Add At/Index class (#43)

[leighman]

Thought I'd give this a try - I think I've understood correctly...
Added At for StrMap and Set based on the monocle module. Noticed that PureScript has set always with a Maybe - do we want that here instead?
Is it worth adding atRecord or atOption as PS does?
Wasn't sure about the composition stuff, would appreciate some guidance on that, then could probably look at doing Index too.

[gcanti]

I think that S.member(setoid), S.insert(setoid) and S.remove(setoid) can be memoized

export function atSet<A = never>(setoid: Setoid<A>): At<Set<A>, A, boolean> {
  const member = S.member(setoid)
  const insert = S.insert(setoid)
  const remove = S.remove(setoid)
  return new At(
    at => new Lens(s => member(s)(at), a => s => (a ? insert(at, s) : remove(at, s)))
  )
}

[gcanti]

Thanks @leighman

Noticed that PureScript has set always with a Maybe - do we want that here instead?

Since this library is a porting of Scala's monocle I'd stick with monocle definitions

Added At for StrMap and Set based on the monocle module

Let's move them in specific files so that users are not forced to load unwanted dependencies, something along the lines of

- src
  - index.ts (<= At definition)
  - At (<= At instances)
    - StrMap.ts (<= atStrMap instance)
    - Set.ts (<= atSet instance)

Is it worth adding atRecord or atOption as PS does?

• atOption: no
• atRecord: not sure what's this, could you please elaborate?

Wasn't sure about the composition stuff

What do you mean, for example fromIso?

[leighman]

Like that?

With atRecord I meant an At over a TS Record or object or something for people who are not using StrMap. I guess they could write one if needed.

What do you mean, for example fromIso

Yup, whether it should be asAt in Iso etc.

[gcanti]

I guess they could write one if needed

Yes, if we add a fromIso method to At

export class At<S, I, A> {
  ...
  /** lift an instance of `At` using an `Iso` */
  fromIso<T>(iso: Iso<T, S>): At<T, I, A> {
    return new At(i => iso.composeLens(this.at(i)))
  }
}

then people can build an At for Record<string, A> pretty easily

const isoStrMap = <A = never>() => new Iso<Record<string, A>, SM.StrMap<A>>(s => new SM.StrMap(s), a => a.value)

const atRecord = atStrMap<string>().fromIso(isoStrMap())

[leighman]

Added Index too.
I used strmapIndex and arrayIndex because that's consistent with monocle but it's atStrMap in At. Should we unify them?

[gcanti]

Added Index too

Great, thanks.

Should we unify them?

Yes please, let's rename them to indexStrMap and indexArray.

---

Are src/At/index.ts, src/Index/index.ts useful? Looks like they are just a burden to maintain

[leighman]

Renamed.

Are src/At/index.ts, src/Index/index.ts useful? Looks like they are just a burden to maintain

I agree. Misunderstood your earlier comment. Removed them.

[gcanti]

Thanks a lot @leighman

[leighman]

Cheers!