What is parameter tuple actually?

[Daniel Keep]

Yang Bo wrote:
> on gdc, this code does not works:

Please do not say "does not work."  That's incredible vague, and doesn't
give us any hints as to *why* something doesn't work.  If you can, post
the output of the compiler.

> template TestLines(alias Line) {
>   const char[] lineString = Line.stringof;
>   const uint line = Line;
> }
> 
> unittest{
>   assert(TestLines!(__LINE__).lineString == __LINE__.stringof);
>   assert(TestLines!(__LINE__).line == __LINE__);
> }
> 
> 
> But, if I use tuple instead, it works:
> 
> 
> template TestLines(Args...) {
>   const char[] lineString = Args[0].stringof;
>   const uint line = Args[0];
> }
> 
> unittest{
>   assert(TestLines!(__LINE__).lineString == __LINE__.stringof);
>   assert(TestLines!(__LINE__).line == __LINE__);
> }
> 
> I wonder why.

I suspect it's because __LINE__ isn't a symbol.  __LINE__ is basically
replaced by the compiler with its value when it's read from the file (at
some stage), and I'll bet it's *before* templates are expanded.  So in
the first instance, you've got:

TestLines!(11).lineString

Which won't work because you can't alias an expression.  The second one
works because it's probably passing the value of __LINE__ as a constant
expression, not a symbol alias.

	-- Daniel