On Fri, Oct 25, 2013 at 11:55 AM, Pavel Levshin <[email protected]>wrote:
> 25.10.2013 13:30, Rainer Gerhards:
>
>
>> And, indeed, it will work as expected. Each message will get it's number
>> in shadow copy.
>>
>> But what if a user tries this (to retain previous value, for example):
>>
>> set $.local = $/var;
>> set $/var = $/var + 1;
>>
>> $/var will be shadowed at first statement, but modified AND propagated at
>> second one.
>>
>> This would violate restriction 1 ("no update after initial access") and
>> lead to a runtime error. But I guess you looked at the pseudocode, and
>> indeed it was missing the reset of the modifiable attribute (which
>> enforces
>> that restriction).
>>
>> Do you agree?
>>
>>
> Yes, it will work.
>
> Overall, it is almost the same restriction as I have proposed earlier.
> Difference is in that there is a shadow, which lets user read the same
> value many times from cached copy. But he still can access the state
> variable only once.
>
Yes, of course. But he doesn't need to care about this (actually not even
know), as it is hidden by the system. And we can issue a warning message
easily if the user tries to do that. We wanted to prevent user errors and
make it look more "natural". With read-only variables, we also have a good
vehicle for keeping config-values (the use case was on the list yesterday).
Also, we may remove some of the restrictions as the engine evolves -- with
the ultimate goal of morphing state vars back into global vars, now with
the expected semantics assigned to them. All of this without the need to
change anything in existing configs. I would also think that what is
currently written for global vars will just continue to work fine with
state vars. The same hopefully holds for the articles David has written and
other docs/snippets. So I think there is great value in trying to preserve
the syntax.
Rainer
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