What are tuples exactly? (D's tuples considered harmful)

[Jonathan M Davis]

On Friday, February 25, 2011 03:39:31 Morlan wrote:
> While trying to understand the expand mechanism presented in the TDPL book
> I tried to read std.typetuple and std.typecons files. I found a true
> nightmare in those files in the form of an almost infinite chain of
> aliases and macro processing. How can one understand a written text if the
> words are redifined in every other line? And people say that goto
> instruction is bad? Please give me a break.
> 
> Anyway, at some point I realized that I cannot understand what is going on
> because there is some language mechanism in action which I do not know. I
> wrote a small program to confirm this. Here it is:
> 
> struct S { TypeTuple!(int, double) field; }
> void main(){
> 	S mys;
> 	mys.field[0] = 4;
> 	mys.field[1] = 4.4;
> }
> 
> It compiles all right. But if you replace the S's definition with {int,
> double field;}
> it does not compile. So tuples are clearly much more than a sequence of
> types and they trigger a completely different semantic action than a plain
> sequence of types. Is there a precise definition of tuples somewhere?

If all you want is a normal tuple, use Tuple from std.typecons. It even has the 
convenience function tuple for creating them. TypeTuple is a different beast 
entirely, and you probably don't want that unless you specifically need it. It 
has to do with processing types and are _not_ generally what you want. If what 
you're looking for is tuples, then use std.typecons.Tuple.

- Jonathan M Davis