Type-level range on TypeScript

README.md

Type-level range on TypeScript

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range.ts

type Nat = unknown[];

type Zero = [];

type Suc<N extends Nat> = [...N, unknown];

type Add<N1 extends Nat, N2 extends Nat> = [...N1, ...N2];
type Sub<N1 extends Nat, N2 extends Nat> = N1 extends [...N2, ...infer Ans] ? Ans : Zero;

type NatToNum<N extends Nat> = N["length"];

type _NumToNat<L extends number, TNat extends Nat> = NatToNum<TNat> extends L ? TNat : _NumToNat<L, Suc<TNat>>;
type NumToNat<L extends number> = _NumToNat<L, Zero>;

type _Range<
  F extends Nat,
  L extends Nat,
  X extends Nat,
  // Ans is expected to be number[], but TypeScript will throw an error if you change it.
  Ans extends unknown[],
> = X extends Sub<L, F> ? Ans : _Range<F, L, Suc<X>, [...Ans, NatToNum<Add<F, X>>]>;
type Range_<F extends number, L extends number> = _Range<NumToNat<F>, NumToNat<L>, Zero, []>;

// Test
// Expect: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
type test1 = Range_<0, 10>;
// Expect: [4, 5, 6, 7, 8, 9, 10, 11, 12]
type test2 = Range_<4, 13>;