Function fold

repos/effect/packages/effect/src/Sink.ts:723–745

(
  s: LazyArg<S>,
  contFn: Predicate<S>,
  f: (s: S, input: In) => Effect.Effect<S, E, R>
)

 * @see {@link forEach} for consuming all input while running an effect for each element
 *
 * @category constructors
 * @since 2.0.0
 */
export const drain: Sink<void, unknown> = fromTransform((upstream) =>
  Pull.catchDone(
    Effect.forever(upstream, { disableYield: true }),
    () => endVoid
  )
)

/**
 * A sink that folds its inputs with the provided function, termination
 * predicate and initial state.
 *
 * **When to use**
 *
 * Use to accumulate stream input element by element with an effectful step and
 * stop based on the accumulated state.
 *
 * **Details**
 *
 * The initial state is evaluated lazily. Each input element is folded with the
 * effectful function, and the sink continues while `contFn` returns `true`. If
 * the sink stops in the middle of a pulled array, the remaining elements from
 * that array are returned as leftovers.
 *