Preliminary submission - std.rational and std.typelist

[Jonathan M Davis]

On Sunday, October 07, 2012 22:57:33 Arlen wrote:
> I can live with that, but, as I explained before, TypeTuples cause
> code duplication in certain cases because they force 'alias' to be
> used in the signature of the metafunctions.
> 
> And how would you return a multi-dimensional TypeTuple?  The result of
> certain metafunctions are multi-dimensional.  For example,
> Permutations and Combinations will return multi-dimensional
> "containers".
> 
> http://arlen.github.com/phobos/std_typelist2.html#Permutations

While this sort of function might be useful from time to time, how often is it 
actually, realistically needed? Certainly, I'd argue that it's rare enough 
that having a slightly more complicated solution specifically for it makes more 
sense than adding a whole new abstraction to the standard library.

Adding something like TypeList on top of TypeTuple is generally overkill and 
overcomplicates things. TypeTuple will do almost everything that TypeList can. 
The question then is how to make TypeTuple do the parts that it can't 
currently do or how to provide a reasonable, alternate solution for those rare 
circumstances. Maybe a multi-dimensional wrapper for TypeTuple would be in 
order to handle those cases?

Regardless, we previously decided that TypeList was unnecessary code 
duplication and an unnecessary cognitive burden. Everything you add to the 
standard library has a cost, even if it's valuable in and of itself, and 
adding something very similar to something else which is already there is that 
much worse, because then you have to understand the differences and constantly 
explain them to those who don't know. So, we're not adding TypeList.

While I sympathize with your desire to get some of the functionality that 
TypeList provides and TypeTuple doesn't, please work with us to improve upon 
TypeTuple rather than trying to add something to Phobos that does almost the 
same thing as TypeTuple. In most cases, it's simply a matter of adding the 
appropriate templates to gain TypeList-like functionality on top of TypeTuple 
rather than adding TypeList, and we've even recently added (post-2.060) some 
of those templates to std.typetuple, thereby improving what can be done with 
std.typetuple.

- Jonathan M Davis