ASP.NET Web PDF Document Viewer/Editor Control Library

How many times have you written some JavaScript validation logic While the edit might be simple in Java or C#, the lack of decent debuggers, combined with JavaScript s weak typing, can make writing them in JavaScript a real pain and error prone How often do these client-side validation rules duplicate edits on the server Using XHR, you can make a call to the server and fire one set of validation rules These rules can be far richer and more complex than anything you would write in JavaScript, and you have the full power of debuggers and integrated development environments (IDEs) We can hear some of you now: I ve been doing that for years with IFRAMES or hidden frames We ve even used this particular technique as a way to post or refresh parts of a page instead of the entire browser, and truth be told, it works.

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This example shows a mutable record in action. A type, couple, is defined where the field him is mutable but the field her is not. Next, an instance of couple is initialized, and then you change the value of him many times, each time displaying the results. I should note that the mutable keyword applies per field, so any attempt to update a field that is not mutable will result in a compile error; for example, the next example will fail on the second line: #light theCouple.her <- "Sybil Williams"; print_any theCouple When attempting to compile this program, you ll get the following error message: prog.fs(2,4): error: FS0005: This field is not mutable

Figure 13-24 shows just the carriage assembly for a little scanner built using the 35DBM10B2U-L linear stepper motor. The lead screw of the stepper is prevented from turning because it has been screwed into the beam end of the two triangle plates that serve as the slider. As the lead screw is pulled (or pushed) through the stepper, these plates slide back and forth and are held straight by two long axles. The movement of the slider is very precise because the total travel is about 2 inches (5cm) and takes more than 1,600 steps to traverse.

The ref type is a simple way for a program to use mutable state, that is, values that change over time. The ref type is just a record type with a single mutable field that is defined in the F# libraries. Some operators are defined to make accessing and updating the field as straightforward as possible. F# s definition of the ref type uses type parameterization, a concept introduced in the previous chapter, so although the value of the ref type can be of any type, you cannot change the type of the value once you have created an instance of the value.

However, many would consider this approach a hack to get around XHR s original lack of cross-browser support The XHR object that is the heart of Ajax is truly designed to allow asynchronous retrieval of arbitrary data from the server As we ve discussed, traditional Web applications follow a request/response paradigm Without Ajax, the entire page (or with IFRAMEs, parts of the page) is reloaded with each request The previously viewed page is reflected in the browser s history stack (though if IFRAMEs are used, clicking the back button doesn t always result in what the user expects) However, requests made with XHR are not recorded in the browser s history This too can pose an issue if your users are familiar with using the back button to navigate within your Web application..

Figure 13-24. 35DBM10B2U-L with linear carriage Figure 13-25 graphically shows the voltage pattern needed to produce continuous motion. The coils are in pairs: yellow/black and orange/brown. The members of a pair are never turned on at the same time because this is a unipolar device. It takes four steps before the pattern repeats, and the four values that need to be sent to the PCF8574 are shown at the bottom of the figure.

Creating a new instance of the ref type is easy; you use the keyword ref followed by whatever item represents the value of ref. The next example is the compiler s output (using the i option, which shows that the type of phrase is string ref, meaning a reference type that can contain only strings): let phrase = ref "Inconsistency"

execution of the code at line 11. When the future breakpoint is encountered, the orange F icon will change to a red B icon, as shown in Figure 7-15.