Friday, July 11, 2008

The Self Language

In my last post I said that I wasn't too impressed by Selfs idea of using prototypes. Well I've changed my mind. My initial complaint was a lack of structure. When you open the Self BareBones snapshot all you get is a graphical representation of a shell object and a waste bin object. You don't get to see all the classes in the image like you do with the Smalltalk Class Browser. There is no Class browser in Self because there aren't any Classes.

This doesn't mean there isn't structure though. If you get the lobby object you will notice a slot called globals, one called traits and one called mixins. As I mentioned in my last post traits are objects that are used to encapsulate shared behaviour (methods). Globals are a parent slot on lobby; inside globals are all the prototypical objects. Each prototype object has a trait object. The prototype holds state whilst the trait holds behaviour. So between the two you have the same structure as a Class. So you create new objects by copying prototypes which inherit shared behaviour through an associated trait object.

Since the traits slot is not a parent slot of lobby you must send the message 'traits' to access trait objects from the lobby. So 'traits list' gets you a reference to the list trait object and 'list' gets you the list prototype. Why is the lobby important? Well all new objects are created in the context of the lobby. So the lobby object acts like a global namespace.

My explanation makes it sound more awkward then it actually is in practice. The bottom line is that Self has a lot of structure, as much as Smalltalk in fact. The structure is just different and more granular. Working with this structure is actually very pleasant. You still think in terms of classes, but only after thinking about the object first. So with Self you create a prototypical instance of what you want then you refactor it into common shared parts (traits) and an instance part (prototype).

The traits are more or less Classes. Self objects support multiple parent slots, but by convention multiple inheritance is not used. Instead usually there is one parent trait and additional shared behaviour is achieved by adding mixins to additional parent slots.

I am beginning to agree with Dave Ungar, that the Self way of thinking about objects is more natural and more simple. What convinced me is the ease in which objects can be created:

( | x <- 100. y <- 200 |)

Is an object literal which you can create at the shell and get an instant graphical representation of. The graphical representation of an object in Self is called an Outliner which is basically an editor that allows you to view, add and modify slots on the associated object. The Outliner also has an evaluator, where you can type in messages and have them sent to the target object.

So in self you create objects by entering literal text, test them out by sending messages and extend them by adding new slots. This is all achieved with instant feedback. You then factor your objects into traits and prototypes by creating new objects and moving slots through drag-and-drop.

Is all this important? I'm not sure, but I think so. The fact that objects have a literal representation that includes their behaviour is quite interesting and I like the drag and drop refactoring. What I can say is that the Self approach is fun and feels more concrete, as if you are using real building blocks to create your program.

Would a Self approach lead to higher productivity? With a bunch of keyboard accelerators so that you didn't need to use the mouse much, then I think so. To me feedback is king, and Self offers plenty of feedback. I think that the Self approach also leads to a more exploratory programming style, which in my opinion is a good thing. Above all, manipulating objects as if they are 'real' is a lot of fun, which as got be be worth something in itself :)

1 comment:

Don Dwoske said...

Some of your readers may not know about which draws much inspiration from Self.