is std.algorithm.joiner lazy?

[Puming via Digitalmars-d-learn]

Hi:

when I use map with joiner, I found that function in map are 
called. In the document it says joiner is lazy, so why is the 
function called?

say:

int[] mkarray(int a) {
    writeln("mkarray called!");
    return [a * 2]; // just for test
}

void main() {
    auto xs = [1, 2];
    auto r = xs.map!(x=>mkarray(x)).joiner;
}

running this will get the output:

mkarray called!
mkarray called!

I suppose joiner does not consume?

when I actually consume the result by writlen, I get more output:

mkarray called!
mkarray called!
[2mkarray called!
mkarray called!
, 4]

I don't understand