/**
 * An array of type T that is guaranteed not to be empty.
 */
export declare type NonEmpty<T> = [T, ...T[]];
/**
 * Helper normalizing handling/converting from an T[] to NonEmpty<T>.
 *
 * @example
 * const ids: string[] = [];
 * const results: string[] = ifEmpty(
 *   ids,
 *   () => [],
 *   (ids) => ids.map((id) => `hello-${id}`)
 * );
 *
 * @param arr An array that may or may not be empty.
 * @param then_ Called if `arr` is empty.
 * @param else_ Called if `arr` is not empty with a properly typed `NonEmpty<T>`.
 */
export declare const ifEmpty: <T, U>(arr: T[], then_: () => U, else_: (nonEmptyArr: NonEmpty<T>) => U) => U;
/**
 * Verifies an array has at least one element.
 *
 * @param arr An array that may or may not be empty.
 * @param [emptyErrorMessage] Error message when array is empty. Default message is 'Unexpected empty array.'.
 */
export declare const assertNotEmpty: <T>(arr: T[], emptyErrorMessage?: string) => NonEmpty<T>;