Allow key path with `Struct.get`

[harrysolovay]

Instead of needing additional Struct.gets...

pipe(
  keypairs,
  Struct.get("box"),
  Struct.get("publicKey"),
  Struct.get("inner"),
)

... could we enable a key path?

pipe(
  keypairs,
  Struct.get("box", "publicKey", "inner"),
)

[harrysolovay]

Perhaps something along the following lines:

export declare const get: <A extends Array<PropertyKey>>(
  ...key: A
) => <S extends Constraint<A>>(
  s: S,
) => Select<A, S>

type Constraint<A extends Array<PropertyKey>> = A extends
  [infer E0 extends PropertyKey, ...infer ERest extends Array<PropertyKey>] ? { [K in E0]: Constraint<ERest> }
  : any

type Select<
  A extends Array<PropertyKey>,
  T extends Constraint<A>,
> = A extends [
  infer E0 extends PropertyKey,
  ...infer ERest extends Array<PropertyKey>,
] ? ERest extends [] ? T[E0] : Select<ERest, T[E0]>
  : never

declare const v: {
  box: {
    publicKey: {
      inner: CryptoKey
    }
  }
}

const inner = get("box", "publicKey", "inner")(v)

inner satisfies CryptoKey

[harrysolovay]

It would be great if there was a way to specify optional chaining. Perhaps a special symbol Struct["?"].

declare const v: {
  box?: undefined | {
    publicKey: {
      inner: CryptoKey
    }
  }
}

const inner = get("box", Struct["?"], "publicKey", "inner")(v)

... although this would demand aliasing when not importing Struct as a namespace.

import { "?" as o } from "effect/Struct"