Understanding Layer Caching and Recreation in Effect TypeScript

import { expect, it } from "@effect/vitest"
import { Context, Effect, Layer } from "effect"

class Counter extends Context.Tag("@app/Counter")<
  Counter,
  { readonly get: () => Effect.Effect<number>; readonly increment: () => Effect.Effect<void> }
>() {
  static readonly layer = Layer.sync(Counter, () => {
    let count = 0
    return {
      get: () => Effect.succeed(count),
      increment: () => Effect.sync(() => void count++),
    }
  })
}

// Each test provides the layer, so each gets its own fresh counter
it.effect("starts at zero", () =>
  Effect.gen(function* () {
    const counter = yield* Counter
    expect(yield* counter.get()).toBe(0)
  }).pipe(Effect.provide(Counter.layer)),
)

it.effect("increments without leaking", () =>
  Effect.gen(function* () {
    const counter = yield* Counter
    yield* counter.increment()
    expect(yield* counter.get()).toBe(1)
  }).pipe(Effect.provide(Counter.layer)),
)