Allow an explicitly specified infer type

[ssalbdivad]

Discussed in #781

^{Originally posted by afoures June 1, 2023}
Hi,

Is there a way to type check schema implementation from an existing type?

Something like this:

import { type, union } from "arktype";

interface Owner {
  name: string;
}

interface Dog {
  name: string;
  age: number;
  owner?: Owner
}

// I would love to be able to create a schema from an existing type in a fully type safe way
const owner = type<Owner>({ name: 'string' });
const dog = type<Dog>({ age: 'integer>0', name: 'string', 'owner?': owner });

// for example this should error, because owner is not defined as possibly undefined
const dog = type<Dog>({ age: 'integer>0', name: 'string', owner });

Related issue from zod -> colinhacks/zod#372

Thanks to @afoures for submitting this! Would definitely be a valuable feature. Currently you can do something like this:

export const user: Type<{ name: string, age: number }> = type({
    name: "string",
    age: "number"
})

But TS's comparison is structural and covariant, i.e. you could add extra fields to your validator and it would still work as long as everything in the type itself is covered.

I tried adding an in out annotation to Type's inference parameter, but it broke a bunch of other things I'd have to look into, so I'm creating this issue and adding it to the backlog to create a dedicated API for this.

If in the meantime you'd benefit from something like the above, feel free to use that, or you could write a custom function to do an equals comparison on the types as an extra validation step if needed.

[Dimava]

I would propose Type<V>.expectType<T>(): Equals<T, V> ? T : TSError<'type doesn't match'>

[ssalbdivad]

Have a working implementation for this! Autocomplete for keys, optional keys, works for tuples, etc.! Will add to the next release (may still change the name) 😊

DeclaredTypes.mp4

[ssalbdivad]

Ended up going with:

type Expected = { a: string; b?: number }
const t = declare<Expected>().type({
    a: "string",
    "b?": "number"
})

Will add declarations for scope in the future: #791

[afoures]

this is really nice, thank you very much !

[afoures]

hello, is there any news regarding this? it seem that declare is not available

[ssalbdivad]

@afoures Yes, very sorry for the delay! This is in the upcoming 2.0 release.

I will close this issue once it is merged and published. In the meantime I've been publishing a few preview dev builds, feel free to check out the ArkType Discord if interested.

[ssalbdivad]

@afoures This is available in 2.0.0-dev.10. Keep an eye out for docs soon!

[afoures]

thanks for the heads up! 🙏🏻