[alankay1]

I should have mentioned a few other things for the later Smalltalks. First, selectors are just objects. Second, you could use the automatic "message not understood" mechanism to field an unrecognized object. I think I'd do this by adding a method called "any" and letting it take care of arbitrary unknown objects ...

[shalabhc]

> adding a method called "any"

Right, I understand there are ways to do this with methods but my question was more about the purity aspect, which you already addressed above.

[alankay1]

A selector is an object -- so that is pure -- and its use is a convention of the messaging, and the message itself is one object, that is an instance of Class message.

What's fun is that every Smalltalk contained the tools to make their successors while still running themselves. In other words, we can modify pretty much anything in Smalltalk on the fly if we choose to dip into the "meta" parts of it, which are also running. In Smalltalk-72, a message send was just a "notify" to the receiver that there was a message, plus a reference to the whole message. The receiver did the actual work of looking at it, interpreting it, etc.

This is quite possible to make happen in the more modern Smalltalks, and would even be an interesting exercise for deep Smalltalkers.

[shalabhc]

> A selector is an object -- so that is pure -- and its use is a convention of the messaging

The selector 'convention' is hard coded in the syntax - this appears to elevate selector based messaging over other kinds. But now I'm rethinking this differently - i.e. selectors isn't part of the essence, but a specific choice that could be replaced (if we find something better.)

[mmiller]

I can't remember if I've brought this up already in this thread, but if you want to "kick the tires" on ST-72, Dan Ingalls has an implementation of it up on the web. It's running off of a real ST-72 image. I wrote about it at https://tekkie.wordpress.com/2014/02/19/encountering-smallta...

I include a link to it, and described how you can use it (to the best of my knowledge), though my description was only current to the time that I wrote it. Looking at it again, Ingalls has obviously updated the emulation.

The nice thing about this version is it includes the original tutorial documentation, written by Kay and Adele Goldberg, so you can download that, and learn how to use it. I found that I couldn't do everything described in the documentation. Some parts of the implementation seemed broken, particularly the class editor, which was unfortunate, and some attempts to use code that detected events from the mouse didn't work. However, you can write classes from the command line (ST-72 was largely a command-line environment, on a graphical display, so it was possible to draw graphics).

If you take a look at it, you will see a strong resemblance to Lisp, if you're familiar with that, in terms of the concepts and conventions they were using. As Kay said in "The Early History of Smalltalk," he was trying to improve on Lisp's use of special forms. I found through using it that his notion of classes, from a Lisp perspective, existed in a nether world between functions and macros. A class could look just like a Lisp function, but if you add parsing behavior, it starts behaving more like a macro, parsing through its arguments, and generating new code to be executed by other classes.

The idea of selectors is still kind of there, informally. It's just that it takes a form that's more like a COND construct in Lisp. So, rather than each selector having its own scope, as in later versions, all of them exist in an environment that exists in the scope of the class/instance.

After using it for a while, I could see why they went to a selector model of message receipt, because the iconic language used in ST-72 allowed you to express a lot in a very small space, but I found that you could make the logic so complex it was hard to keep track of what was going on, especially when it got recursive.

[shalabhc]

> I wrote about it at https://tekkie.wordpress.com/2014/02/19/encountering-smallta....

Sweet, thanks! There's also the ST-78 system at https://lively-web.org/users/bert/Smalltalk-78.html

> existed in a nether world between functions and macros

Macros are just functions that operate on functions at 'read-time', from my POV. So if you eliminate the distinction between read-time and run-time, they're the same.

> It's just that it takes a form that's more like a COND construct in Lisp.

And even COND isn't special, it's just represented as messaging in Smalltalk, right?

> you could make the logic so complex it was hard to keep track of what was going on

Interesting, I see.

[mmiller]

"And even COND isn't special, it's just represented as messaging in Smalltalk, right?"

Right. What I meant was that the parsing would begin with "eyeball" (ST-72 was an iconic language, so you would get a character that looked like an eye viewed sideways), and then everything after that in the line was a message to "eyeball," talking about how you wanted to parse the stream--what patterns you were looking for--and if the patterns matched, what messages you wanted to pass to other objects. That was your "selector" and method. What felt weird about it, after working in Squeak for a while, is these two concepts were combined together into "blobs" of symbolic code. You would have a series of these "messages to eyeball" inside a class. Those were your methods.

The reason I said it was similar to COND was it had a similar format: A series of expressions saying, "Conditions I'm looking for," and "actions to take if conditions are met." It was also similar in the sense that often that's all that would be in a class, in the same way that in Lisp, a function is often just made up of a COND (unless you end up using a PROG instead, which I consider rather like an abomination in the language).

In ST-72, there's one form of conditional that uses a symbol like "implies" in math (can't represent it here, I don't think), and another where you can be verbose, saying in code, "if a = b then do some stuff." But what actually happens is "if" is a class, and everything else ("a = b then do some stuff") is a message to it. Of course, you could create a conditional in any form you want.

In ST-80, they got rid of the "if" keyword altogether (at least in a "standard" system), and just started with a boolean expression, sending it a message.

a = b ifTrue: [<do-one-thing>] ifFalse: [<do-something-else>].

They introduced lambdas (the parts in []'s) as objects, which brought some of the semantics "outside of the class" (when viewed from an ST-72 perspective). It seems to me that presents some problems to its OOP concept, because the receiver is not able to have complete control over the meaning of the message. Some of that meaning is determined by partitioned "blocks" (lambdas) that the receiver can't parse (at least I don't think so). My understanding is all it can do with them is either pass parameter(s) to the blocks, executing them, or ignore them.

One of the big a-ha moments I had in Smalltalk was that you can create whatever control structures you want. The same goes for Lisp. This is something you don't get in most other languages. So, a temptation for me, working in Lisp, has been to spend time using that to work at trying to make code more expressive, rather than verbose. A positive aspect of that has been that it's gotten me to think about "meanings of meaning" in small doses. It creates the appearance to outsiders, though, that I seem to be progressing on a problem very slowly. Rather than just accepting what's there and using it to solve some end goal, which I could easily do, I try to build up from the base that's there to what I want, in terms of expression. What I have just barely scratched the surface of is I also need to do that in terms of structure--what you have been talking about here.

[alankay1]

It's an extensible language with a meta system so you can make each and every level of it do what you want. And, as I mentioned, the first version of Smalltalk (-72) did not have a convention to use a selector. The later Smalltalks wound up with the convention because using "keywords" to make the messages more readable for humans was used a lot in Smalltalk-72.