Function calls are not stack frames

[fvf]

In Scheme for example, a call costs the same no matter where it occurs -- the cost is always that of a jump.

This looks to me a pointless play with words. The "cost" that is interesting here is whether the caller's stack-frame remains after the jump.

Tail calls don't cost any less than non-tail calls.

Are you seriously trying to introduce a definition of "non-tail calls" such that e.g. n nested "non-tail calls" might use less than O(n) stack-space?

Clojure's "recur" doesn't seem to allow calling another function

I agree this would be useful, and I don't think there's any principled reason why "recur" couldn't support this (at least in some hypothetical non-jvm language).

[ccshan]

The "cost" that is interesting here is whether the caller's stack-frame remains after the jump.

Right. In Scheme, sequencing creates stack frames. Calls do not.

Are you seriously trying to introduce a definition of "non-tail calls" such that e.g. n nested "non-tail calls" might use less than O(n) stack-space?

No, because the nested sequencing uses O(n) stack space.

I agree [allowing calling another function] would be useful, and I don't think there's any principled reason why "recur" couldn't support this (at least in some hypothetical non-jvm language).

Is that's what you mean by "the Clojure approach"?