import type { MatcherResult } from './shared/types';
declare global {
    namespace jest {
        interface Matchers<R> {
            toBeEither(): R;
        }
    }
}
/**
 * Asserts that the received value is an `Either` instance.
 *
 * @example
 * ```typescript
 * import { left, right } from 'fp-ts/lib/Either'
 *
 * test('passes when value is an Either', () => {
 *   expect(left(true)).toBeEither()
 *   expect(right(true)).toBeEither()
 * })
 *
 * test('passes when value is not an Either', () => {
 *   expect(undefined).not.toBeEither()
 * })
 * ```
 */
export declare function toBeEither(actual: unknown): MatcherResult;