Store mutable indirections in immutable data with this one weird trick!

[Andrei Alexandrescu]

On 2021-11-13 12:14, Paul Backus wrote:
> On Saturday, 13 November 2021 at 16:57:47 UTC, Imperatorn wrote:
>> On Saturday, 13 November 2021 at 14:43:00 UTC, Paul Backus wrote:
>>> On Saturday, 13 November 2021 at 14:35:36 UTC, Imperatorn wrote:
>>>> Many use the "compromise edition" to pure, I think it's ok. Pure in 
>>>> that sense is just same out for a given in, doesn't matter what the 
>>>> function does inside.
>>>
>>> [`pureMalloc`][1] does not return the same output for a given input.
>>>
>>> [1]: https://druntime.dpldocs.info/core.memory.pureMalloc.html
>>
>> Yeah, these words are not precise enough.
> 
> To be fair, the language spec is also not very precise about this:
> 
>> An implementation may assume that a strongly pure function that 
>> returns a result without mutable indirections will have the same 
>> effect for all invocations with equivalent arguments. It is allowed to 
>> memoize the result of the function under the assumption that 
>> equivalent parameters always produce equivalent results.
> 
> What does "equivalent" mean, here? The intent seems to be something like 
> "equal, if you ignore differences in memory addresses," but the term is 
> never actually defined.

The upside of these missing definitions is you have the opportunity to 
introduce them. I'm sure related pull requests would be received well.