rivo/Ary/IsFixedLength.d.ts

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import type { Ary } from ".";
import type { Args, Fn } from "../HKT";
import type { Nat } from "../Num";
import type { Extends, If, IsAny, Not } from "../helpers";

/**
 * Check if an {@link Ary} (i.e., `Array` or `ReadonlyArray`) is a fixed length one (i.e., the
 * `length` property is a number literal).
 *
 * Sig: `(a: Ary) => boolean`
 *
 * @example
 * ```typescript
 * type R1 = IsFixedLength<string[]>;
 * //   ^?: false
 * type R2 = IsFixedLength<readonly number[]>;
 * //   ^?: false
 * type R3 = IsFixedLength<["foo", "bar"]>;
 * //   ^?: true
 * type R4 = IsFixedLength<readonly [42, 1337, ...string[]]>;
 * //   ^?: false
 * ```
 */
export type IsFixedLength<A extends Ary> = If<
  IsAny<A>,
  boolean,
  A extends { length: infer N } ? Not<Extends<Nat, N>> : never
>;

/**
 * [Fn] Check if an {@link Ary} (i.e., `Array` or `ReadonlyArray`) is a fixed length one (i.e., the
 * `length` property is a number literal).
 *
 * Sig: `(a: Ary) => boolean`
 */
export default interface IsFixedLengthFn extends Fn<[Ary], boolean> {
  def: ([a]: Args<this>) => IsFixedLength<typeof a>;
}