validation.ts

/**
 * We could use `Either<string, T>` as a validaiton result. But the problem is,
 * we wouldn't be able to accumulate the errors. The `Validation` types enables
 * that for us.
 *
 * ```ts
 * type Validation<E, T> = Either<NonEmptyArray<E>, T>
 * ```
 *
 * @example
 * import * as Either from 'fp-ts/Either';
 * import { pipe } from 'fp-ts/function';
 * import { sequenceT } from 'fp-ts/Apply';
 * import { getSemigroup, NonEmptyArray } from 'fp-ts/NonEmptyArray';
 *
 * const notAllowedUrls = (s: string) =>
 *   s.includes('facebook') ? Either.left('Not allowed') : Either.right(s);
 *
 * const wordLimit = (s: string) =>
 *   s.split(' ').length > 5 ? Either.left('Too many words') : Either.right(s);
 *
 * const hasBannedWords = (s: string) =>
 *   s.includes('BAD_WORD') ? Either.left('Includes bad word') : Either.right(s);
 *
 * const applicativeValidation =
 *   Either.getApplicativeValidation(getSemigroup<string>());
 *
 * const validateText = (s: string) =>
 *   pipe(
 *     sequenceT(applicativeValidation)(
 *       liftValidation(notAllowedUrls)(s),
 *       liftValidation(wordLimit)(s),
 *       liftValidation(hasBannedWords)(s)
 *     ),
 *     Either.map(() => s)
 *   );
 *
 * validateText('BAD_WORD one two three four five six');
 * //=> Either.left(['Too many words', 'Includes bad word'])
 *
 * validateText('I am good');
 * //=> Either.right('I am good')
 */
export function liftValidation<E, A>(
  check: (a: A) => Either.Either<E, A>
): (a: A) => Either.Either<NonEmptyArray.NonEmptyArray<E>, A> {
  return (a) =>
    pipe(
      check(a),
      Either.mapLeft((a) => [a])
    );
}