Why map return [] ?

[cym13 via Digitalmars-d-learn]

On Friday, 14 April 2017 at 15:29:33 UTC, Suliman wrote:
> auto x = MySQLTablesRange.map!(a=>a);
> writeln(x);
>
> return: []
>
> while next code:
> MySQLTablesRange.each!(a=>a.writeln);
>
> return data line by line.
>
> Why?

What library is that supposed to be?