Thursday, October 29, 2009

Functional .Net : Currying

Currying is another functional technique that is possible to achieve with C#. The technique basically allows the rewriting of a function that takes in multiple arguments to one that takes in one argument and returns a function which may in turn take more arguments, the basic premise being able to build up composite function by splitting functions down and reducing the number of parameters dealt with. I will be honest and say that  I have found the language you use (C#, F#, Haskell etc) would be more influential to your predisposition  in using this technique, as it is with many of the function patterns and IMO C# does not lend itself nearly as well as F# for example. That being said it still can be done so lets look at a basic example. For starters currying is something that is not catered for explicitly out the box in C# but can easily be done using extension methods eg:

public static Func<TArg1, Func<TArg2, TResult>> Curry<TArg1, TArg2, TResult>(this Func<TArg1, TArg2, TResult> func)
return a1 => a2 => func(a1, a2);
public static Func<TArg1, Action<TArg2>> Curry<TArg1, TArg2>(this Action<TArg1, TArg2> action)
return a1 => a2 => action(a1, a2);

These extension method now allow you to take a 2 parameter delegate and split it into a one parameter argument return an Action or Func that take one argument. This can obviously be extended and helps facilitate the separation and composition of functions.

Now from my understanding of currying it is a specific form of partial application, being that currying splits its functions down to single argument delegates while partial application make no such claim, i.e. a three argument function be be reduced to a single argument function returning a 2 argument delegate. As this is purely academia I don't really care, the principle is the same.

A trivial example of currying (I'm lazy I stole it from Matt P and it is using the extension method above) :

Func<int, int, int> multiply = (x, y) => x * y;
var curriedMultiply = multiply.Curry();
var curriedMultiplyThree = curriedMultiply(3);
var curriedMultiplyResult = curriedMultiplyThree(15);
Console.WriteLine("Result of 3 * 15 = {0}", curriedMultiplyResult);

Unfortunately the verbosity of C# when approaching this style of coding very quickly begins to put me off.  The equivalent in F# is much more readable, but hey its what the language is strong at so it really should be a nicer experience. Either way its good to know the facilities are there if one day I do ever need to use it.

Basically the take away from this post is the extension methods at the top, with out these there will be no curry love.

Links forwarding (yeah this was a lazy post):

Tuesday, October 13, 2009

Functional .Net : Closures

One of the more commonly used functional techniques that can be used in C# is the use of Closures, a technique that if your are currently using lambdas, you may be using them inadvertently. My understanding of closure may be different to others as there seems to be so many subtlety different definitions especially when comparing languages. Anyway, in my mind the comments of Javascript closure best align with my understanding (

A closure is a delegate that has references to a variable not passed to it and in an scope outside the delegates immediate scope.

Like any delegate its definition and execution are not the same thing. You can define a closure and never use it or just call it later

A simple closure example I can think of is:

static void Main(string[] args)
var timesToRepeat = 100;
//Declare the Action
Action<string> print = text => //text (string) is the only parameter
//using varb declared outside of Action
for (int i = 0; i < timesToRepeat; i++)
timesToRepeat = 3;//Lets modify the variable
print("Hello!");//Call the action/evaluate the expression

Note that the timeToRepeat variable is declared outside of the declaration of the lambda statement. Think about this; the Action 'print' can be passed out side of this scope, it could be passed to another class which does not have visibility of the locally declared variable. The 'print' expression is bound to that variable declared outside of its scope. This obviously has ramification in terms of holding reference to that object. Please also note that the expression 'print', like all delegates is evaluated when it is called, not when it is declared; Stepping over the above code will not print when declaring the 'print' Action but at the last line when it is called. One last thing to note is that the variable timetoRepeat is modified after defining the print Action and this is carried when we call 'print' in the last line; "Hello!" is printed 3 times, not 100 times as the variable would imply when the closure was declared.

You may have been using closures with out knowing it. Javascript and the associated libraries like jQuery use this technique a lot, as do many open source library such as TopShelf, MassTransit etc.

Monday, October 12, 2009

Functional .Net : First Class Functions

One thing I notice in .Net is that many developers do not think of functions as first class citizens. I guess in the OO world classes or more appropriate the instance representations are the real hero's, however, in my mind, functions deserve much more appreciation than they perhaps get.

Since .Net 1.0 delegates have been around and I still think many developers do not fully understand how they work. I have previously made a post with regard to delegates showing how they can be used in a real world way to save code duplication here. I guess one of the first steps to being comfortable with functional programming is being comfortable with functions as first class citizens; The best way for a typical C# developer to do this is get comfortable with delegates. Before I continue on with my Functional programming journey I want others following with me to be on the same page. Please be sure you understand what a method & delegate are; I feel I describe them reasonably well on the previous mentioned post.

Functional .Net: The Beginning

Of late I have (along with a few colleagues and friends) started to make a bit more of a concerted effort to up skill in the area of functional programming. I admit that my knowledge is  of functional programming is high level (at best) although have in advertently been using several of the core concepts due to the language feature I am exposed by the C# language I use on a day to day basis.

What has spurred me on is the talks from Dr Erik Miejer on channel 9 (the first of 13 can be found here). The talks plan on tackling Functional programming by working through the benchmark Functional book: Programming in Haskell

I am keen to see how the series progresses, I am only up to episode 2 but am already seeing value, more in the "why" as opposed to the "how", which is fine for this early stage of my journey.

I also want a bit of commercial return on investment with relation to what I can do in my day to day job with functional programming. As I have mentioned C# actually handles several of the Functional paradigms (although perhaps not as elegantly as F# and the like) and thanks to .Net resident Functional voice-to-the-masses, a bunch of Functional programming samples in C# can be found here to download; Cheers Matt! Along with just raw C# code he has a bunch of Wiki links to highlight what each example is actually doing; you may be surprised that you are inadvertently using some of these techniques!

Any way I will keep you posted as to how I progress as I move forward on the beginning on what is hopefully a fruitful journey!