void distance(picture pic=currentpicture, pair A, pair B, Label L="", real n=0,

size(12cm,0);

void distance(picture pic=currentpicture, pair A, pair B, Label L="", real n=0,
pen p=currentpen)
{
real d=3mm;
path g=A--B;
transform T=shift(-n*d*unit(B-A)*I);
pic.add(new void(frame f, transform t) {
picture opic;
path G=T*t*g;
draw(opic,Label(L,Center,UnFill(1)),G,p,Arrows(NoFill),Bars,PenMargins);
add(f,opic.fit());
});
pic.addBox(min(g),max(g),T*min(p),T*max(p));
}

pair A=(0,0), B=(3,3);

dot(A);
dot(B);

distance(A,B,"$\ell$",1);