@wrog
> but I can't see how more than 2 would ever be necessary for a copying GC

It's not "necessary", it's "to make GC performance a negligeable percentage of overall CPU".

It was about a theoretical worst case as I recall, certainly not about one particular algorithm.

And IIRC it was actually a factor of 7 -- 5 is merely a good mnemonic which may be close enough. (e.g. perhaps 5-fold keeps overhead down to 10-20% rather than 7's 1%, although I'm making it up to give the flavor -- I haven't read the book for 10-20 years)

But see the book (may as well use the second edition) if and when you care; it's excellent. Mandatory I would say, for anyone who wants to really really understand all aspects of garbage collection, including performance issues.

@screwlisp @kentpitman @cdegroot @ramin_hal9001

@wrog @dougmerritt
The LispM did a nice thing (at some tremendous cost in hardware, I guess, but useful in the early days) by having various kinds of forwarding pointers for this. At least you knew you were going to incur overhead, though, and pricing it properly at least said there was a premium for not side-effecting and tended to cause people to not do it. And the copying GC could fix the problem eventually, so you didn't pay the price forever, though you did pay for having such specific hardware or for cycles in systems trying to emulate that which couldn't hide the overhead cost. I tend to prefer the pricing model over the prohibition model, but I see both sides of that.

If my memory is correct (so yduJ or wrog please fix me if I goof this): MOO, as a language, is in an interesting space in that actual objects are mutable but list structure is not. This observes that it's very unlikely that you allocated an actual object (what CL would call standard class, but the uses are different in MOO because all of those objects are persistent and less likely to be allocated casually, so less likely to be garbage the GC would want to be involved in anyway).

I always say "good" or "bad" is true in a context. It's not true that side effect is good or bad in the abstract, it's a property of how it engages the ecology of other operations and processes.

And, Ramin, the abolishing of mutable variables has other intangible expressional costs, so it's not a simple no-brainer. But yes, if people are locked into a mindset that says such changes couldn't improve performance, they'd be surprised. Ultimately, I prefer to design languages around how people want to express things, and I like occasionally doing mutation even if it's not common, so I like languages that allow it and don't mind if there's a bit of a penalty for it or if one says "don't do this a lot because it's not aesthetic or not efficient or whatever".

To make a really crude analogy, one has free speech in a society not to say the ordinary things one needs to say. Those things are favored speech regardless because people want a society where they can do ordinary things. Free speech is everything about preserving the right to say things that are not popular. So it is not accidental that there are controversies about it. But it's still nice to have it in those situations where you're outside of norms for reasonable reasons. 😀

> Ultimately, I prefer to design languages around how people want to express things, and I like occasionally doing mutation even if it's not common, so I like languages that allow it and don't mind if there's a bit of a penalty for it or if one says "don't do this a lot because it's not aesthetic or not efficient or whatever".

Me too -- although I remain open to possibilities. Usually such want me to switch paradigms, though, not just add to my toolbox.

@ramin_hal9001 @screwlisp @wrog @cdegroot

“the abolishing of mutable variables has other intangible expressional costs, so it’s not a simple no-brainer.”

@kentpitman I prefer the term “constraint” to “expressional cost,” because constraints are the difference between a haiku and a long-form essay. For example, I am very curious what the code for the machine learning algorithm that trains an LLM would look like expressed as an APL program. I don’t know, but I get the sense it would be a very beautiful two or three lines of code, as opposed to the same algorithm expressed in C++ which would probably be a hundred or a thousand lines of code.

Not that I disagree with you, on the contrary, that is why I was convinced to switch to Scheme as a more expressive language than Haskell. I like the idea of starting with Scheme as the untyped lambda calculus, and then using it to define more rigorous forms of expression, working your way up to languages like ML or Haskell, as macro systems of Scheme.

@dougmerritt @screwlisp @wrog @cdegroot

I'm not 100% positive I understand your use of constraint here, but I think it is more substantive than that. If you want to use the metaphor you've chosen, a haiku reaches close to theoretical minimum of what can be compressed into a statement, while a long-form essay does not. This metaphor is not perfect, though, and will lead astray if looked at too closely, causing an excess focus on differential size, which is not actually the key issue to me.
I won't do it here, but as I've alluded to more than once I think on the LispyGopher show, I believe that it is possible to rigorously assign cost to the loss of expression between languages.

That is, that a transformation of expressional form is not, claims of Turing equivalence notwithstanding, cost-free both in terms of efficiency and in terms of expressional equivalence of the language. It has implications (positive or negative) any time you make such changes.

Put another way, I no longer believe in Turing Equivalence as a practical truth, even if it has theoretical basis.

And I am pretty sure the substantive loss can be expressed rigorously, if someone cared to do it, but because I'm not a formalist, I'm lazy about sketching how to do that in writing, though I think I did so verbally in one of those episodes.

It's in my queue to write about. For now I'll just rest on bold claims. 😀 Hey, it got Fermat quite a ways, right?

But also, I had a conversation with ChatGPT recently where I convinced it of my position and it says I should write it up... for whatever that's worth. 😀

cc @screwlisp @wrog @dougmerritt @cdegroot

> That is, that a transformation of expressional form is not, claims of Turing equivalence notwithstanding, cost-free both in terms of efficiency and in terms of expressional equivalence of the language. It has implications (positive or negative) any time you make such changes.

I hope everyone here is already clear that "expressiveness" is something that comes along on *top* of a language's Turing equivalence.

Indeed Turing Machines (and pure typed and untyped lambda calculus and SKI combinatory calculus and so on) are all *dreadful* in terms of expressiveness.

And for that matter, expressiveness can be on top of Turing incomplete languages. Like chess notation; people argue that the algebraic notation is more expressive than the old descriptive notation. (People used to argue in the other direction)

@ramin_hal9001 @screwlisp @wrog @cdegroot