Re: call-with-current-continuation
Klaus Schilling (Klaus.Schilling@home.ivm.de)
Thu, 13 Aug 1998 08:31:51 +0200 (MET DST)
Maciej Stachowiak writes:
>
> aarchiba@calum.csclub.uwaterloo.ca writes:
> > Telford Tendys <telford@triangle.triode.net.au> writes:
> >
> > > > Guile currently implements its error handling with exceptions; these
> > > > are a very restrictive form of call/cc. There is an explanation of
> > > > this in the docs. Basically, they're more efficient than call/cc.
> > >
> > > Are you talking speed efficient or memory efficient?
> >
> > Both, to some extent. Here's the story: call/cc is implemented
> > essentially by copying the stack. This is expensive but necessary
> > because you might continue the same way many times, from anywhere.
> > ISTR there have been thread systems implemented using call/cc.
> > Exceptions, OTOH, just need to be able to unwind the stack (and call
> > dyn-winds). So they don't have to make a stack copy.
> >
>
> That's how Guile does it, in particular to help it interoperate with C
> code nicely. Using a very different implementation strategy, call/cc
> can be a small constant cost and invoking a continuation can be
> essentially a jump that passes a value. Doing things that way makes it
> much harder to integrate with C, though.
So call-with-current-continuation can only be unexpensive in scheme implemen-
tations that are not based on a C-written library, but implement themselves?
Klaus Schilling