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

[Paul Backus]

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.