Proposal: Relax rules for 'pure'

[Robert Jacques]

On Fri, 24 Sep 2010 13:09:05 -0400, Steven Schveighoffer  
<schveiguy at yahoo.com> wrote:

> On Fri, 24 Sep 2010 12:16:23 -0400, Robert Jacques <sandford at jhu.edu>  
> wrote:
>
>> On Fri, 24 Sep 2010 10:47:16 -0400, Steven Schveighoffer  
>> <schveiguy at yahoo.com> wrote:
>>
>>> On Fri, 24 Sep 2010 10:33:10 -0400, Robert Jacques <sandford at jhu.edu>  
>>> wrote:
>>>
>>>> On Fri, 24 Sep 2010 09:32:40 -0400, Steven Schveighoffer  
>>>> <schveiguy at yahoo.com> wrote:
>>>>> structs can have value copy semantics.  But for a struct that  
>>>>> contains a reference, the references have reference semantics, which  
>>>>> makes the struct a reference type.
>>>>
>>>> A struct is never a reference type. It may have reference semantics,  
>>>> but this is a high-order feature that is uncheckable by the compiler.  
>>>> At best the compiler can detect that a struct contains references,  
>>>> not how those references are exposed/managed by the API.
>>>
>>> Containing references means it cannot be cast to immutable, the only  
>>> important thing when determining the strength of purity here.
>>>
>>> This whole discussion is going nowhere, you haven't made any real  
>>> points.  Can you think of a case where the compiler would be wrong in  
>>> determining purity strength for "value types" as you define them?  An  
>>> example would be most helpful.
>>>
>>> -Steve
>>
>> I feel like I've upset you and would like to make amends. First, this  
>> discussion was never about the compiler being able to determine purity.  
>> That's easy. The point raised was about whether the programmer could  
>> enforce strong-purity for any strongly-pure fucntion. The answer is  
>> that you can by making all arguments immutable, which is a fairly minor  
>> restriction on the callee. Second, this thread (fiber?), has veered a  
>> little off-topic due to a misunderstanding/debate about value-types vs  
>> value-semantics and reference-types vs reference-semantics. Perhaps  
>> it's because I work with large, 'hybrid' structs a lot, but the  
>> difference between value-type and value-semantics is very large in my  
>> mind. Third, this fiber started, in part, due to a question by  
>> regarding value-types being answered with value-semantics. Forth, I  
>> raised the question regarding value-types in part due to  
>> half-remembered old posts in which value-semantics were discussed but  
>> under the label of value-types, thus causing confusion in my brain. And  
>> because it was late at night, I decided to ask a simple question  
>> instead of testing DMD's current behavior myself. I did end up testing  
>> DMD the next day (and then banging my head on a wall), but by then all  
>> this had started.
>
> You haven't upset me, I just couldn't follow what you were saying :)
>
> When you said "what about value types," I thought it was a suggestion  
> that value types would make it hard to determine whether a function was  
> strongly pure or weakly pure, and my interpretation of what "value type"  
> means sent us off on this tangent.
>
> I think we both agree that the proposed changes by Don will work with  
> the compiler, and I understand your desire to ensure a function is  
> strongly-pure, and why the proposal does not give an easy solution to  
> that.
>
> So no hard feelings, we can move on and forget this whole thread ever  
> happened :)
>
> -Steve

Agreed :)