DIP54 : revamp of Phobos tuple types

[Jakob Ovrum]

On Tuesday, 24 December 2013 at 10:30:47 UTC, Dmitry Olshansky 
wrote:
> As long as algorithms that operate say on 2 lists expect 
> arguments to have '.expand' we'd be in a good shape just 
> providing a `packList` or some such together with `list`.

I think so too. Of course, it is predicated on the fact that such 
algorithms are added to Phobos. Note that when an algorithm 
always expects 2 lists we can use nested templates instead, which 
has some benefits.

The issue, of course, is that this proposal suggests having only 
`packList` and changing all list algorithms to expect packed 
lists, which I think (and have elaborated upon) is a nasty case 
of fighting the language for no benefit, but for serious code 
breakage and lots of extra code on lines that tend to be long 
already[1].

> It would even work out of the box with current user-defined 
> pack types if they have .expand.

Right, std.typecons.Tuple happens to be one such type.

[1] It's not the case that we can simply search and replace 
`TypeTuple!(...)` => `TemplateArgumentList!(...).expand` and call 
it a day; consider code like:
---
auto fun(Ranges...)(Ranges ranges)
     if (!is(CommonType!(staticMap!(ElementType, Ranges)) == void)
{
     ...
}
---
If list algorithms are changed to receive packed list, the 
`Ranges` argument to `staticMap` must be changed to 
`TemplateArgumentList!Ranges`.