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bject-oriented programming is the third major programming paradigm. At its heart, object-oriented programming has a few simple ideas, some of which you ve already encountered. Possibly the most important idea is that the implementations and state should be encapsulated, that is, hidden behind well-defined boundaries. This makes the structure of a program easier to manage. In F#, things are hidden by using signatures for modules and type definitions and also by simply defining them locally to an expression or class construction (you ll see examples of both in this chapter). The second idea is that you can implement abstract entities in multiple ways. In OOP this is known as polymorphism. You ve met a number of simple abstract entities already, such as function types. A function type is abstract because a function with specific type can be implemented in many different ways; for example, the function type int -> int can be implemented as a function that increments the given parameter, a function that decrements the parameter, or any one of millions of mathematical sequences. Other abstract entities can be built out of existing abstract components such as the interface types defined in the .NET BCL. More sophisticated abstract entities are modeled using user-defined interface types. Interface types have the advantage that they can be arranged hierarchically; this is called interface inheritance. For example, the .NET BCL includes a hierarchical classification of collection types, available in the System.Collections and System.Collections.Generic namespaces. In OOP you can sometimes arrange implementation fragments hierarchically. This is called implementation inheritance. This tends to be less important in F# programming because of the flexibility that functional programming provides for defining and sharing implementation fragments. However, it is significant for domains such as graphical user interface (GUI) programming.

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Casting is a way of explicitly altering the static type of a value by either throwing information away, upcasting, or rediscovering it, downcasting. In F#, upcasts and downcasts have their own operators. The type hierarchy starts with obj (or System.Object) at the top and all its descendants below it. An upcast moves a type up the hierarchy, and a downcast moves a type down the hierarchy. Upcasts change a value s static type to one of its ancestor types. This is a safe operation since the compiler can always tell whether this will work because the compiler always knows all the ancestors of a type so is able to work out through static analysis whether an upcast will

Figure 14-17. Track electrical connections The LEGO train track ties have two rows of regular LEGO studs that you can use to mount the sensor using plates, as can be seen in Figure 14-18. You must set the sensor back far enough to avoid being hit by the train as it goes by, but close enough for the reflected light from the train car to be registered by the NXT.

This book is just a starting point we can t cover everything having to do with Ajax (if we wanted it to be shorter than, say, a printed copy of the federal budget, that is). In addition, this space is rapidly evolving. Luckily, some excellent resources can help keep you up-to-date on Ajax. The first place we start nearly every day is Ajaxian (www.ajaxian.com). Billed as Ben and Dion s AJaX Mission, Ajaxian spots pretty much everything that s new in the Ajax world. (It s not unusual for Ben or Dion to post several times a day.) Industry pundits Ben Galbraith and Dion Almaer, both frequent speakers at conferences such as No Fluff Just Stuff, maintain Ajaxian.com. Lest you doubt their credentials, both were attendees of the inaugural O Reilly/Adaptive Path Ajax Summit, and having seen both present in person, we vouch for their knowledge and true passion for the field. Without a doubt, Ajaxian.com is the first stop for news and events in the Ajax space.

be successful An upcast is represented by a colon followed by the greater-than sign (:>) The following code shows an example of using an upcast to convert a string to an obj: #light let myObject = ("This is a string" :> obj) Generally, upcasts are required when defining collections that contain disparate types If an upcast is not used, the compiler will infer that the collection has the type of the first element and give a compile error if elements of other types are placed in the collection The next example demonstrates how to create an array of controls, a pretty common task when working with WinForms Notice that all the individual controls are upcast to their common base class, Control #light open SystemWindows.

An NXT-G program to control a train station is shown in Figure 14-19. The train automatically stops at the station long enough to pick up passengers and then pulls away. Power is supplied to the train motor till the Light Sensor or Color Sensor detects the reflection from the locomotive. It then cuts the power to make the engine stop. It waits one second at the station and then pulls away. The on time in the last Motor block guarantees that the train will be clear of the station before the program loops back to the beginning. If the train were on a circular track, it would stop every time it came around to the station.