Excel list functions

List

/**
 * Tests whether the input is a list.
 * A list is an array with one row or column.
 */
isList = LAMBDA(input, AND(TYPE(input) = 64, OR(ROWS(input) = 1, COLUMNS(input) = 1)));

/**
 * Applies an array-returning function to each element of the input list,
 * returning a flattened array. The result are arranged vertically.
 * ```
 * flatMap({1; 2; 3}, LAMBDA(x, SEQUENCE(x))) ==> { 1; 1; 2; 1; 2; 3}
 * ```
 */
flatMap = LAMBDA(input, function,
    IF(
        NOT(isList(input)),
        #VALUE!,
        DROP(REDUCE("", input, LAMBDA(prev, current, VSTACK(prev, function(current)))), 1)
    )
);

/**
 * Returns a list that is `count` repetitions of the original list.
 * ```
 * repeat({1; 2}, 3) ==> { 1; 2; 1; 2; 1; 2}
 * ```
 */
repeat = LAMBDA(input, count, flatMap(SEQUENCE(count), LAMBDA(_, input)));

/**
 * Tests whether the input list contains the element.
 * Comparision function `equals` can be provided to control equality testing.
 * If `equals` is omitted then exact matching is used.
 */
contains = LAMBDA(input, element, [equals],
    IF(
        ISOMITTED(equals),
        IF(TYPE(input) = 16, input, NOT(ISERROR(XMATCH(element, input)))),
        TYPE(FILTER(input, MAP(input, LAMBDA(x, equals(x, element))), FALSE)) = 64
    )
);