Saturday, October 18, 2008

Breaking Down The Code


Last week we walked through creating our first F# application using TDD (Test Driven Development). Today we will break down that first example to identify what each of our commands are doing.

Here's a review of the extremely simple application that we wrote:

public void CanCalculateNumberSquared()
int result = FSharpSample.CalculateNumberSquared(2);
Assert.AreEqual(result, 4);


let CalculateNumberSquared x = x*x;;

Breaking Down The Code:

So what does this all mean?

The #light (pronounced "Hash" light) compiler directive provides various features that simplify the F# language. It also allows compatibility with the language OCaml. At the time of this writing, most experts recommend the use of this directive.

The "let" keyword allows the programmer to associate any value or formulate to an "Identifier". Here's how Robert Pickering describes identifiers in his book "Foundations of F#". "To most people coming from an imperative programming background, this will look like a variable assignment. There are a lot of similarities, but there are key differences. In pure functional programming, once a value is assigned to an identifier, it never changes. This is why I will refer to them throughout this book as identifiers and not variables." If you have not read this book, I strongly recommend it.

In our example, we are setting up an identifier named "CalculateNumberSquared", specifying a parameter named "x", and finally providing instructions on what to do with that parameter. This identifier can then be passed around to other identifiers and/or functions.

This ability to pass around functions and identifiers is one of the great features in F#. While it may take some time to get use to this concept, you will soon wonder how you ever got along without it.

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