ASP.NET Web PDF Document Viewer/Editor Control Library

Identifiers are the way you give names to values in F# so you can refer to them later in a program. You define an identifier using the keyword let followed by the name of the identifier, an equals sign, and an expression that specifies the value to which the identifier refers. An expression is any piece of code that represents a computation that will return a value. The following expression shows a value being assigned to an identifier: let x = 42 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. You will see in the Scope section later in this chapter that, under some circumstances, you can redefine identifiers and that in imperative programming in F#, in some circumstances, the value of an identifier can change. An identifier can refer to either a value or a function, and since F# functions are really values in their own right, this is hardly surprising. (I discuss this relationship in detail in the Functions and Values section later in this chapter.) This means F# has no real concept of a function name or parameter name; they are all just identifiers. You write a function definition the same way as a value identifier, except a function has two or more identifiers between the let keyword and the equals sign, as follows: let raisePowerTwo x = x ** 2.0 The first identifier is the name of the function, raisePowerTwo, and the identifier that follows it is the name of the function s parameter, x.

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startUpdateStockQuoteInterval function. If the check box becomes unchecked in response to a user s click, then the automatic refreshing is disabled by calling the clearStockTickerUpdateInterval function. The implementation for automatically refreshing the weather forecast and headline news components is similar to what was described for the stock quote component. As you can see, the automatic refreshing is not terribly difficult to implement. The key is breaking tasks out into small, reusable functions and then building on top of those reusable functions. The entire file is less than 40 lines long. Better yet, thanks to the Taconite framework, only five lines of JavaScript are necessary to perform the Ajax request!

Bring up the View menu on the NXT and select the Touch Sensor on Port 1 using this sequence: NXT Menu View Touch Port 1. It should display the value 0, as in Figure 4-2. Now touch the ends of the wires together. Somehow the NXT knows you re doing that, and it changes the display to a 1, as in Figure 4-3. We know it isn t rocket science, but we can make a little game out of this.

module definition, you can nest submodules. The following code defines three submodules, FirstModule, SecondModule, and ThirdModule. ThirdModule is nested within SecondModule. #light module FirstModule = let n = 1 module SecondModule = let n = 2 module ThirdModule = let n = 3 Note that different submodules can contain the same identifiers without any problems. Modules affect the scope of an identifier. To access an identifier outside of its module, you need to qualify it with the module name so there is no ambiguity between identifiers in different modules. In the previous example, the identifier n is defined in all three modules. The following example shows how to access the identifier n specific to each of the modules: let x = FirstModule.n let y = SecondModule.n let z = SecondModule.ThirdModule.n A module will be compiled into a .NET class, with the values becoming methods and fields within that class. You can find more details about what an F# module looks like to other .NET programming languages in Calling F# Libraries from C# in 13.

Namespaces help organize your code hierarchically. To help keep module names unique across assemblies, the module name is qualified with a namespace name, which is just a character string with parts separated by dots. For example, F# provides a module named List, and the .NET BCL provides a class named List. There is no name conflict, because the F# module is in namespace Microsoft.FSharp, and the BCL class is in namespace System.Collections.Generic. It s important that namespace names be unique. The most popular convention is to start namespace names with the name of a company or organization, followed by a specific name that indicates functionality. Although it s not obligatory to do this, the convention is so widely followed that if you intend to distribute your code, especially in the form of a class library, then you should adopt it too.