2012/1/31 Andy Wingo <wi...@pobox.com>: > Hi Tobias, > > On Mon 30 Jan 2012 20:17, Tobias Gerdin <tger...@gmail.com> writes: > >> 2012/1/23 Andy Wingo <wi...@pobox.com>: >>> Did you ever run into problems with non-resumable continuations? >> >> At times I was scratching my head and wondering what was going on but >> if you mean continuations that wouldn't resume due to a bug I do not >> think so. > > If a partial continuation includes a trip through C and then back to the > VM, it won't be resumable. Basically if you call a function that is > implemented in C and then that function does a scm_call(...), and you > abort from within the scm_call(...), the abort works but the > continuation will not be resumable. > > This is an implementation restriction. We're trying to get around it by > having more things implemented in Scheme rather than C. > > The reason for this restriction is that you can't capture part of the C > stack, then compose it with some other C continuation (i.e., splat it at > some other stack position). > > E.g.: > > scheme@(guile-user)> (call-with-prompt 'foo (lambda () > (call-with-output-string (lambda args (abort-to-prompt 'foo)))) (lambda (k) > k)) > $1 = #<partial-continuation 2b63200> > scheme@(guile-user)> ($1) > ERROR: In procedure #<partial-continuation 2b63200>: > ERROR: Throw to key `vm-error' with args `(vm-run "Unrewindable partial > continuation" (#<vm-continuation 2b73aa0>))'. > > Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue. > scheme@(guile-user) [1]> > > I asked you this because I was wondering I wanted to know how we were > doing in practice: if you happened to run into this. So it's nice that > you didn't run into it :)
So if a Scheme primitive the program calls happens to be implemented in C you would run into this problem? As you saw my examples were quite minimal and maybe not representative of what you do "in practice". But I agree that given this implementing more things in Scheme definitely makes sense. >> [...] > > There is a prompt in the server. The handler is applied to the request, > body, and state values in a thunk, and that thunk is called in one of > these: > > (define (with-stack-and-prompt thunk) > (call-with-prompt (default-prompt-tag) > (lambda () (start-stack #t (thunk))) > (lambda (k proc) > (with-stack-and-prompt (lambda () (proc k)))))) Great, all set then! -Tobias
