Branded types

[colinhacks]

Branded types

[colinhacks]

This is the recommended way to do tagged/branded types in TypeScript/Zod:

type Tagged<T, Tag> = T & { __tag: Tag };
type UUID = Tagged<string, 'UUID'>;

const uuid: z.Schema<UUID> = z.string().uuid() as any;
uuid.safeParse('ae6cd9c2-f2e0-43c5-919c-0640b719aacf'); // Success, data has type UUID
uuid.safeParse(5); // Fail
uuid.safeParse('foo'); // Fail

[danielo515]

This is the recommended way to do tagged/branded types in TypeScript/Zod:

type Tagged<T, Tag> = T & { __tag: Tag };
type UUID = Tagged<string, 'UUID'>;

const uuid: z.Schema<UUID> = z.string().uuid() as any;
uuid.safeParse('ae6cd9c2-f2e0-43c5-919c-0640b719aacf'); // Success, data has type UUID
uuid.safeParse(5); // Fail
uuid.safeParse('foo'); // Fail

Is there any way without using any?

[jraoult]

@danielo515 did you happen to find a type safe workaround?

There is now a native way for this https://github.com/colinhacks/zod#brand

[jraoult]

@colinhacks what did you decide regarding branded type? I've just attempted migrating from Runtypes because I really want strict/excess property check (which Runtypes lacks) but I'm stuck on branded types.

We use them mainly for simple types like UnsignedInteger, JsonDate etc. that are respectively number and string with extra checks:

const zJsonDate = z.string().refine((str) => isValid(parseJSON(str)));

With branding, we "know" down the line that they have been checked. Without them, we have to recheck locally and then cast to the branded type, but then it defeats the purpose of the initial check, we might as well just expect string for JsonDate for example and then have:

type JsonDate = z.infer<typeof zJsonDate> & {__jsonDateBrand: true};
function checkJsonDate(val: string) {
  return zJsonDate.parse(val) as JsonDate;
}

I don't love all the boilerplate associated with io-ts branded type

Might be worth checking the Runtypes implementation, I'm not an expert but it didn't look too convoluted:

https://github.com/pelotom/runtypes/blob/69b8724302ebe780270ded2b7cb94e24f3259e70/src/runtype.ts#L164-L167

[colinhacks]

@jraoult someone provided an example of how to implemented branded types on top of Zod here, might be useful for you. #210 (comment)

[ryasmi]

Hey @vriad, I actually made an internal library where we had to solve for this because we validate large nested objects. I've wrote up some stuff that may be of use https://github.com/ryansmith94/rulr/blob/v6/readme.md

[jraoult]

@colinhacks great thx for pinging me! I'll try that.

[colinhacks]

My "option 2" is misguided; it doesn't actually achieve the same thing as true branded/opaque types. @krzkaczor

[bradennapier]

You can do opaque types with intersections upon any types - and can be very useful if you use symbols throughout. The way they show there is less useful as it can't be saved and queried (you cant add multiple "brands" or "flags" dynamically for the type system to later use to infer from).

For example, I am playing right now with seeing if I can build out a syntax that would still type statically just for fun, doubting I'd actually try to go past typing it all out -- but this is what i came up with, which basically utilizes opaque typing to achieve it:

const one = { one: wrap.optional.string(), four: 3 } as const;
const two = { two: 2 } as const;

const validator = {
  foo: wrap.optional.string(),
  bar: wrap.nullable.optional.literal('bar'),
  baz: 123,
  qux: 'qux',
  blah: wrap.unequal(2),
  union: wrap.optional.nullable.union([wrap.string(), wrap.number()]),
  intersect: wrap.intersection([one, two]),
  key: wrap.forbidden(),
  undef: wrap.undefined(),
} as const;

Notice that it is simply property accessing - which actually does not need to be done in any given order.

Technically just using unique symbol does work -- using interface like they did just seems excessively verbose

type CheckNullable<
  T extends AnyObj,
  V
> = T['__nullable'] extends typeof NULLABLE ? null | V : V;

type CheckOptional<
  T extends AnyObj,
  V
> = T['__optional'] extends typeof OPTIONAL ? undefined | V : V;

type CheckFlags<T extends AnyObj, V> = V extends infer O
  ? CheckNullable<T, O> | CheckOptional<T, O>
  : never;

type TypedPrototype<T extends AnyObj> = Readonly<{
  assert(value: unknown): asserts value is T;
  guard(value: unknown): value is T;
  union<V extends unknown[]>(arr: V): CheckFlags<T, V[number]>;
  intersection<A extends unknown[] | readonly unknown[]>(
    arr: A,
  ): IntersectionOf<A[number]>;

  optional: TypedPrototype<T & { readonly __optional: typeof OPTIONAL }>;
  nullable: TypedPrototype<T & { readonly __nullable: typeof NULLABLE }>;

  negative: TypedPrototype<T>;
  nonnegative: TypedPrototype<T>;

  positive: TypedPrototype<T>;
  nonpositive: TypedPrototype<T>;

  string<V extends string>(): CheckFlags<T, V>;
  number<V extends number>(): CheckFlags<T, V>;
  bigint<V extends bigint>(): CheckFlags<T, V>;
  regexp<V extends RegExp>(): CheckFlags<T, V>;
  symbol<V extends symbol>(): CheckFlags<T, V>;
  primitive<V extends Primitive>(): CheckFlags<T, V>;
  unknown<V extends unknown>(): CheckFlags<T, V>;
  undefined(): undefined;
  forbidden(): never;
  any<A extends any>(): A;

  literal<V>(value: V): CheckFlags<T, V>;
  equal<V>(value: V): V;
  unequal<R, V = unknown>(value: V): R;
}>;

which has a schema like:

but finally infers into:

---

All that being said, I am not sure I personally see that much use in having this library provided branded types. I think they just sound like they'd be cool but in practice have very little actual use. The larger issue usally becomes once they are branded they no longer pass as their wider type, so it makes your entire application insanely complex trying to manage all the cases and potential brands things may have.

A brand may be useful for the top-level object which indicates that it has been validated, however -- and that can even work at runtime. For example, you could return a new object when validate() is called which is Inferred & typeof VALIDATED then a fn can add the brand to any so that the compiler will only let you call the fn if the validation has ran:

const obj = {
  positive: 2,
  foo: 'one',
  bar: 'bar',
  baz: 123,
  qux: 'qux',
  blah: 3,
  union: 2,
  intersect: {
    one: 'hi',
    four: 3,
    two: 2,
  },
} as const;

const validatedSchema = schema.validate(obj);

function example(o: Inferred & typeof VALIDATED) {
  console.log('Validated Only Can Call');
}

validatedSchema.union;

example(validatedSchema);
example(obj); // <--- not assignable to typeof VALIDATED

[maneetgoyal]

Hi @colinhacks, is the support for ReadOnly types planned?

[krzkaczor]

@maneetgoyal this approach should work: #210 (comment)

[OmgImAlexis]

@danielo515 did you happen to find a type safe workaround?