[qwertyuiop924]

I was talking about the mechanics of the compiler/interpreter, at least the original Sussman/Steele implementation.

Let's look at Factorial in Scheme. You mentioned Fib, but Factorial is a simpler example, and will suffice.

  (define (fact n)
    (if (= n 0)
        1
        (* n (fact (- n 1)))))

The Scheme compiler/interpreter will detect that the last expression to be evaluated, and thus the return value will be:

  (* n (fact (- n 1)))

Now, you might think that Scheme would detect this as ineligible for TCO, but that's not what happens. In fact, what happens (in pseudo-asm) is this:

  push *n
  push $1
  call subtract ;;returns t1 on the stack, taken as arg by fact
  call fact ;;both n, and t1, the args to mult, are on the stack, followed by the return address.
  jmp mult ;;mult re-uses the stack frame, so it's a tail call.

obviously, in a real asm, sub and mul are primitives, not funcalls, but you get the idea.

If that didn't make sense, just read, "LAMBDA: The Ultimate Declarative." it explains it better than I could.

So yes, all functions in Scheme are TCOed (at least, in the original implementation), it's just that some of the build of stack frames. I don't know if that's how other languages work, but it might be.

[taeric]

I find this... still an odd definition. To quote Sussman/Abelson[1], "An evaluator that can execute a procedure such as sqrt-iter without requiring increasing storage as the procedure continues to call itself is called a tail-recursive evaluator."

Specifically, the caller of this function will have to clear out the stack of all operations that were pushed. Something you wouldn't have to do on a tail or non-recursive function.

[1] https://mitpress.mit.edu/sicp/full-text/book/book-Z-H-34.htm...

[qwertyuiop924]

It is an odd definition, I won't deny that. I was trying to talk about possible implementation, as opposed to the typical definition, so it's possible that I should have used another term.

By the way, this is the paper I was talking about. It won't explain my bizarre choice of words, but it might explain what I meant when I said that Scheme tail-call optimizes unconditionally:

http://repository.readscheme.org/ftp/papers/ai-lab-pubs/AIM-...

[taeric]

I think I see the difference. There are tail call optimizations, and then there is tail call recursion. They are two different things, potentially.

Thanks for linking the paper!

[qwertyuiop924]

Yeah, there you go.

That paper is part of a collection of papers, known as The Lambda Papers. If you enjoyed this one, you'll probably want to read the rest:

http://library.readscheme.org/page1.html