I grew up drinking the Object Oriented Kool-Aid, which imbues the drinker with the idea that functional programming languages are arcana whose use and utility are limited to academia and programming language research.
Despite this I remained vaguely curious about functional programing (probably because I secretly suffer from “academic envy”), and every so often would do some informal research on cool functional programming language constructs; closures, continuations, monads, list comprehension, higher-order functions and pattern matching being good examples. I was eager to use the aforementioned in anger but I never had what I thought was the “killer app” for a predominantly functional language.
Note: I say “predominantly functional” because many modern functional programming languages are not purely functional. For example F# is a best-of-breed functional programming language, with object oriented features. In comparison C# is a best-of-breed object oriented language that has functional programming language features. F# and C# seem to be slowly converging.
Another reason that I did not make a serious foray into functional programming was the lack of good tools. I tried my hand at Standard ML, but since I had been so spoiled (in a good way) by Visual Studio, I could not bring myself to go back to a generic text editor and the command line. I was also not happy about the prospect of having to learn yet another set of APIs.
In 2005 Microsoft Research released F# 1.0 into the wild. F# is in the ML family of languages, so the little knowledge I had acquired playing with SML was applicable, and F# is a native .NET language, so I was able to leverage all of my existing knowledge of the .NET Framework. Initially the Visual Studio integration was very limited, but improved steadily with subsequent releases of F# and Visual Studio. When Visual Studio 2010 was released along with F# 2.0, F# finally became a bona fide 1st class .NET language. So with the release of Visual Studio 2010 I officially ran out of excuses for not learning and using functional programming.
I had played with F# before the VS 2010 release but my code always landed up looking very C#-like, with ref and mutable keywords all over the place. I miserably failed to embrace a functional style, and therefore missed the real magic offered by functional programming.
I recently wrote what I would consider my first “real” functional program using F#. I decided to port an InfoPath/SharePoint/Excel(VBA) application to .NET and provide a number of enhancements in the process. I also thought it was a good opportunity to use functional programming in anger, so I decided to write a couple of the modules in F# and embrace a functional style from the outset. This meant writing the code as a hierarchy of functions, avoiding side effects and mutable types unless necessitated by the use of existing non-functional .NET APIs, and making as much possible use of F# types, e.g. Lists, Tuples and Records, and F# language constructs, e.g. Active Patterns.
One of the modules that I wrote was a deserializer for InfoPath XML forms, which uses Partial Active Patterns to parse the XML. I will describe the implementation of the deserializer in detail in a future post.
I am not sure when it was that I achieved Functional Programming Satori; perhaps it was around the thousandth line of F# code; but it was profound and left me with a lingering love for F# and functional programming in general. There is something about functional code that is just so bloody elegant! I excitedly showed my F# code to a colleague, not because I wanted him to review it for correctness, but because I wanted him to appreciate the aesthetics of it! And that aesthetic appeal did not come at the price of performance; the resulting code performed as well as an imperative C# implementation that I had written as a benchmark.
I highly recommend learning and using F# to all software engineers actively developing on the .NET platform. For those developers who have mastered LINQ they will find that many of the concepts they have learned will translate directly to F#. You can find everything you need to know to get started at the Microsoft F# Developer Center.
For those developers who are developing in Java I recommend taking a look at Scala. It is a general purpose programming language that includes many functional programming elements, and targets the Java Virtual Machine. Scala is what Java might have become if it had not been hobbled by politics.
Be warned though, once you have learned functional programming you may never want to go back!