function factoryWhitespace(effects, ok) {
  /** @type {boolean} */
  let seen

  return start

  /** @type {State} */
  function start(code) {
    if (markdownLineEnding(code)) {
      effects.enter(types.lineEnding)
      effects.consume(code)
      effects.exit(types.lineEnding)
      seen = true
      return start
    }

    if (markdownSpace(code)) {
      return factorySpace(
        effects,
        start,
        seen ? types.linePrefix : types.lineSuffix
      )(code)
    }

    return ok(code)
  }
}
Parse spaces and tabs.

There is no `nok` parameter:

*   line endings or spaces in markdown are often optional, in which case this
    factory can be used and `ok` will be switched to whether spaces were found
    or not
*   one line ending or space can be detected with
    `markdownLineEndingOrSpace(code)` right before using `factoryWhitespace`

@param {Effects} effects
  Context.
@param {State} ok
  State switched to when successful.
@returns {State}
  Start state.