`Lens#composeAt` method?

[wbadart]

🚀 Feature request

Current Behavior

It seems that most of the optics include a couple of methods for composing each of the other kinds of optics in order to drill down into the structure another level. Lens, for example, has

• compose
• composeIso
• composeLens
• composeOptional
• composePrism
• composeTraversal

I was wondering: is there a technical or theoretical limitation that prevents composeAt from being implemented? Or just a question of resources? (If the latter, I'm happy to take a stab at the implementation).

Who does this impact? Who is this for?

I bring it up since it's personally useful, but I suspect that since At is such a useful tool, I won't be the last person to want to use it in a more deeply-nested setting. I'm not sure where At (and composing it with other optics) falls on the beginner-advanced spectrum, though it seems to be a pretty important tool for working with immutable maps/ records. (Do any of the other optics allow you to "delete" keys? If so, that could be the basis of another workaround.)

Describe alternatives you've considered

For context, I have a structure like this:

interface Foo {
    bars: Readonly<Record<string, Bar>>
}

And am trying to write something like

import * as A from 'monocle-ts/At';
import * as L from 'monocle-ts/Lens';
import { some, none } from 'fp-ts/Option';

const FooL = {
    barAt: pipe(L.id<Foo>(), L.prop('bars'), L.composeAt(A.atReadonlyRecord()),
}

FooL.barAt('a').set(none)({bars: {a: 10, b: 20}})  // {bars: {b: 20}}

So, to bring it back to the workarounds I tried: I wasn't sure where At falls in the optics hierarchy, so I tried using atReadonlyRecord with composeTraversal and composeOptional. As I should have guessed, these didn't typecheck, and I couldn't figure out a straight-forward conversion.

Really, I'm just trying to include Ats in my pipelines. Is there an existing approach to this that would make the addition of composeAt moot?

Many thanks!