The class definition message

Q: What is a class definition in Smalltalk?

A: Just another message send!

I love that simplicity… We don’t need control structures because we send messages and as we have an abstraction that represents some piece of code everything is happiness (yeah… blocks are cool stuff).

But still knowing it I go to the System Browser… I see the class definition and I don’t think that is another common message send, in fact when I realize that I can write that “magic code” in a workspace and have the same effect (creating the class) and see it in the system browser I said wow! After a while I thought “so lame… of course is just a message send”.

But however I go to the class definition again select an instance variable and I want to apply the rename refactoring… so… I ask for suggestions… and I got… nothing… just selecting a string and that confused me and then I realize, ok it’s just a message send and is just a string as argument so… that’s logic. The thing is that is not true that is just a message send… because for some reason we expect more in some points, by example when we are in the class definition…

So I did some prototypes for changing the representation and meanwhile I change my mind in the process.

1. The class definition is not represented as a message node

I was radical… I said: A Class Definition is always a class definition, so I changed the parser and when parsing I tried to parse a class definition
The prototype for this can be found in:

MCHttpRepository
location: ‘http://smalltalkhub.com/mc/gisela/ParsingClassDefinition/main’
user: ”
password: ”

I started with some issues here… The main one is:
How do I know I’m in a class definition, in the prototype you will see a very “innocent” implementation for realizing this… I just assume that if the message identifier I parsed is #subclass: it’s because a full message definition. There is no way to know if you are in a class definition until you finish the parsing.
Lot of the behavior is similar to a message send, and to implement the program node API we have to see how to share this code, it seems an efort without a real motive, because all the problems I was having relies in the fact that I assumed that a class definition is not a message send…

2.The class definition is created from a message send

Something in between, I still changed the parser but… transforming the message node parsed only if it was a class definition.
The prototype is in:

MCHttpRepository
location: ‘http://smalltalkhub.com/mc/gisela/MessageClassDefinition/main’
user: ”
password: ”

I had less problems 🙂 but still some strange behaviors, it was because I changed the AST and all the users of the ast should change, specially the visitors because now you have classDefinitionNode, categoryNode, variableDefinitionNode (…).

And then I have lots of failling test and for olving in the major part I endedUp delegating in the message node reference I had hold.
But this was expected, since I changed the AST structure in some point all the users of that structures need to be modified, but ended up having like the same that before… lot of behavior shared between a message node and a class definition, and sometimes the “new representation wasn’t even desiarable.
By example asking for suggestions… if I’m writting a method and write somthing like:

Object
subclass: #JunkClass
instanceVariableNames: ‘zzz’
classVariableNames: ”
poolDictionaries: ”
category: ‘DeleteMe-1’.

If I ask for suggestions in zzz I expect something like… “Extract local” and no “Rename” but in the definition space… I want that behavior, so… since that I decided that in some ocassions you look the class definition message as a special stuf, and that depends on the context where you are, so changed the parser wasn’t so great because at that point you don’t how they would like to treat that particular message, that it’s mostly user-decision.

3. Request the class definition explicitly

I don’t change the parser, I add a message to the nodes “asClassDefinition”

MCHttpRepository
location: ‘http://smalltalkhub.com/mc/gisela/ClassDefEnhaceTree/main’
user: ”
password: ”

And then let the user of that tree decide in which case he is and if it’s who apply to do the transformation or not.

With this approach the direct user of the ast must know if he wants this representation or not, the original tree doesn’t change, and we can think that is that we let that two representation exists, is a similar approach to the semantic analysis by default you don’t have the information and if you want that you explicitly need to say… ok do the analysis.
And in the end

I finish in the beginning, the AST represents the syntax structure and that is how we understand it and how we manipulate it, sometimes we want a transformation, we want more and we enhance that structure for better understanding.

At least for the suggestions it was only in the class definition view. And after dealing with the consequences of change I get convinced that we send message and that’s it, we don’t have and neither need an special syntax for defining a class, is just sending the right message to the right object, it’s simple and it’s very good.

And when we want another approach now we can have it, very easily.

 

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