Andy Wingo <wi...@pobox.com> writes: > Hi David, > > On Sun 04 Mar 2012 13:01, David Kastrup <d...@gnu.org> writes: > >> The global symbol space is a different identity space than heap >> equality, and it never gets garbage collected: the lifetime of a >> gensym is eternal. > > This is not true in Guile, where symbols can be garbage collected.
The symbol name is not garbage collected. That is the difference between gensym and make-symbol. >> And frankly: the manual talks about prompts being composable and >> gives an example which seems utterly wrong to me since it does not >> actually abort a computation but rather half-finishes it. It is >> unclear what part of the computation will finish and what will >> complete. > > That is an interesting point. I guess there are two ways of answering > it. One is to note that in Scheme, it's difficult in general to > determine whether a computation is finished or will finish, because of > call/cc. > > But you ask about a specific point, here: an abort to a prompt is > basically boils down to a longjmp to the prompt's handler. The > partial continuation is logically passed as an argument to the > handler. But where does the "partial continuation" start and where does it end? If I am doing a "longjmp to the prompt's handler", how can it be that the calling stack frame inside of the thunk that is supposed to be exited can finish a calculation? Where is the difference between (+ 34 (abort-to-prompt 'foo)) and (let ((x (abort-to-prompt 'foo))) (+ 34 x)) ? Why is the first allowed to complete and return a result, and the second (presumably) not? Or _if_ the second is allowed to complete, what does "abort" in "abort-to-prompt" even mean? All this does not really make discernible sense to me. Whereas call/ec has rather clear semantics and usage. The one thing that is not self-evident is its behavior in case of misuse, namely when it is asked to do a job only call/cc can. -- David Kastrup
