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A nice freeware program called nxtRICedit by Andreas Dreier lets you create and modify the graphics files used for the NXT display (see Figure 14-26). These files carry the RIC extension, and can be found in a subdirectory of the LEGO software. In fact, all RIC files in that directory will show up in the image selection list for the NXT-G Display block. Conveniently, the default directory for nxtRICedit is also that folder.

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It is possible to use the record types you met in 3 to simulate object-like behavior. This is because records can have fields that are functions, which you can use to simulate an object s methods. This technique was first invented before functional programming languages had object-oriented constructs as a way of performing tasks that lent themselves well to object-oriented programming. Some programmers still prefer it, because only the function s type (or as some prefer, its signature) is given in the record definition, so the implementation can easily be swapped without having to define a derived class as you would in objectoriented programming. I discuss this in greater detail in Object Expressions and again in Inheritance later in this chapter. Let s take a look at a simple example of records as objects. The next example defines a type, Shape, that has two members. The first member, reposition, is a function type that moves the shape, and the second member, draw, draws the shape. You use the function makeShape to create a new instance of the shape type. The makeShape function implements the reposition functionality for you; it does this by accepting the initPos parameter, which is then stored in a mutable ref cell, which is updated when the reposition function is called. This means the position of the shape is encapsulated, accessible only through the reposition member. Hiding values in this way is a common technique in F# programming. #light open System.Drawing type Shape = { reposition: Point -> unit; draw : unit -> unit } let makeShape initPos draw = let currPos = ref initPos in { reposition = (fun newPos -> currPos := newPos); draw = (fun () -> draw !currPos); }

Throughout this book, we ve given you a fair number of Ajax examples. And the astute observer may have noticed a fair amount of, well, duplicate code. For instance, how many times have you seen something like Listing 8-1 2 Listing 8-1. The Most Repeated Chunk of Code in This Book var xmlHttp; function createXMLHttpRequest() { if (window.ActiveXObject) { xmlHttp = new ActiveXObject("Microsoft.XMLHTTP"); } else if (window.XMLHttpRequest) { xmlHttp = new XMLHttpRequest(); } } Of course, in a production application, we d abstract this little routine. Actually, we d probably go further and create a special library that encapsulated the messy, repetitive aspects of Ajax. Then again, we might do a quick Google search and discover quite a few hits for Ajax frameworks. For a snapshot of available frameworks, check out Appendix B.

Figure 14-26. NxtRICedit by Andreas Dreier In addition to creating images, the program can convert photographic images to the RIC format. The NXT display is pure black and white with no grayscale, so contrast is a critical adjustment. Figure 14-27 shows part of the Import window where the contrast can be adjusted.

let circle initPos = makeShape initPos (fun pos -> printfn "Circle, with x = %i and y = %i" pos.X pos.Y) let square initPos = makeShape initPos (fun pos -> printfn "Square, with x = %i and y = %i" pos.X pos.Y) let point (x,y) = new Point(x,y) let shapes = [ circle (point (10,10)); square (point (30,30)) ] let moveShapes() = shapes |> List.iter (fun s -> s.draw()) let main() = moveShapes() shapes |> List.iter (fun s -> s.reposition (point (40,40))) moveShapes() main()