Function exactlyOnce

packages/effect/test/Effect/filtering.test.ts:10–21

(
  value: A,
  f: (_: Effect.Effect<A>) => Effect.Effect<A1, string, R>
)

import { strictEqual } from "node:assert"

const exactlyOnce = <R, A, A1>(
  value: A,
  f: (_: Effect.Effect<A>) => Effect.Effect<A1, string, R>
): Effect.Effect<A1, string, R> => {
  return Effect.gen(function*() {
    const ref = yield* (Ref.make(0))
    const res = yield* (f(pipe(Ref.update(ref, (n) => n + 1), Effect.zipRight(Effect.succeed(value)))))
    const count = yield* (Ref.get(ref))
    yield* (count !== 1 ? Effect.fail("Accessed more than once") : Effect.void)
    return res
  })
}

describe("Effect", () => {
  it.effect("filter - filters a collection using an effectual predicate", () =>