[gambit-list] Does gambit-c have call/ec?
christian at pflanze.mine.nu
Wed Jun 27 11:21:14 EDT 2007
Bradley Lucier wrote:
> On Jun 27, 2007, at 8:35 AM, Marc Feeley wrote:
>> On 25-Jun-07, at 4:27 PM, naruto canada wrote:
>>> Does gambit-c have call/ec? Thanks.
>> Call/ec is not predefined. You can define it yourself like this:
>> (define call/ec call/cc)
>> Obviously, in this case the implementation of call/ec is not as fast
>> as it could be.
> Is there some combination of continuation-capture and continuation-
> graft that gives the semantics of call/ec?
> To be honest, I don't understand what any of call/ec, continuation-
> capture, and continuation-graft do.
In my honest experience:
##continuation-capture is just call/cc but gives the raw continuation
data type without wrapping as a procedure. (It's a bit faster because of
##continuation-graft (there is no |continuation-graft|, right, or has
this been added recently?) is quite nifty, it runs the given thunk in
the context of a given continuation (it needs the raw continuation as
given by ##continuation-capture). See
http://scheme.mine.nu/gambit/scratch/cj-exception/ for an example (the
idea of this module is to provide exception values which are restartable
later, e.g. to be serialized and stored to disk and provided for
debugging later on). Actually you should use the -with-winding functions
as shown so that dynamic-wind works. What the shown code does: capture
the (raw) continuation from where an exception happened, wrap it
together with the exception value in a cj-exception structure, and call
the user-provided exception handler with it in the context of the code
outside of the with-exception/continuation-catcher (e.g. the dynamic
environment from outside has already been reinstantiated and exceptions
thrown from |handler| will thus call the next outer
current-exception-handler instead of looping into the inner one).
As far as I understand, call/ec would return a continuation which can
only be instantiated once and only from the dynamic scope of their
capture (like exceptions in languages without 1st class continuation
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