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CHAPTER 6 WORKING WITH OBJECTS AND MODULES
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> LabelInfo (text="Hello World");; val it : LabelInfo = {Font = [Font: Name=Microsoft Sans Serif, Size=12]; Text = "Hello World"} > LabelInfo("Goodbye Lenin");; val it : LabelInfo = {Font = [Font: Name=Microsoft Sans Serif, Size=12];
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Text = "Goodbye Lenin"}
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> LabelInfo(font=new Font(FontFamily.GenericMonospace,36.0f), text="Imagine");; val it : LabelInfo = {Font = [Font: Name=Courier New, Size=36]; Text = "Imagine"} Optional arguments must always appear last in the set of arguments accepted by a method. They are usually used as named arguments by callers. The implementation of LabelInfo uses the F# library function defaultArg, which is a useful way to specify simple default values for optional arguments. Its type is as follows:
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val defaultArg : 'a option -> 'a -> 'a
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Note The second argument given to the defaultArg function is evaluated before the function is called. This means you should take care that this argument is not expensive to compute and doesn t need to be disposed. In the previous example, we used a match expression to specify the default for the font argument for this reason.
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Using Optional Property Settings
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Throughout this book we have been using a second technique to specify configuration parameters when creating objects, which is using initial property settings for objects. For example, in 2 we used the following code: open System.Windows.Forms let form = new Form(Visible=true,TopMost=true,Text="Welcome to F#") The constructor for the System.Windows.Forms.Form class takes no arguments, so in this case the named arguments actually indicate post-hoc set operations for the given properties. The code is shorthand for this:
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CH A PT ER 6 WO RKI NG WI TH O BJEC TS AN D M OD ULES
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open System.Windows.Forms let form = let tmp = new Form() tmp.Visible <- true tmp.TopMost <- true tmp.Text <- "Welcome to F#" tmp The F# compiler interprets unused named arguments as calls that set properties of the returned object. This technique is widely used for mutable objects that evolve over time such as graphical components, since it greatly reduces the number of optional arguments that need to be plumbed around. Here s how to define a version of the LabelInfo type used earlier that is configurable by optional property settings: open System.Drawing type LabelInfoWithPropertySetting() = let mutable text = "" // the default let mutable font = new Font(FontFamily.GenericSansSerif,12.0f) member x.Text with get() = text and set(v) = text <- v member x.Font with get() = font and set(v) = font <- v > LabelInfoWithPropertySetting(Text="Hello World");; val it : LabelInfo = {Font = [Font: Name=Microsoft Sans Serif, Size=12]; Text = "Hello World"} We use this technique in 11 when we show how to define a Windows Forms control with configurable properties. We cover mutable objects in more detail in the Defining Object Types with Mutable State section later in this chapter.
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Adding Method Overloading
.NET APIs and other OO design frameworks frequently use a notational device called method overloading. This means a type can support multiple methods with the same name, and uses of methods are distinguished by name, number of arguments, and argument types. For example, the System.Console.WriteLine method of .NET has 19 overloads! Method overloading is used relatively rarely in F#-authored classes, partly because optional arguments and mutable property setters tend to make it less necessary. However, method overloading is still permitted in F#. First, methods can easily be overloaded by the number of arguments. For example, Listing 6-3 shows a concrete type representing an interval of numbers on the number line. It includes two methods called Span, one taking a pair of intervals and the other taking an arbitrary collection of intervals. The overloading is resolved simply according to argument count.
CHAPTER 6 WORKING WITH OBJECTS AND MODULES
Listing 6-3. A Vector2D Type with Length Precomputation /// Interval(lo,hi) represents the range of numbers from lo to hi, /// but not including either lo or hi. type Interval(lo,hi) = member r.Lo = lo member r.Hi = hi member r.IsEmpty = hi <= lo member r.Contains(v) = lo < v && v < hi static member Empty = Interval(0.0,0.0) /// Return the smallest interval that covers both the intervals /// This method is overloaded. static member Span(r1:Interval,r2:Interval) = if r1.IsEmpty then r2 else if r2.IsEmpty then r1 else Interval(min r1.Lo r2.Lo,max r1.Hi r2.Hi) /// Return the smallest interval that covers all the intervals /// This method is overloaded. static member Span(ranges: #seq<Interval>) = Seq.fold (fun r1 r2 -> Interval.Span(r1,r2)) Interval.Empty ranges Second, multiple methods can also have the same number of arguments and be overloaded by type. One of the most common examples is providing multiple implementations of overloaded operators on the same type. The following example shows a Point type that supports two subtraction operations, one subtracting a Point from a Point to give a Vector and one subtracting a Vector from a Point to give a Point: type Vector = { DX:float; DY:float } member v.Length = sqrt(v.DX*v.DX+v.DY*v.DY) type Point = { X:float; Y:float } [<OverloadID("SubtractPointPoint")>] static member (-) (p1:Point,p2:Point) = { DX=p1.X-p2.X; DY=p1.Y-p2.Y } [<OverloadID("subtractPointVector")>] static member (-) (p:Point,v:Vector) = { X=p.X-v.DX; Y=p.Y-v.DY } The version of F# at the time of writing (1.9.2.9) asks for a little help here. You should give a full type signature for each overload (by specifying the types for all arguments) and also annotate each overload with an OverloadID attribute, thus giving a different name for each overload. The OverloadID name is used internally by the F# compiler but should match the OverloadID used in the signature of the type should you give one (see 7 for more details
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