ASP.NET PDF Viewer using C#, VB/NET

let actualValue = Lazyforce lazyValue print_int actualValue print_newline() On the first line, you delay a simple expression for evaluation later The next line forces evaluation Then you print the value The value has been cached, so any side effects that take place when the value is computed will occur only the first time the lazy value is forced This is fairly easy to demonstrate In the next example, you create a lazy value that has a side effect when it is calculated: it writes to the console To show that this side effect takes place only once, you force the value twice, and it is plain to see from the result that writing to the console takes place only once #light let lazySideEffect = lazy ( let temp = 2 + 2 print_int temp print_newline() temp ) print_endline "Force value the first time: " let actualValue1 = Lazy.

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You probably don t want to only blink LEDs and play games with your I2C bus. Making the outputs useful requires adding circuitry that allows higher voltages and currents than the PCF8574 can handle. One of the easiest ways is to use a relay, and you can buy reed relays for example, Radio Shack #275232 or Digi-Key #306-1019-ND that operate directly off the feeble 25mA available from the output pin. Figure 13-19 is a schematic for an I2C interface with two relay outputs. Obviously, you could control all eight outputs in the same way. As you learned in 12, diodes D1 and D2 (1N4148) are necessary to protect the PCF8574 when the relays RL1 and RL2 are turned off.

force lazySideEffect print_endline "Force value the second time: " let actualValue2 = Lazyforce lazySideEffect The results of this example are as follows: Force value the first time: 4 Force value the second time: Laziness can also be useful when working with collections The idea of a lazy collection is that elements in the collection are calculated on demand Some collection types also cache the results of these calculations, so there is no need to recalculate elements F# provides the LazyList collection type, which caches computed results and is useful for functional programming and search The second lazy collection is the seq type, a shorthand for the BCL s IEnumerable type This plays a similar role to LazyList but does not cache computed results LazyList and seq values are created and manipulated using functions in the LazyList and Seq modules, respectively.

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Many other values are also compatible with the type seq; for example, all F# lists and arrays are compatible with this type, as are most other collection types The next example shows how to use the LazyList module Possibly its most important function, and probably the most difficult to understand, is unfold This function allows you to create a lazy list What makes it complicated is that you must provide a function that will be repeatedly evaluated to provide the elements of the list The function passed to LazyList unfold can take any type of parameter and must return an option type An option type is a union type that can be either None or Some(x), where x is a value of any type None is used to.

The active HTML page opens in read-only mode. In other words, you can search through the document, but you are unable to make changes to the document. The page shows the entire contents of the HTML file including any embedded scripts. You can open scripts that are not embedded directly in the HTML but are included as a separate .js file by using the Running Documents window (available via the View Running Documents menu), as shown in Figure 7-7. With the current document open, you can now start debugging. Select a line on which you d like to set a breakpoint. Place the cursor on the desired line, and then press F9 or click the Toggle Breakpoint button on the menu bar. The line should become highlighted, with a dot appearing in the left gutter of the window. Start the debugging sequence by performing the Figure 7-7. Finding the current action on the page that causes the script to be run. The Web page and any included script should pause during execution when the selected script files in the Running breakpoint is encountered. Documents window The script is now stopped, but what can you do with it The first simple task is to view the call stack. Click the Call Stack window on the menu bar to view the course of execution of the nested function calls. Double-clicking each item in the Call Stack list will bring you to that function call. If multiple threads are running, then each will have its own call stack. The most useful feature of Script Debugger is the ability to view and modify variable values at runtime. The Command Window is the portal for inspecting and modifying variable

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