z0=(0,0);
z1=(1.5gu,0);
z2=(0,-gu);
z3=(1.5gu,-gu);
path ac,ab;

label(btex $\mathtt{[a]}$ etex,z0);
label(btex $\mathtt{[b]}$ etex,z1);
label(btex $\mathtt{Maybe\ a}$ etex,z2 shifted (-u,0));
label(btex $\mathtt{Maybe\ b}$ etex,z3 shifted (u,0));

drawEdge(z0,z1,"\mathtt{fmap_{[]}\ f}");
drawEdge(z2,z3,"\mathtt{fmap_{Maybe}\ f}");

path abb,acb;
abb:=edge(z2 shifted (.2u,0),z0 shifted (.2u,0));
acb:=edge(z3 shifted (-.2u,0),z1 shifted (-.2u,0));
drawarrow abb;
drawarrow acb;
label.rt(btex $\mathtt{mToList}$ etex,midpoint(abb));
label.lft(btex $\mathtt{mToList}$ etex,midpoint(acb));