2. No tail recursion

3. There is no cons, cdr or car stuff. Lists are just vector objects and you use push, cat, slice etc to manipulate elements.

Only this much is radical enough to say that it isn't a Lisp but disguised in Lisp syntax. Lisp is as much about semantics as the syntax. The power of Lisp to do amazing things comes from the language capabilities. If you cannot reasonably translate powerful programs from Lisp textbooks, then is it a Lisp?

Some languages (example: JavaScript, Python) make it easy to create an inefficient Lisp interpreter that could execute common textbook programs. Minimax, A-star search, unification matching, etc.

(IIUC the "garbage collector" is refcounting; I'm not sure what "All objects used by the Lisp are reference counted objects from the class library." means in practice, and specifically I'm not sure if that means "you basically have a GC in lisp as long as you don't make cycles".)

All objects used by the Lisp are instances of classes from the class library, ie that stuff in class/ folder.

These instances are ref counted objects, as they are derefed and they drop to 0 refs they get deinited and freed.

The Lisp is constructed from these, for example Lists in the Lisp are an instance of the class/vector, numbers are an instance of class/num, the environment is a chain of class/hmap etc.

So yes, you do have GC if you don't make cycles, and you do have to care about not doing that just as you would in C++.

Clojure doesn't because it's a Lisp in Java, and Java doesn't. Back when Clojure was just starting to get some attention I watched a video [1] of Rich doing a demo for a user group. Tail call optimization was one of their first questions. He gave them an answer that sounded like he had given it multiple times before.

[1] can't find it, sorry.

I keep hearing this and I have no idea where it comes from. x86_64 doesn't either, Clojure has recur, and prefers consistent var semantics to silently specialcasing some tail calls. There's no particularly good reason the compiler couldn't just do it.

x86_64 instruction set gives the programmer complete control over the organization of memory: the stack, use of registers, calling conventions and so on.

It has few safety features.

x86_64 doesn't specifically support tail calls, but it makes tail calls possible by way of jump instructions being able to go anywhere in the address space. An instruction in the middle of one function can branch to an instruction in the middle of another function, and without doing anything with the registers or stack.

The JVM byte code doesn't allow such a thing.

The JVM supports iteration. Therefore local tail calling is possible, because it's a syntactic sugar for local control transfers. That's presumably why Clojure can have recur.

I'm going to rephrase my own argument because it doesn't appear you interacted with it: what part of your argument doesn't work for x86_64? "One would need to compile the code in complex ways: function calls would no longer map directly to CALL/RET instructions." Sure! That's arguably what makes it an optimization. Who cares? CPUs can loop, and so can the JVM.

I would maybe see your point if Clojure didn't already know how to do tail calls (and so would need to implement the allegedly complicated compilation step), but as I have pointed out several times: it already has `recur`, which lets you call a function without the JVM thinking there is a function call going on.

That one needs to manually annotate it in Clojure just means that the compiler is manually instructed to generate a loop-like construct instead of a function call.

The advantage in Clojure is that these loops are explicitly marked, which IMHO improves readablity.

Many Lisps don't bother to implement tail recursion optimization, because they support the more general tail call optimization - by adjusting/reusing the current stack frame and using a JMP instruction.