Utility types for manipulating and traversing lists.

util-lists.ts

// Find the first item of a list.
export type Head<List extends any[]> = List['length'] extends 0 ? never : List[0];

// Find the last item of a list.
export type Last<List extends any[]> = number extends List['length'] ? List[1e+309] : _Last<List>;
type _Last<
	Tuple extends any[],
	_I extends number = Tail<Tuple>['length']
> = Tuple extends [] ? never : _I extends keyof Tuple ? Tuple[_I] : never;

// Produce a list skipping the first item.
export type Tail<List extends any[]> = ((..._: List) => any) extends ((_: any, ...tail: infer U) => any) ? U : never;

// Produce a list skipping the last item.
export type Init<List extends any[]> = number extends List['length'] ? List : _Init<List>;
type _Init<
	Tuple extends any[],
	_Tail extends any[] = Tail<Tuple>
> = {
	[I in keyof _Tail]: I extends keyof Tuple ? Tuple[I] : never;
};

// True when the list has a prefix that is a tuple.
export type HasTuplePrefix<List extends any[]> =
	// Only lists having a ...T[] component have (number extends List['length'])
	number extends List['length']
		// If the list is equivalen to an array of its first element, then the
		// list has no leading required elements.
		? List extends List[0][] ? List[0][] extends List ? false : true : true
		: true;

// "unshift" the type H onto the beginning of List.
export type Unshift<List extends any[], H> =
	((_: H, ...args: List) => any) extends (...args: infer U) => any ? U : [];

// Extract the required elements from the list.
export type ExtractTuplePrefix<List extends any[]> =
	// If the list is finite, avoid doing recursive work and return the list directly.
	number extends List['length']
		? _ExtractTuplePrefix<List>
		: List;

// Use only on Lists with non-finite components.
type _ExtractTuplePrefix<
	List extends any[],
	C extends number[] = [],
	_Return = {
		0: C,
		1: _ExtractTuplePrefix<Tail<List>, Unshift<C, 0>>
	}[List extends List[0][] ? List[0][] extends List ? 0 : 1 : 1]
> = _Return extends any[] ? _Return : never;

// Find all the suffixes of a list
export type Suffixes<
	List extends any[],
	_Tail extends any[] = Tail<List>
> = {
	0: List,
	1: List | Suffixes<_Tail>,
	2: List[1e309][]
}[
	_Tail['length'] extends 0
		? 0
		: number extends List['length']
			? List extends List[0][] ? List[0][] extends List ? 2 : 1 : 1
			: 1
];

// Find all the prefixes of a tuple, including the tuple itself.
export type Prefixes<List extends any[]> =
	number extends List['length']
		? List extends List[0][]
			? List[0][] extends List
				? List
				: _Prefixes<List, Suffixes<_ExtractTuplePrefix<List>>> | List
			: _Prefixes<List, Suffixes<_ExtractTuplePrefix<List>>> | List
		: _Prefixes<List>;
type _Prefixes<
	Tuple extends any[],
	_Suffixes = Suffixes<Tuple>
> = {
	[I in keyof _Suffixes]: I extends keyof Tuple ? Tuple[I] : never;
};