How to break const

[Timon Gehr]

On 06/19/2012 12:49 PM, Iain Buclaw wrote:
> On 19 June 2012 10:47, Christophe Travert<travert at phare.normalesup.org>  wrote:
>> Iain Buclaw , dans le message (digitalmars.D:170145), a écrit :
>>> On 19 June 2012 09:18, Don Clugston<dac at nospam.com>  wrote:
>>>> So would I. Can you think of one?
>>>> It was the best name I could come up with, given that the 'pure' was the
>>>> keyword.
>>>> We want a word that means 'no hidden state'.
>>>
>>> I thought that was what pure was for. :~)
>>>
>>> --
>>> Iain Buclaw
>>>
>>> *(p<  e ? p++ : p) = (c&  0x0f) + '0';
>>
>>
>> A delegate does have a frame pointer, and it's not that well hidden. If
>> you want no such 'hidden state', you do not want a delegate, you want a
>> function pointer. That means all delegates are weakly pure (until they
>> have an immutable frame pointer qualifier).
>>
>> If you want that this 'hidden state' does not change, that is another
>> story. pure for a delegate could mean that the frame pointer does not
>> change, but then, pure methods wouldn't allow you to make pure
>> delegates:
>>
>> struct S
>> {
>>   int i;
>>   int foo() pure { return i; }
>> }
>>
>> S s;
>> int delegate() pure dg =&s.foo;
>> // error if pure change meaning when applied to a delegate
>>
>
> So we have a few combinations then:
>
> pure  - as in weakly pure, guarantees not to change global state, but
> may alter it's own hidden state.
>
> pure nothrow - as in strongly pure, where is guaranteed not to have
> any side effects, so is suitable for constant folding

This would need to be qualified 'pure immutable nothrow', otherwise it
might change it's own hidden state.

> / the usual optimisations for a function typically marked as __pure__ in C.
>

Some of them can even be applied if it is just 'pure immutable '. loop
invariant code motion, for instance.

> pure const - similar to strongly pure, as is guaranteed not to be able
> to alter its own state (as it's const), but still may have side
> effects / throw an exception.
>

Therefore, you think 'pure const' should be allowed to have side effects?

>
> Make much sense? :-)
>