👍 Add isParametersOf

[Milly]

Motivation

I want to validate method arguments with optional parameters.

Comparison with isTupleOf()

• isTupleOf(): the length of the tuple is fixed.

const a: unknown = ["a"];
if (is.TupleOf([is.String, is.Optional(is.Number)] as const)(a)) {
  // don't reach here
  const _: [string, number | undefined] = a;
}

• isParametersOf(): the tuple is truncated length if trailing predicates are optional.

const a: unknown = ["a"];
if (is.ParametersOf([is.String, is.Optional(is.Number)] as const)(a)) {
  // reach here
  const _: [string, number?] = a;
}

Naming

If there is a better name, please suggest.

[lambdalisue]

It seems the function is quite resemble with isTupleOf with predElse.

import { is } from "https://deno.land/x/unknownutil@$MODULE_VERSION/mod.ts";

const isMyType = is.TupleOf(
  [is.Number, is.String, is.Boolean],
  is.ArrayOf(is.Number),
);
const a: unknown = [0, "a", true, 0, 1, 2];
if (isMyType(a)) {
  // a is narrowed to [number, string, boolean, ...number[]]
  const _: [number, string, boolean, ...number[]] = a;
}

What is the difference and advantage compared to the existing one?

[Milly]

How can I use isTupleOf() with predElse to resolve the following args type?

It is possible by defining a custom Predicate isOpts, but I would like to avoid implementing type guards.

import { is } from "https://deno.land/x/unknownutil@$MODULE_VERSION/mod.ts";

function f(name: string, optCount?: number, optFlag?: boolean): void {}

const arglist: unknown[] = [
  ["a", 1, true],
  ["a", 1],
  ["a"],
  ["a", true, 1], // Expect skip
  ["a", 1, true, true, true], // Expect skip
];

for (const args of arglist) {
  if (is.TupleOf([is.String] as const, is.ArrayOf(is.UnionOf([is.Number, is.Boolean])))(args)) {
    // args is narrowed to [string, ...(number, boolean)[]]
    f(...args); // deno-ts:Error:2345:Argument of type 'number | boolean' is not assignable to parameter of type 'number | undefined'. Type 'boolean' is not assignable to type 'number'.
  }

  if (is.UnionOf([is.TupleOf([is.String]), is.TupleOf([is.String, is.Number]), is.TupleOf([is.String, is.Number, is.Boolean])])(args)) {
    // args is narrowed to [string] | [string, number] | [string, number, boolean]
    f(...args); // deno-ts:Error:2556:A spread argument must either have a tuple type or be passed to a rest parameter.
  }

  const isOpts = (_x: unknown): _x is [number?, boolean?] => /* nice implements */ true;
  if (is.TupleOf([is.String], isOpts)(args)) {
    // args is narrowed to [string, (number | undefined)?, (boolean | undefined)?]
    f(...args); // No error
  }

  if (is.ParametersOf([is.String, is.OptionalOf(is.Number), is.OptionalOf(is.Boolean)] as const)(args)) {
    // args is narrowed to [string, (number | undefined)?, (boolean | undefined)?]
    f(...args); // No error
  }
}

[lambdalisue]

Ah got it.　

[lambdalisue]

I think it's better to make the description a bit more clear but LGTM.

[lambdalisue]

💯