[Swift-user] Re: [Swift-devel] Problem with iterate

[Ioan Raicu]

But wouldn't the single assignment part prevent your example from 
dead-locking? In other words, the compiler should throw an error on line 
"b = cat(a)" as its trying to modify an existing value b.

Mihael Hategan wrote:
> On Thu, 2010-02-18 at 17:08 +0000, Ben Clifford wrote: 
>   
>>> I'm unsure. I have a feeling that static dealdock detection is
>>> undecidable in swift. I need to think some more about that.
>>>       
>> I think doing it statically (i.e. at compile time) is equivalent to 
>> executing the code (including external apps) - I think there's probably a 
>> straightforward example where something depends on an integer read from an 
>> externally executed app which then influences an if statement that decides 
>> which of two different arrays is being written to; or something like that.
>>     
>
> Right. So termination is undecidable by virtue of it being a turing
> complete language. What I'm unsure is whether deadlocks follow as a
> consequence of that. They are a different beast.
>
> We can probably start with simpler things. The following deadlocks with
> the current swift:
> a = cat(b);
> b = cat(a);
>
> So it seems that loops in the graph will create a deadlock and detecting
> loops requires knowing the graph. We can probably detect the static
> parts and we may be able to handle alternatives by issuing warnings of
> the "branch 1 in that 'if' combined with branch 2 in that other 'if'
> leads to a deadlock" sort.
>
> Arrays may be a bit tougher.
>
>
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>
>   

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Ioan Raicu, Ph.D.
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