TypeFunction example creatiing a conversion matrix

[Andrei Alexandrescu]

On 10/1/20 7:10 PM, Paul Backus wrote:
> On Thursday, 1 October 2020 at 21:34:50 UTC, Adam D. Ruppe wrote:
>>
>> So some facility to turn a ctfe array back into a tuple - the dereifiy 
>> step basically - with compiler assistance I think will be practical.
> 
> What if we could just mutate tuples locally?
> 
> template staticMap(alias F, Args...) {
>      static foreach(ref Arg; Args)
>          Arg = F!Arg;
>      alias staticMap = Args;
> }

Interesting - this is akin to D's relaxed purity whereby a pure function 
can mutate its arguments. I'm trying to wrap my head around it, e.g. is 
Args considered a private copy of the argument much like a static array 
passed by value?

> The obvious objection is "the spec says order of declarations doesn't 
> matter", but this is a case where the spec is just flat-out wrong (see 
> previous discussion of "compile-time race conditions" [1]). So, given 
> that declaration order is in fact significant in D, we may as well 
> officially acknowledge it and reap the benefits.
> 
> [1] 
> https://forum.dlang.org/thread/swbmgrhtoiqtqokmqniu@forum.dlang.org?page=2#post-rccfbm:24sjv:241:40digitalmars.com 

I don't get the interaction. How does mutating the argument affect order 
of declaration?