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> almost a non-issue considering Almost. It's just that that many implementations support it (ABCL not) and that the language does not support it. TCO interact

by lispm 12y ago

> almost a non-issue considering Almost. It's just that that many implementations support it (ABCL not) and that the language does not support it. TCO interacts badly with many Lisp language features: dynamic binding, unwind-protect, ... > Everyone else can assume it, at least for local calls, which represent 98% of the times that it matters anyway. If the right optimizations settings are set.

ScottBurson 12y ago
> TCO interacts badly with many Lisp language features: dynamic binding, unwind-protect, ... I've never understood this claim. I mean, I understand the technical point: that use of dynamic binding or UNWIND-PROTECT creates a barrier in the stack across which TCO cannot be done. But in practice it seems to me a non-issue. I can't recall ever, in 35 years of Lisp hacking, wanting to write a tail-recursive routine that used one of them on each iteration, in such a way as to cause unbounded stack growth. It just doesn't happen, at least not to me. Maybe that's partly because I try to keep my usage of dynamic binding to a bare minimum. I think some people overuse it.
ScottBurson 12y ago
Ah right, I forgot about ABCL. I haven't used it, though I keep meaning to give it a try. Hmm, it can't do TCO even on local calls? My code may not run in it, then.
- lispm 12y ago
  Btw., if you haven't seen it, here is an overview: http://0branch.com/notes/tco-cl.html http://0branch.com/notes/tco-cl.html Btw., Symbolics Genera also did not support TCO. ;-)
  - ScottBurson 12y ago
    The overview is useful -- thanks. I see Allegro now does full TCO. Either that wasn't true when I was using it (a dozen years ago) or we just didn't have the right optimize switch settings to turn it on. In any case, I thought that the strategy of doing TCO only on local calls was pretty good -- it let me write loops tail-recursively with LABELS, but didn't significantly impact debugging because only local call frames got deleted. I'm well aware that the LispMs did not do TCO -- I owned several (starting with a CADR) and still have an XL1200 rusting in my closet :-)

by lispm 12y ago

ScottBurson 12y ago

> TCO interacts badly with many Lisp language features: dynamic binding, unwind-protect, ... I've never understood this claim. I mean, I understand the technical point: that use of dynamic binding or UNWIND-PROTECT creates a barrier in the stack across which TCO cannot be done. But in practice it seems to me a non-issue. I can't recall ever, in 35 years of Lisp hacking, wanting to write a tail-recursive routine that used one of them on each iteration, in such a way as to cause unbounded stack growth. It just doesn't happen, at least not to me. Maybe that's partly because I try to keep my usage of dynamic binding to a bare minimum. I think some people overuse it.

ScottBurson 12y ago

Ah right, I forgot about ABCL. I haven't used it, though I keep meaning to give it a try. Hmm, it can't do TCO even on local calls? My code may not run in it, then.

lispm 12y ago
Btw., if you haven't seen it, here is an overview: http://0branch.com/notes/tco-cl.html http://0branch.com/notes/tco-cl.html Btw., Symbolics Genera also did not support TCO. ;-)
- ScottBurson 12y ago
  The overview is useful -- thanks. I see Allegro now does full TCO. Either that wasn't true when I was using it (a dozen years ago) or we just didn't have the right optimize switch settings to turn it on. In any case, I thought that the strategy of doing TCO only on local calls was pretty good -- it let me write loops tail-recursively with LABELS, but didn't significantly impact debugging because only local call frames got deleted. I'm well aware that the LispMs did not do TCO -- I owned several (starting with a CADR) and still have an XL1200 rusting in my closet :-)