I adapted DrBoolean's lens implementation, so that it works without introspection/duck typing/relying on prototype identities. The computation should be exclusively determined by continuations and function arguments of higher order functions. I've come this far:
const id = x => x;
// compose n functions (right to left)
const $$ = (f, ...fs) => x =>
fs.length === 0
? f(x)
: f($$(...fs) (x));
// function encoded Const type
const Const = x => k => k(x);
const constMap = f => fx => fx(x => Const(x));
const runConst = fx => fx(id);
// lens constructor
const objLens = k => cons => o =>
constMap(v => Object.assign({}, o, {[k]: v})) (cons(o[k]));
const arrLens = i => cons => xs =>
constMap(v => Object.assign([], xs, {[i]: v})) (cons(xs[i]));
const view = fx =>
$$(runConst, fx(Const));
const user = {name: "Bob", addresses: [
{street: '99 Maple', zip: 94004, type: 'home'},
{street: '2000 Sunset Blvd', zip: 90069, type: 'work'}]};
// a lens
const sndStreetLens =
$$(objLens("addresses"), arrLens(1), objLens("street"));
console.log(
view(sndStreetLens) (user) // 2000 Sunset Blvd
);
In this version the functor constraint constMap
is hard coded in the lens constructor. When view
invokes a lens, the right type ($$(runConst, fx(Const))
) is already passed, but in order to obtain an ad-hoc polymorphic lense, I need to pass the corresponding functor instance too.
I already tried the most obvious thing $$(runConst, fx(constMap) (Const))
, however, this interferes with the composition. I just can't figure it out.