Flatten and intersperse a dynamic menu in Typescript; type-level and runtime code

dynamicMenu.ts

type Group = 'item'[];
type NEGroup = NonEmptyArray<'item'>;
// whatever our dynamic code produced; can be empty or non-empty groups
type DynamicMenu = Group[];
type FilterEmptyGroups<T> = T extends DynamicMenu ?
  (T extends [[], ...infer R] ? FilterEmptyGroups<R> : T extends [] ? T : T extends [infer NG, ...infer R] ? (
    NG extends NEGroup ? [NG, ...FilterEmptyGroups<R>] : [NEGroup, ...FilterEmptyGroups<R>]
    ) : NEGroup[])
  : never;
type Joined<T> =
  T extends NEGroup[] ?
    (
      T extends [infer NG1, infer NG2, ...infer R] ? (
        NG1 extends NEGroup ? (
          NG2 extends NEGroup ? [NG1, 'separator', ...Joined<[NG2, ...R]>] : [NG1, 'separator', ...Joined<[NEGroup, ...R]>]
          ) : [NEGroup, 'separator', ...Joined<[NEGroup, ...R]>]
        ) : T extends [] ? T : T extends [infer NG] ? [NG] : (NEGroup | 'separator')[]
      )
    : never;
type MenuInvariants<T> =
  T extends DynamicMenu ? Joined<FilterEmptyGroups<T>> : never;

type FlattenMenu<T> = T extends [infer NG, ...infer R] ? (
    NG extends NEGroup ? [...NG, ...FlattenMenu<R>] : NG extends 'separator' ? [NG, ...FlattenMenu<R>] : never
    ) :
  T extends [] ? [] :
    T extends ['separator', ...infer R] ? ['separator', ...FlattenMenu<R>] :
      T extends (NEGroup | 'separator')[] ? ('item' | 'separator')[] : never;
const menu1: FlattenMenu<MenuInvariants<[['item'], [], ['item', 'item'], ['item', 'item'], [], []]>> = [
  'item',
  'separator',
  'item', 'item',
  'separator',
  'item', 'item'
] as const;

const ITEM = 'item' as const;
const SEPARATOR = 'separator' as const;
const renderMenu = <M extends DynamicMenu>(dm: M) =>
  pipe(dm,
    A.filter(A.isNonEmpty),
    A.intersperse(SEPARATOR as typeof SEPARATOR | NonEmptyArray<'item'>),
    A.flatMap(x => x === SEPARATOR ? [SEPARATOR] : x)
  ) as FlattenMenu<MenuInvariants<M>>;
const dynamicMenu: DynamicMenu = [[ITEM], [], [ITEM, ITEM], [ITEM, ITEM], [], []];
const menu = renderMenu(dynamicMenu);
console.log(menu);
// [ 'item', 'separator', 'item', 'item', 'separator', 'item', 'item' ]