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CHAPTER 7 ENCAPSULATING AND PACKAGING YOUR CODE
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Later, this chapter explains how encapsulation applies when you re building assemblies, frameworks, and applications. In the extreme, you may even be ensuring that your code is secure when used in partial trust mode in other words, that it can t be inadvertently or deliberately used to achieve malicious results when used as a library by code that doesn t have full permissions. However, the most important kind of encapsulation is the day-to-day business of hiding the internal implementation details of functions, objects, types, and modules. The primary techniques used to do this are as follows: Local definitions Accessibility annotations Explicit signatures
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We cover the first two of these techniques next, and we cover explicit signatures in the Using Signature Types and Files section later in this chapter.
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Hiding Things with Local Definitions
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The easiest way to hide definitions is to make them local to expressions or constructed class definitions using inner let bindings. These aren t directly accessible from outside their scope. This technique is frequently used to hide state and other computed values inside the implementations of functions and objects. Let s begin with a simple example. Here is the definition of a function that incorporates a single item of encapsulated state: let generateTicket = let count = ref 0 (fun () -> incr count; !count) If you examine this definition, you see that the generateTicket function isn t defined immediately as a function but instead first declares a local element of state called count and then returns a function value that refers to this state. Each time the function value is called, count is incremented and dereferenced, but the reference cell is never published outside the function implementation and is thus encapsulated. Encapsulation through local definitions is a particularly powerful technique in F# when used in conjunction with object expressions. For example, Listing 7-1 shows the definition of an object interface type called IPeekPoke and a function that implements objects of this type using an object expression. Listing 7-1. Implementing Objects with Encapsulated State type IPeekPoke = abstract member Peek: unit -> int abstract member Poke: int -> unit let makeCounter initialState = let state = ref initialState { new IPeekPoke with member x.Poke(n) = state := !state + n member x.Peek() = !state } The type of the function Counter is as follows: val makeCounter : int -> IPeekPoke
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CHAPTER 7 ENCAPSULATING AND PACKAGING YOUR CODE
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As with the earlier generateTicket function, the internal state for each object generated by the makeCounter function is hidden and accessible only via the published Peek and Poke methods. The previous examples show how to combine let bindings with anonymous functions and object expressions. You saw in 6 how let bindings can also be used in constructed class types. For example, Listing 7-2 shows a constructed class type with private mutable state count and that publishes two methods: Next and Reset. Listing 7-2. A Type for Objects with Encapsulated State type TicketGenerator() = // Note: let bindings in a type definition are implicitly private to the object // being constructed. Members are implicitly public. let mutable count = 0 member x.Next() = count <- count + 1; count member x.Reset () = count <- 0 The variable count is implicitly private to the object being constructed and is hence hidden from outside consumers. By default, other F# definitions are public, which means they re accessible throughout their scope. Frequently, more than one item of state is hidden behind an encapsulation boundary. For example, the following code shows a function makeAverager that uses an object expression and two local elements of state, count and total, to implement an instance of the object interface type IStatistic: type IStatistic<'T,'U> = abstract Record : 'T -> unit abstract Value : 'U let makeAverager(toFloat: 'T -> float) = let count = ref 0 let total = ref 0.0 { new IStatistic<'a,float> with member stat.Record(x) = incr count; total := !total + toFloat x member stat.Value = (!total / float !count) } The inferred types here are as follows: type IStatistic<'T,'U> = abstract Record : 'T -> unit abstract Value: 'U val makeAverager : ('T -> float) -> IStatistic<'T,float> The internal state is held in values count and total and is, once again, encapsulated.
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