From Newsgroup: comp.lang.lisp

Paul Rubin [2026-08-27 13:01:44] wrote:

antispam@fricas.org (Waldek Hebisch) writes:

And normally functions do not return, simply tail-call to passed
"continuation".

Continuations are nontrivial precisely because of the non-tail-call
case, where the continuation is re-used. So you can use them to
implement coroutines, multitasking, etc. If they can't do that, they're
not "full" continuations. I don't see how to implement full
continuations using CL macros.

Like most cases of "impossibility" the result depends on the details of
the question:

AFAIK it's impossible to implement support for `call/cc` via macros in
Common Lisp *if* you want the affected code to be able to transparently
call arbitrary "external" functions like (say) `mapcar`.

OTOH, you can definitely implement a Common Lisp macro that takes as
argument code written in a DSL which happens to be a language that looks exactly like Common Lisp except it includes a `call/cc` primitive
(and in which you can (re)implement `mapcar`).

There is some sentiment[1] in the Scheme world that mandating full continuations in Scheme was a mistake and that it would have been
preferable to stop with "delimited" or "one-shot" continuations. Those
are maybe closer to catch/throw than to coroutines. I don't know if
those can be done with CL macros but it seems more plausible.

AFAIK they bump into the same problem as `call/cc` w.r.t implementing
them as macros.


=== Stefan
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From Newsgroup: comp.lang.lisp

Stefan Monnier <monnier@iro.umontreal.ca> writes:

AFAIK it's impossible to implement support for `call/cc` via macros in
Common Lisp *if* you want the affected code to be able to transparently
call arbitrary "external" functions like (say) `mapcar`.

OTOH, you can definitely implement a Common Lisp macro that takes as
argument code written in a DSL which happens to be a language that looks exactly like Common Lisp except it includes a `call/cc` primitive
(and in which you can (re)implement `mapcar`).

There is some sentiment[1] in the Scheme world that mandating full
continuations in Scheme was a mistake and that it would have been
preferable to stop with "delimited" or "one-shot" continuations. Those
are maybe closer to catch/throw than to coroutines. I don't know if
those can be done with CL macros but it seems more plausible.

AFAIK they bump into the same problem as `call/cc` w.r.t implementing
them as macros.

I have always wondered if using the environment would suffice as a inter-procedural reality. common-lisp is not very informative about
this.

(defmacro that (&env environment)
(push 'this-is-true environment)
(this))

(defmacro this (&env environmenr)
(how to you push or pop environment)..


I suppose you could always use &aux ?
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