function flatMap<ItemType, ProjectedItemType>(projectorFn: (item: ItemType) => ProjectedItemType[]): (aggregate: ProjectedItemType[], item: ItemType) => ProjectedItemType[] {
	return (aggregate: ProjectedItemType[], current: ItemType) => aggregate.concat(projectorFn(current) || []);
}

['alice', 'bob'].reduce(flatMap(name => name.split('')), []);
['alice', 'bob'].reduce(flatMap<string, string>(name => name.split('')), []); // this works fine