Code shared from run.dlang.io. Run with '-unit test'

main.d

/+dub.sdl:
dependency "sumtype" version="~>1.2.0"
dependency "emsi_containers" version="~>0.9.0"
+/
void main()
{
    import std.range, std.stdio;

    auto sum = 0.0;
    auto count = stdin //Get an input range set up to read one line at a time
    .byLine //Perform a transparent operation (as in the shell command tee)
    .tee!(l => sum += l.length).walkLength;

    writeln("Average line length: ", count ? sum / count : 0);
}