You can typically simulate laziness by lifting variables into functions. A 0-arity function works fine, but for lists it's slightly more elegant to define them

You can typically simulate laziness by lifting variables into functions. A 0-arity function works fine, but for lists it's slightly more elegant to define them as functions from an index to a value (in that case the functor map is just composition, which is neat). So the following works let loeb = (structure => { let inner = (i) => structure(i)(inner); return inner; }) let fs = (i) => {switch(i) { case 0: return _ => 1 case 1: return (x) => x(0) + 1 case 2: return (x) => x(1) + 1 case 3: return (x) => x(2) + 1 }} If javascript was sensible you could just do let fs = [ _ => 1 , x => x(0) + 1 , x => x(1) + 1 , x => x(2) + 1 ].at but alas that doesn't work, you can do let fs = (i) => [ _ => 1 , x => x(0) + 1 , x => x(1) + 1 , x => x(2) + 1 ][i] though. In that case javascript may end up rebuilding the list every time, I'm not quite sure.