.NET/ASP.NET/C#/VB.NET PDF Document SDK

embed functions within data structures so F# expressions can become part of your language, usually as an action in response to some particular condition of the language. You ve already seen a great example of this style of programming in 7. There you looked at a module that provides a simple way to create a command-line argument processor. It is simple because it allows the user to specify a data structure, such as the one shown here, that describes what the arguments should be without really having to think about how they will be parsed: let argList = [ ("-set", Arg.Set myFlag, "Sets the value myFlag"); ("-clear", Arg.Clear myFlag, "Clears the value myFlag"); ("-str_val", Arg.String(fun x -> myString := x), "Sets the value myString"); ("-int_val", Arg.Int(fun x -> myInt := x), "Sets the value myInt"); ("-float_val", Arg.Float(fun x -> myFloat := x), "Sets the value myFloat") ] I am particularly fond of this kind of DSL because I think it makes it really clear what arguments the program is expecting and what processing should take place if that argument is received. The fact that the help text is also stored in the structure serves a double purpose; it allows the function processing command-line arguments to automatically print out a help message if anything goes wrong, and it also reminds the programmer what the argument is in case they forget. I also like this method of creating a command-line interpreter because I have written several command-line interpreters in imperative languages, and it is not a satisfying experience you end up having to write lots of code to detail how your command line should be broken up. If you are writing it in .NET, then you usually spend way too much time calling the string type s IndexOf and Substring methods.

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Figure 5-31. Homebrew Light Sensor step 4 Run the connecting wires through the hole in the Technic right-angle beam, as shown in Figure 5-32. Using a friction pin and a blue axle pin, sandwich the photocell with a three-hole beam. This beam holds the LDR in place and makes the light sensitivity of the sensor more directional.

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WinForms are based on the System.Windows.Forms.Form class. By creating an instance of this class, you essentially create a new window. You must then create an event loop, a way of ensuring user interactions with the window are responded to. You do this by calling the System.Windows.Application.Run method and passing it the form object you have created. You can control the look of the form by setting its properties and calling its methods. The following example demonstrates this: #light open System.Drawing open System.Windows.Forms let form = new Form(BackColor = Color.Purple, Text = "Introducing WinForms") Application.Run(form) This example will not work with F# interactive, fsi, because you cannot start an event loop from within fsi. So to work with forms in fsi, you simply call the form s Show method or set the form s Visible property to true. This example shows the second technique:

Figure 5-32. Homebrew Light Sensor assembly Your finished Light Sensor should look like Figure 5-33. When you re making more than one of these, it s a good idea to reverse the direction of the right-angle beam. That way you end up with two symmetrical sensors.

So far you ve seen how you can use Ajax techniques to send requests to the server and the various ways the client can parse the server s response. The only thing missing in the previous examples is that you re not sending any data as part of the request to the server. For the most part, sending a request to the server without any request parameters doesn t do much good. Without any request parameters, the server has no contextual data by which to create a personalized response for the client, and in essence the server will send the identical response to every client. Unlocking the real power of Ajax techniques requires that you send some contextual data to the server. Imagine an input form that includes a section for entering mailing addresses. You can use Ajax techniques to prepopulate the name of the city that corresponds to the ZIP code entered by the user. Of course, to look up the ZIP code s city, the server needs to know the ZIP code entered by the user. Somehow you need to pass the ZIP code value entered by the user to the server. Fortunately, the XMLHttpRequest object works much the same as the old HTTP techniques you re used to working with: GET and POST. The GET method passes the value as name/value pairs as part of the request URL. The end of the resource URL ends with a question mark ( ), and after the question mark are the name/ value pairs. The name/value pairs are in the form name=value, and they are separated by an ampersand (&). The following line is an example of a GET request. The request is sending two parameters, firstName and middleName, to the application named yourApp on the server named localhost.

> #light open System.Drawing open System.Windows.Forms let form = new Form(BackColor=Color.Purple, Text="Introducing WinForms", Visible=true);; Either way, you have the advantage that you can dynamically interact with your form object. For example: > form.Text <- "Dynamic !!!";; When working with WinForms, you can take one of two approaches: drawing forms yourself or using controls to build them. First you ll look at drawing your own forms, and then you ll move on to using controls.