For a long time the “Hello, World!” program was the first app a programmer tried with a new language or situation. It’s a quick way to test the general operation of things.

When it comes to object-oriented languages, an object-oriented first program needs a simple object. I’ve long thought that the “XY Point” made an outstanding “Hello, World!” object. It’s basically very simple, so it’s easy to do. And you can extend on it nicely once you get your Point working. You can extend a 2D Point to a 3D Point, or use Points to create Shapes.

This article is about how you create a Point Model in BOOL.

We start by defining a (2D) Point Model:

**2dPoint . *number x = 0 . *number y = 0 . . @@reset:: . << ! . . set: !x 0 ; . . set: !y 0 ; . . @@set:: . >> *number x . >> *number y . << ! . . set: !x x ; . . set: !y y ; . . @@move:: . >> *number x . >> *number y . << ! . . set: !x add: !x x ; . . set: !y add: !y y ;

The code above defines a new Model, called `*2dPoint`

. This illustrates an interesting feature of BOOL: Model names can start with a digit! (Model names are always prefixed with at least one asterisk, so there is no risk of mistaking them for a number.)

This Model has two data members, both instances of the `*number`

Model (which is a generic numeric type). I could also have used the `*int`

or `*real`

Models for a specific binary representation. By default, a new `2dPoint`

Object has both data members initialized to zero.

The Model also has three Model Actions (methods): `@reset`

, `@set`

and `@move`

. Since these are Model Actions, they are automatically bound to the `reset:`

, `set:`

and `move:`

Messages, respectively. The first takes no parameters, the other two both take an **x** and **y** `*number`

object. All three return the Object itself.

Now we can create some *2dPoint instances:

@@2dPoint-Demo . *2dPoint p1 = 10,10 --create new point at [10,10]. *2dPoint p2 = p1 --create a duplicate of p1. move: p1 3,6 ; --move p1 to [3,6]. reset: p2 ; --reset p2 (back to [0,0])

It might be nice to get back a distance between two points, so let’s say we’d added a Model Action for that (the fragment below could appear anywhere before the one directly above):

**2dPoint@@distance:: . >> *2dPoint p . << *number h . . *number x2 = sqr:sub:!x [x]p ;; . . *number y2 = sqr:sub:!y [y]p ;; . . set:h sqrt:add: x2 y2 ;;

Now we could continue with the `@2dPoint-Demo`

code:

. *number dist . set:dist distance:p1 p2 ;; . print: "Distance is: %n",dist ;

I realize most of this seems fairly opaque at this point. It will all be made clear over time!