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Defining Partially Implemented Class Types
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In this chapter, you ve seen how to define concrete types, such as Vector2D in Listings 6-2 and 6-3, and you ve seen how to define object interface types, such as IShape in Listing 6-5. Sometimes it s useful to define types that are halfway between these types: partially concrete types. Partially implemented types are class types that also have abstract members, some of which may be unimplemented and some of which may have default implementations. For example, consider the following class:
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CHAPTER 6 WORKING WITH OBJECTS AND MODULES
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/// A type whose members are partially implemented [<AbstractClass>] type TextOutputSink() = abstract WriteChar : char -> unit abstract WriteString : string -> unit default x.WriteString s = s |> String.iter x.WriteChar This class defines two abstract members, WriteChar and WriteString, but gives a default implementation for WriteString in terms of WriteChar. Because WriteChar isn t yet implemented, you can t create an instance of this type directly; unlike other concrete types, partially implemented types still need to be implemented. One way to do this is to complete the implementation via an object expression. For example: { new TextOutputSink() with member x.WriteChar c = System.Console.Write(c) }
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Using Partially Implemented Types via Delegation
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This section covers how you can use partially implemented types to build complete objects. One approach is to instantiate one or more partially implemented types to put together a complete concrete type. This is often done via delegation to an instantiation of the partially concrete type; for example, the following example creates a private, internal TextOutputSink object whose implementation of WriteChar counts the number of characters written through that object. You use this object to build the HtmlWriter object that publishes three methods specific to the process of writing a particular format: /// A type which uses a TextOutputSink internally type HtmlWriter() = let mutable count = 0 let sink = { new TextOutputSink() with member x.WriteChar c = count <- count + 1; System.Console.Write c } member member member member x.CharCount = count x.WriteHeader() = sink.WriteString("<html>") x.WriteFooter() = sink.WriteString("</html>") x.WriteString(s) = sink.WriteString(s)
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Using Partially Implemented Types via Implementation Inheritance
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Another technique to use partially implemented types is called implementation inheritance, which is widely used in OO languages despite being a somewhat awkward technique. Implementation inheritance tends to be much less significant in F# because it comes with major drawbacks: Implementation inheritance takes base objects and makes them more complex. This is against the spirit of functional programming, where the aim is to build simple, composable abstractions. Functional programming, object expressions, and delegation tend to provide good alternative techniques for defining, sharing, and combining implementation fragments.
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CHAPTER 6 WORKING WITH OBJECTS AND MODULES
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Implementation hierarchies tend to leak across API boundaries, revealing how objects are implemented rather than how they can be used and composed. Implementation hierarchies are often fragile in response to minor changes in program specification.
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If implementation inheritance is used, you should in many cases consider making all implementing classes private or hiding all implementing classes behind a signature. For example, the Microsoft.FSharp.Collections.Seq module provides many implementations of the seq<'T> interface but exposes no implementation inheritance. Nevertheless, hierarchies of classes are important in domains such as GUI programming, and the technique is used heavily by .NET libraries written in other .NET languages. For example, System.Windows.Forms.Control, System.Windows.Forms.UserControl, and System.Windows.Forms.RichTextBox are part of a hierarchy of visual GUI elements. Should you want to write new controls, then you must understand this implementation hierarchy and how to extend it. 11 shows a complete example of extending UserControl. However, even in this domain, implementation inheritance is often less important than you may think, because these controls can often be configured in powerful and interesting ways by adding function callbacks to events associated with the controls. Here is a simple example of applying the technique to instantiate and extend the partially implemented type TextOutputSink: /// An implementation of TextOutputSink, counting the number of bytes written type CountingOutputSinkByInheritance() = inherit TextOutputSink() let mutable count = 0 member sink.Count = count default sink.WriteChar c = count <- count + 1; System.Console.Write c The keywords override and default can be used interchangeably; both indicate that an implementation is being given for an abstract member. By convention, override is used when giving implementations for abstract members in inherited types that already have implementations, and default is used for implementations of abstract members that didn t previously have implementations. Implementations are also free to override and modify default implementations such as the implementation of WriteString provided by TextOutputSink. Here is an example: { new TextOutputSink() with member sink.WriteChar c = System.Console.Write c member sink.WriteString s = System.Console.Write s } You can also build new partially implemented types by extending existing partially implemented types. The following example takes the TextOutputSink type from the previous section and adds two abstract members called WriteByte and WriteBytes, adds a default implementation for WriteBytes, adds an initial implementation for WriteChar, and overrides the implementation of WriteString to use WriteBytes. The implementations of WriteChar and WriteString use the .NET functionality to convert the Unicode characters and strings to bytes under System.Text.UTF8Encoding, documented in the .NET Framework class libraries:
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