Focuses on a part A of S that may not be present (e.g. a union variant or a validated subset).

When to use

Use when the focus is conditional — reading can fail (wrong variant, failed validation).

• Building a new S from A does not require the original S.

Details

• getResult(s) returns Result.Success<A> when the focus matches, or Result.Failure<string> with an error message.
• set(a) always succeeds and returns a new S.
• Extends Optional.
• Composing two Prisms produces a Prism; composing a Prism with a Lens produces an Optional.

Example (Narrowing a tagged union)

import { Optic, Result } from "effect"

type Shape =
  | { readonly _tag: "Circle"; readonly radius: number }
  | { readonly _tag: "Rect"; readonly width: number }

const _circle = Optic.id<Shape>().tag("Circle")

console.log(Result.isSuccess(_circle.getResult({ _tag: "Circle", radius: 5 })))
// Output: true

console.log(Result.isFailure(_circle.getResult({ _tag: "Rect", width: 10 })))
// Output: true