DIP54 : revamp of Phobos tuple types

[Jakob Ovrum]

On Sunday, 29 December 2013 at 11:10:00 UTC, Dicebot wrote:
> Despite my initial desire to standardize API between library 
> types it seems that breakage will be much more damaging than I 
> have initially expected. Jakob example 
> (http://forum.dlang.org/post/vkeqyorptcobufzmmhgm@forum.dlang.org) 
> does not look _that_ bad but imagining is(typeof()) error 
> suppression on top is pain.

Breakage that causes errors in secondary locations and cannot be 
automatically repaired (e.g. search & replace) is the worst kind 
of breakage, after silent failures, that I can think of.

> As an alternative, I propose signatures of existing templates 
> as-is but augment them all with overloads taking single `Pack` 
> (and make it requirement when introducing new ones).

Not a single of the existing algorithms need packed lists. 
Further, this DIP doesn't present a single algorithm that *does* 
need packed lists. It adds a huge maintenance cost and cognitive 
cost for no benefit.

> New multilist utilities added are expected to use `Pack`s (I am 
> still convinced that using nested templates for that is ugly)

I don't consider "it's ugly" with no elaboration a valid argument.

> Point about omitting expanding library type still remains. I am 
> still looking for use case when those are really necessary (as 
> a library type).

As elaborated upon in an earlier post[1], I think auto-expansion 
is *the* use case for template argument lists, as a natural 
consequence of language features. That's what makes this DIP so 
absurd - you are asking everyone to type `.expand` literally 
everywhere such a list is actually applied. The only tangible 
rationale for this was that auto-expansion is hard to learn, 
which I think is not only unsubstantiated, but rather moot 
because I think *expansion* is what needs grokking, and there's 
no way around that because at the end of the line, expansion is 
the only use case of template argument lists.

By fixing the naming issue, we'd be making strides in easing 
teaching of list expansion, without making lists harder to use by 
sometimes requiring `.expand` and sometimes not, sometimes 
requiring `TemplateArgumentPack!Args` and sometimes just `Args`, 
and without breaking code in non-trivial ways.

[1] 
http://forum.dlang.org/post/glqrqbynhpeidktrbjca@forum.dlang.org