[Swift-devel] swift write once array/struct

[Ketan Maheshwari]

Hi Tim,

Thanks for the clarification and code snippet. To clarify, in my
understanding, the implementation is 'extrinsic'.

To be intrinsic, I imagined the array is implemented as an object which has
a function that monitors and controls its write-once property and gets
invoked every time anything is written to any element of the array.

Very rudimentary code below to clarify the idea:

bool checklock(val_t val){
  if (self.item) return ERRNOWRITE;
  dowrite(self.item, val);
 return OK;
}

About Swift semantics work: thanks, I will try to take a look and comment
if I can.

--
Ketan

On Sat, Mar 21, 2015 at 10:14 AM, Tim Armstrong <tim.g.armstrong at gmail.com>
wrote:

> Also, if you're curious about semantics of Swift data structures I've been
> doing some work on that recently, would be happy to talk about it in more
> detail.
>
> - Tim
>
> On 21 March 2015 at 10:06, Tim Armstrong <tim.g.armstrong at gmail.com>
> wrote:
>
>> Hi Ketan,
>>   I'm not sure that I fully understand the distinction between
>> internal/external implementation.  In terms of externally observable
>> behaviour, writing an already written cell will cause a runtime error
>> immediately.  There's no way you can cause a deadlock or observe
>> inconsistent values.  Deadlocks with array cells should only happen if
>> something is waiting for a cell that has been assigned 0 times.
>>
>> If you're curious, i copied and pasted the relevant code fragment for
>> Swift/T (it's in data.c in lb).  It's relatively straightforward, even
>> though it's handling a few cases.  Essentially it's if (A[i] exists)
>> return error code;.  The data structure is only accessed by a single
>> thread so no race conditions are possible.
>>
>> I believe there's something similar in swift/k.
>>
>> - Tim
>>
>>       // Does the link already exist?
>>       adlb_container_val t = NULL;
>>       bool found = container_lookup(c, curr_sub, &t);
>>
>>       if (found && (value == NULL || t != NULL))
>>       {
>>         // Can overwrite reserved (unlinked) entries with actual data, but
>>         // cannot double reserve entries.
>>
>>         // Don't print error by default: caller may want to handle
>>         DEBUG("already exists: "ADLB_PRIDSUB,
>>               ADLB_PRIDSUB_ARGS(id, d->symbol, subscript));
>>         return ADLB_DATA_ERROR_DOUBLE_WRITE;
>>       }
>>       // Following code actually assigns the array cell
>>       ...
>>
>>
>>
>>
>>
>> On 20 March 2015 at 23:33, Ketan Maheshwari <ketan at mcs.anl.gov> wrote:
>>
>>> Hi,
>>>
>>> So, in the meeting today, I was curious about wether the Swift write
>>> once array/struct property is enforced via some external function or is it
>>> intrinsic to the array/struct.
>>>
>>> In other words, is  the array/struct implemented in such a way that it
>>> will automatically not allow anything to write once its i-th item has been
>>> written or some external entity keeps track of this property and gets
>>> triggered every time a write happens.
>>>
>>> I thought with intrinsic property, write operations will be atomic and
>>> free of deadlocks as opposed to externally tracked/enforced property.
>>>
>>> --
>>> Ketan
>>>
>>> _______________________________________________
>>> Swift-devel mailing list
>>> Swift-devel at ci.uchicago.edu
>>> https://lists.ci.uchicago.edu/cgi-bin/mailman/listinfo/swift-devel
>>>
>>>
>>
>
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