export interface optional<S extends Constraint> extends optionalKey<UndefinedOr<S>> {
  readonly "Rebuild": optional<S>
}

interface optionalLambda extends Lambda {
  <S extends Constraint>(self: S): optional<S>
  readonly "~lambda.out": this["~lambda.in"] extends Constraint ? optional<this["~lambda.in"]> : never
}

/**
 * Marks a struct field as optional, allowing the key to be absent or
 * `undefined`.
 *
 * **Details**
 *
 * The resulting property may be absent or explicitly set to `undefined`.
 * Equivalent to `optionalKey(UndefinedOr(S))`.
 *
 * Use {@link optionalKey} instead if you want exact optional semantics (absent
 * only, not `undefined`).
 *
 * **Example** (Defining an optional field accepting undefined)
 *
 * ```ts
 * import { Schema } from "effect"
 *
 * const schema = Schema.Struct({
 *   name: Schema.String,
 *   age: Schema.optional(Schema.Number)
 * })
 *
 * // { readonly name: string; readonly age?: number | undefined }
 * type Person = typeof schema.Type
 * ```
 *
 * @category combinators
 * @since 3.10.0
 */
export const optional = Struct_.lambda<optionalLambda>((self) => optionalKey(UndefinedOr(self)))