> Languages that rely on recursion like LISPs often specify that TCO must be implemented in their language spec.
This is inaccurate. As far as I can tell, it's only Scheme specifications like r5rs that have a hard requirement on tail-call optimization. The Common Lisp specification makes no mention of tail-call optimization, and Clojure -- arguably the most widely used Lisp today -- does not have it either (but it has special forms and macros that let you write recursive code and produce trampolines, thunks, etc. to avoid blowing up the stack).
With that said, at least in the case of Common Lisp, it is true that many compilers will implement TCO, even though it is *not* required at all to conform to the specification.
Common Lisp is not a Lisp which 'relies on' recursion. It has various loop statements (DO, DOTIMES, DOLIST, ...), various mapping functions (map, mapcar, mapc, maphash, ...) and a complex LOOP. To prevent stack overflows those can all be built on top of a goto operator (tagbody + go). One of the reason was that compilation targets (cpus, virtual machines, other languages, ...) don't have native TCO support. For example the various Lisp Machines had no TCO support, and many C compilers did not support TCO for quite some time. C is a popular compilation target (KCL, AKCL, ECL, CLASP, CLICC, ... were (some are) all Lisp to C compilers).
> With that said, at least in the case of Common Lisp, it is true that many compilers will implement TCO, even though it is not required at all to conform to the specification.
The specification does not require it, it actually says nothing about it. TCO isn't even mentioned. Means, it also says nothing how a compiler should support it, if it wanted to. The language supports recursion, but something like tail recursion support is entirely an implementation specific optimization. Since TCO has some advantages (less stack space needed, possibly some faster loops, ...), many compilers implement it, but mostly no interpreter does it.
Also, most of them support GC, even though exactly nothing is said about it in the specification. ;-)
This is inaccurate. As far as I can tell, it's only Scheme specifications like r5rs that have a hard requirement on tail-call optimization. The Common Lisp specification makes no mention of tail-call optimization, and Clojure -- arguably the most widely used Lisp today -- does not have it either (but it has special forms and macros that let you write recursive code and produce trampolines, thunks, etc. to avoid blowing up the stack).
With that said, at least in the case of Common Lisp, it is true that many compilers will implement TCO, even though it is *not* required at all to conform to the specification.