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