hunk.code_window:  *
    * @param {S} initialState Initial state.
    * @param {function (state: S): boolean} condition Condition to terminate generation (upon returning false).
    * @param {function (state: S): S} iterate Iteration step function.
    * @param {function (state: S): T} resultSelector Selector function for results produced in the sequence.
    * @param {SchedulerLike} [scheduler] A {@link SchedulerLike} on which to run the generator loop. If not provided, defaults to emit immediately.
    * @returns {Observable<T>} The generated sequence.
    */
     export function generate<T, S>(initialState: S,
  
hunk.labels: keep keep keep keep replace keep keep keep keep
hunk.after_edit:  * @param {function (state: S): T} resultSelector Selector function for results produced in the sequence. (deprecated)
  
hunk.file_path: src/internal/observable/generate.ts
hunk.type: replace
file: import { Observable } from '../Observable';
  import { Operator } from '../Operator';
  import { Subscriber } from '../Subscriber';
  import { EmptyError } from '../util/EmptyError';
  import { OperatorFunction } from '../../internal/types';
  import { filter } from './filter';
  import { take } from './take';
  import { defaultIfEmpty } from './defaultIfEmpty';
  import { throwIfEmpty } from './throwIfEmpty';
  import { identity } from '../util/identity';
  
  /* tslint:disable:max-line-length */
  export function first<T, D = T>(
    predicate?: null,
    defaultValue?: D
  ): OperatorFunction<T, T | D>;
  export function first<T, S extends T>(
    predicate: (value: T, index: number, source: Observable<T>) => value is S,
    defaultValue?: S
  ): OperatorFunction<T, S>;
  export function first<T, D = T>(
    predicate: (value: T, index: number, source: Observable<T>) => boolean,
    defaultValue?: D
  ): OperatorFunction<T, T | D>;
  /* tslint:enable:max-line-length */
  
  /**
   * Emits only the first value (or the first value that meets some condition)
   * emitted by the source Observable.
   *
   * <span class="informal">Emits only the first value. Or emits only the first
   * value that passes some test.</span>
   *
   * ![](first.png)
   *
   * If called with no arguments, `first` emits the first value of the source
   * Observable, then completes. If called with a `predicate` function, `first`
   * emits the first value of the source that matches the specified condition. It
   * may also take a `resultSelector` function to produce the output value from
   * the input value, and a `defaultValue` to emit in case the source completes
   * before it is able to emit a valid value. Throws an error if `defaultValue`
   * was not provided and a matching element is not found.
   *
   * ## Examples
   * Emit only the first click that happens on the DOM
   * ```javascript
   * const clicks = fromEvent(document, 'click');
   * const result = clicks.pipe(first());
   * result.subscribe(x => console.log(x));
   * ```
   *
   * Emits the first click that happens on a DIV
   * ```javascript
   * const clicks = fromEvent(document, 'click');
   * const result = clicks.pipe(first(ev => ev.target.tagName === 'DIV'));
   * result.subscribe(x => console.log(x));
   * ```
   *
   * @see {@link filter}
   * @see {@link find}
   * @see {@link take}
   *
   * @throws {EmptyError} Delivers an EmptyError to the Observer's `error`
   * callback if the Observable completes before any `next` notification was sent.
   *
   * @param {function(value: T, index: number, source: Observable<T>): boolean} [predicate]
   * An optional function called with each item to test for condition matching.
   * @param {R} [defaultValue] The default value emitted in case no valid value
   * was found on the source.
   * @return {Observable<T|R>} An Observable of the first item that matches the
   * condition.
   * @method first
   * @owner Observable
   */
  export function first<T, D>(
    predicate?: ((value: T, index: number, source: Observable<T>) => boolean) | null,
    defaultValue?: D
  ): OperatorFunction<T, T | D> {
    const hasDefaultValue = arguments.length >= 2;
    return (source: Observable<T>) => source.pipe(
      predicate ? filter((v, i) => predicate(v, i, source)) : identity,
      take(1),
      hasDefaultValue ? defaultIfEmpty<T | D>(defaultValue) : throwIfEmpty(() => new EmptyError()),
    );
  }
  