.NET/Java PDF, Tiff, Barcode SDK Library

Figure 13-20. Relay output on solderless breadboard Finally, Figure 13-21 shows how to use one of the relays in the project to control an old RCX-style motor. The LEGO battery box has a built-in switch that can be used to turn the power off when the PCF8574 is not under program control.

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After inspecting and possibly modifying variable values, you can continue stepping through the script, line by line. The F8 key or Step Into toolbar button will step into the next line of code, stepping into any functions that are called; the Shift+F8 key or Step Over toolbar button will execute the next line of code without stepping into any called functions. The F5 key or Run button will run the rest of the script, stopping only when another breakpoint is encountered or the script completes. Microsoft Script Debugger is a simple but useful tool for debugging client-side scripts. Because of the primitive interface of the Command Window, the Script Debugger is probably best left for those working in an all Internet Explorer environment or even those who are using VBScript instead of JavaScript. Still, it s another tool in the developer s toolbox that can provide a significant amount of help when needed.

#light let fibs = Seq.unfold (fun (n0, n1) -> Some(n0, (n1, n0 + n1))) (1I,1I) let first20 = Seq.take 20 fibs print_any first20 The results of this example are as follows: [1I; 1I; 2I; 3I; 5I; 8I; 13I; 21I; 34I; 55I; 89I; 144I; 233I; 377I; 610I; 987I; 1597I; 2584I; 4181I; 6765I] These examples are too simple to really demonstrate the power of lazy evaluation. You ll look at lazy evaluation again in several places in this book, notably in 7, where you ll see how to use lazy evaluation in user-interface programming to make user interfaces more responsive by ensuring computations do not happen until really needed.

Figure 13-21. Relay output controlling an RCX-style stepper motor Stepper motors turn in small controlled steps in which a typical step is fewer than 10 degrees of rotation. Accurate motion is achieved by making only a precise number of steps. Stepper motors are available with a shaft like most motors for rotary output or a threaded lead screw that creates linear motion. There are bipolar and unipolar coil configurations, but we will pursue only unipolar types here because they are the easiest to drive. The PCF8574 can drive LEDs or small low-voltage relays directly, but stepper motors need something a little more substantial. The ULN2003A (see Figure 13-22) is an array of seven inverting amplifiers that can switch up to 50V. Inverting means that when the input to the amplifier is 1, its output will be zero. One nice thing about the ULN2003A is that the diode needed with inductive loads like motors or relays is built right in.

Summary

Venkman is the code name for the JavaScript debugging environment available for Mozillabased browsers such as Firefox. Venkman is available as an extension for these browsers; you can install it from www.hacksrus.com/~ginda/venkman/. Venkman development started in April 2001 by Robert Ginda. Venkman is based on the Mozilla JavaScript debugging API known as js/jsd. The js/jsd API formed the basis of the Netscape JavaScript Debugger 1.1 that was available for the 4.x series of Netscape browsers. Once you ve installed it, you can start Venkman from the Tools JavaScript Debugger menu item on Firefox s main menu bar. Figure 7-9 shows the default layout for Venkman.

In this chapter, you looked at the major functional programming constructs in F#. This is the core of the language, and I hope you ve developed a good feel for how to approach writing algorithms and handling data in F#. The next chapter will cover imperative programming, and you ll see how to mix functional and imperative programming techniques to handle tasks such as input/output (I/O).