On Tuesday, 11 March 2014 at 05:37:25 UTC, Timothee Cour wrote:

void writelnIfNotEmpty(T)(T a){
auto file_pos=get_filepos(stdin);
write(a);
if(get_filepos(stdin)!=file_pos)
writeln;
}

You could simply create an sink that forwards to stdout, while keeping state:

//----
import std.stdio, std.format;

bool writelnIfNotEmpty(T)(T a)
{
    bool written = false;
    void checkWriter(in char[] s)
    {
        if (s.length)
        {
            written = true;
            write(s);
        }
    }
    formattedWrite(&checkWriter, "%s", a);
    if (written)
    {
        writeln();
        return true;
    }
    return false;
}

void main()
{
    writelnIfNotEmpty(1);
    writelnIfNotEmpty("");
    writelnIfNotEmpty(2);
}
//----

This prints:
//----
1
2
//----

Also, this didn't work up until a few releases ago. I am really really happy to see code like this finally "just work". yay!