in the early morning (at 4 AM...) after the talk.

Links & Resources

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Sunday, June 27, 2010

Reactive Programming with F# in London

If you’re a registered member of the F#unctional Londoners user group, then you maybe already know that I'll be visiting London on June 23 and I’ll be talking about Reactive programming with F#. If you're not a registered member and occasionally visit London, then you should definitely register. The user group is organized by Carolyn Miller and Phil Trelford (whom I met some time ago at Microsoft Research in Cambridge). Among others, previous speakers include Robert Pickering (who also presented some samples based on my F# and Accelerator series). Finally, another reason for joining the group is that it has a great name (as well as a logo)!

When, what & where?

By the way, I'll also have a free copy of my Real-World Functional Programming book to give away during the talk!

Reactive Programming with F#

I'm sure you're already convinced to come. Nevertheless, you may still want to know what I'm going to talk about. There are many areas where F# offers clear benefits such as parallel & concurrent programming. I believe that reactive programming is another great area for F#. In reactive programming, we face quite different problems than in other programming styles. We (as the authors of the application) can no longer specify what the application should do. Instead, the application needs to be able to handle many possible combinations of events. This aspect of programming is sometimes called inversion of control.

Reactive programming is important for programming user interfaces, especially today when user interfaces are becoming more interactive and more "fancy". To demonstrate this, I'm working on some nice Silverlight demos for the talk! However, it is also needed to handle other kinds of events such as completion of background task or message from other application. We'll look at the two essential techniques that F# provides for reactive programming:

  • Declarative event combinators - one way of writing reactive applications is to specify the whole event processing declaratively by saying "what" should be done with occurrences of events. This is particularly useful when we need to encode simpler logic with a clear data-flow.
  • Imperative using workflows - for more complicated interactions, we can use asynchronous workflows. This makes the code more explicit, but we get full control over the control-flow of the application. Even though this approach is more "imperative" it can be used for writing nicely composable functional code as well.

I'm looking forward to seeing you at the talk next week!

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Tuesday, June 15, 2010

ASP.NET and F# (I.) - Creating MVC web applications in F#

Some time ago, I wrote a couple of examples of developing web applications in F# using ASP.NET. Since then, the F# language and runtime has changed a little bit and there are also new technologies available in ASP.NET, so I thought I'd write a more up-to-date article on this topic. In this article, I'll present a simple "demo" F# web application that you can use as a starting point when creating your own projects (you'll also find a convenient Visual Studio 2010 template below). The article shows the following interesting things:

  • ASP.NET MVC - We're going to use ASP.NET MVC Framework to create the web application. As the article name suggests, most of the actual program code including models and controllers will be implemented in F#.
  • F# LINQ to SQL - The application uses a sample Northwind database and we'll write queries for selecting data from the database using LINQ support that's available in the F# PowerPack.
  • F# features - The application also uses some nice F# features that are quite useful for developing web applications. We'll use modules and records to implement the model and we'll also use advanced meta-programming features for constructing LINQ queries.

If you want to use F# for creating an MVC application, you have a few options. It should be possible to create the web application solely as an F# project. However, we'll use a more convenient approach. We'll create a standard C# MVC project and move all the actual implementation to an F# library. We'll look at the application structure shortly. The following screenshot shows a page that lists products in the Northwind database:

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Sunday, May 09, 2010

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!)

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Thursday, February 18, 2010

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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Sunday, February 07, 2010

Deal of the day: Real-World Functional Programming

Some time ago, I received my copies of Real-World Functional Programming. I started working on it back in May 2008 and as many people who had more experience with writing books told me, it took longer than I was expecting! Anyway, I have to say, it was worth it, holding the actual printed book with my name on the cover is just fantastic!


The goal of the book is to present functional programming concepts and ideas in a readable form. I wanted to create a book that will teach you how to think functionally without using the usual shock therapy that people usually feel when seeing functional programming for the first time. There are already a couple of reviews that suggest I was quite successful:

  • Functional Programming for the Real World, by Tomas Petricek and Jon Skeet, has been a very helpful book for moving to F# from C#, as the authors do a fantastic job of helping to explain the differences between OOP and FP.
    James Black at Amazon.com
  • This book isn’t just a simple introduction to programming in F#; it’s an introductory text on functional programming covering the many reasons why it is time for this programming paradigm to finally be accepted by mainstream programmers. And it also contains much more...
    CliveT, Software Engineer at Red Gate Software
  • ... and there are many other great comments about the book at Manning book page.

Deal of the day (January 24)

Finally, here is one great news if you're interested in getting the book! Real-World Functional Programming is Manning's Deal of the Day this Sunday, January 24. On this day, the print book is available for $20 from the Manning website, with code dotd0124.

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Sunday, January 24, 2010

Accelerator and F# (IV.): Composing computations with quotations

In this article series, we're talking about the Accelerator project and I'm presenting an F# library that I implemented, which allows you to use Accelerator [references] in a more sophisticated way. We've seen two examples of using Accelerator directly (see also introduction and Game of Life). In the previous article I introduced my F# library for working with Accelerator. We've seen F# functions from the DataParallel module, we implemented an algorithm that rotates an image using these functions and finally, we've seen that we can take this ordinary F# code and run it using Accelerator. This is all possible thanks to F# quotations, which we can use to get an AST (a source code) of an F# function we wrote (if the function is marked in some special way).

Blurred photo of Prague

In this part of the series, we're going to look at working with quotations explicitly. We'll use meta-programming techniques to work with Accelerator. Meta-programming means writing programs that manipulate with other programs or pieces of code. This is exactly what we're going to do in this article. We'll write an F# function (running on CPU) that builds a program, which we'll then run using Accelerator.

This is quite interesting approach, which isn't possible when we call Accelerator methods as standard F# functions or .NET methods. The benefit is that we'll clearly see which parts of program run on CPU and what parts execute on GPU or using X64 multi-core target. We could also perform more complicated optimizations with the code (because this wouldn't affect the readability). Just for your reference, here is the list of articles in this series in case you missed some of them:

However, enough with theory and let's take a look at some code samples! This time, we'll implement blurring of an image (also called convolution). Another example how to write this in F# using Accelerator is Satnam Singh's blog post [4]. Our example will be different, because we'll write the code as standard F# program and then have it translated to Accelerator automatically using quotations. We'll also talk about the architecture of the library that we're using and look at some performance results.

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Tuesday, January 12, 2010