/**
 * Index getter
 */
export function at<T>(idx: number): (array: ReadonlyArray<T>) => T | undefined {
  if (idx < 0) {
    return (array) => array[array.length + idx];
  } else {
    return (array) => array[idx];
  }
}

/**
 * Array first item getter
 */
export const first: <T extends ReadonlyArray<any>>(
  array: T,
) => T[0] | undefined = at(0);

/**
 * Array second item getter
 */
export const second: <T extends ReadonlyArray<any>>(
  array: T,
) => T[1] | undefined = at(1);

/**
 * Array third item getter
 */
export const third: <T extends ReadonlyArray<any>>(
  array: T,
) => T[2] | undefined = at(2);

/**
 * Array last item getter
 */
export const last: <T>(array: ReadonlyArray<T>) => T | undefined = at(-1);

/**
 * Applies map and filter on an array, using a single function
 */
export function fmap<A, T extends A>(
  m?: undefined,
  f?: (val: A) => val is T,
): (array: A[]) => T[];
export function fmap<A>(
  m?: undefined,
  f?: (val: A) => boolean,
): (array: A[]) => A[];
export function fmap<A, B, T extends B>(
  m: (val: A) => B,
  f?: (val: B) => val is T,
): (array: A[]) => T[];
export function fmap<A, B>(
  m: (val: A) => B,
  f?: (val: B) => boolean,
): (array: A[]) => B[];
export function fmap<A, B>(
  m?: (val: A) => B,
  f?: (val: A | B) => boolean,
): (array: A[]) => (A | B)[] {
  return (array) => {
    if (m && f) {
      return array.map(m).filter(f);
    } else if (m) {
      return array.map(m);
    } else if (f) {
      return array.filter(f);
    } else {
      return array.slice();
    }
  };
}