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Postings from November 2004
At 3:39 PM -0500 11/16/04, Matt Fowles wrote:
>Dan~
>
>
>On Tue, 16 Nov 2004 15:25:24 -0500, Dan Sugalski <dan@sidhe.org> wrote:
>> At 3:12 PM -0500 11/16/04, Matt Fowles wrote:
>>
>>
>> >Dan~
>> >
>> >On Tue, 16 Nov 2004 13:41:25 -0500, Dan Sugalski <dan@sidhe.org> wrote:
>> >> At 10:32 AM -0800 11/16/04, Jeff Clites wrote:
>> >> >The continuation preserves the frame (the mapping from logical
>> >> >variables to their values), but not the values of those variables at
>> >> >the time the continuation was created.
>> >>
>> >> This is one of the fundamental properties of continuations, but it
>> >> does throw people. And it's why register contents have to be thrown
>> >> away when a continuation is invoked, since the registers have values,
>> >> and continuations don't preserve values.
>> >
>> >I think right here we have the crux of my failure to understand. I
>> >was/am under the impression that the continuation will restore the
>> >register frame to exactly as it was when the continuation was taken.
>> >Thus those registers which are values (I,N) will continue to have the
>> >value they had when the continuation was taken, while those registers
>> >which are pointers/references (S, P) will still point to the same
>> >place, but that data may have changed. Is this correct?
>>
>> No. The registers are just about the only thing that *isn't* restored.
>>
>> Continuations put the environment back. This includes things like the
>> lexical pad stack, the namespace stack, the stack itself, any
>> security credentials... basically everything that describes the
>> environment. *Data*, on the other hand, is *not* restored. Data stays
>> as it is.
>>
>> Registers are a special case of data, and they're just declared crud
>> by fiat, since otherwise things get nasty and unpredictable.
>
>Then I am not sure what you mean by "The return continuation PMC type,
>used to create return continuations used for call/return style
>programming, guarantees that registers 16-31 will be set such that the
>contents of those registers are identical to the content of the
>registers when the return continuation was I<created>."
>
>I read that as saying that registers will be restored by
>continuations. If that is not what it is intended to mean, could you
>clarify for me.
Return continuations are special, basically. There are a number of
specialized continuation forms, and this is one of 'em. Which makes
things a bit more confusing but, well, there you go.
--
Dan
--------------------------------------it's like this-------------------
Dan Sugalski even samurai
dan@sidhe.org have teddy bears and even
teddy bears get drunk
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