Crafting software

This is the 2nd part of my series on everyday functional programming.

Filtering

Suppose you have a collection of items and need to grab just a subset that match a certain criteria. Programming C# in an imperative style, you could use a for or foreach loop as follows:

Functional programming recognises this common scenario as a higher order function known as a Filter [3]; where you want to create a new list for every element that evaluates to true when a predicate function is applied to it.

In C#, filter is implemented as the LINQ Where(Func)) extension method, allowing one to write the following:

Apart from the obvious reduction in number of lines; notice how much clearer the intent of the filter is, and the many opportunities for error we have eliminated.

In Javascript we use the filter function:

Mapping

In many instances you find yourself wanting to perform the same operation on every item in an existing collection. In Javascript in an imperative style we might want to do the following

In functional programming this is know as the higher order function: Map [4] and in Javascript can be applied using a single statement map()

In C#, map is implemented by the LINQ Select(Func) extension method:

[1] – http://railspikes.com/2008/7/29/functional-loops-in-ruby-each-map-inject-select-and-for
[2] – http://msdn.microsoft.com/en-us/library/system.linq.enumerable.aspx
[3] – http://en.wikipedia.org/wiki/Filter_(higher-order_function)
[4] – http://en.wikipedia.org/wiki/Map_(higher-order_function)

This is the first of a series of blog posts where I will be exploring how functional programming techniques are useful in the daily life of a working “enterprise software” developer.

If you, like me, began programming in the 1990′s, then you will probably have started in a procedural programming style with simple task orientated scripts, and then progressed to an object oriented style for its better fit with event orientated GUI applications. As the software craftsmanship movement has grown over the past few years, you will have honed your S.O.L.I.D. OO skills; and focussed on making your code maintainable & testable/ed.

Whilst functional programming has an academic & computational background; functional elements have begun finding their way into mainstream languages. In this series of articles I’ll be concentrating on the two languages I am most familiar with – C# and Javascript – and how exploiting the functional programming features in these can help to make the code you and I write every day more expressive & maintainable, and in some instances dramatically more scalable.

I won’t be trying to explain the concepts behind functional programming – others have done an exellent job of that already; so I’ll just link to them [1] [2]. Rather, I’ll be curating practical examples where a functional style can be applied to everyday programming problems.

1. Higher order functions – simplifying loops
2. Implementing the strategy pattern without an explosion of classes
3. Side effect free functions – code that is easy to test & reuse

[1] – http://en.wikipedia.org/wiki/Functional_programming
[2] – http://www.defmacro.org/ramblings/fp.html