Recursive Types Cannot be Defined

I might have a Typescript type like this:

type T = string | T[]

But if I try to define this type in valita I cannot pass the identifier to its own instantiator:

const T = v.union(v.string(), v.array(T)); // error: identifier used before it is defined

I mulled over this a bit, but I can’t think of a smart way to implement it that doesn’t involve somehow the union after it is instantiated, and then having to trick typescript into giving it a different type…

[Bubz43]

I’m using Valita quite heavily in a not yet public rework of that project with plenty of v.optionals and I didn’t have to change anything, so it shouldn’t be too breaking.

[jviide]

Good discussion! Yes, recursive types are indeed something that we ourselves have needed for some time now. So I added lazy to the library, it works quite similarly to @Bubz43’s suggestion with some additional guardrails:

type T = string | T[];
const t: v.Type<T> = v.lazy(() => v.union(v.string(), v.array(t)));

The main problem here was trying to solve was the following scenario:

type T = string | T[];
const t: v.Type<T> = v.lazy(() => v.union(v.string(), v.array(t)).optional());

// Uh oh, t's optionality is erased:
const o = v.object({ t });
type X = v.Infer<typeof o>; // { t: T }, should be { t?: T }

So I took some drastic measures and restricted where .optional() types can be used! 🙂 For example, you now can’t return an optional from lazy, so this is a type error:

const t: v.Type<T> = v.lazy(() => v.union(v.string(), v.array(t)).optional());

As is this:

const t: v.Type<T> = v.lazy(() => v.union(v.string(), v.array(t))).optional();

The most common usecase for .optional() is still supported:

type T = string | T[];
const t: v.Type<T> = v.lazy(() => v.union(v.string(), v.array(t)));

const o = t.object({
  x: t.optional()
});

I believe this a pretty good tradeoff between expressivity and providing safe guardrails against accidents. This is a backwards incompatible change, for which I apologize. Looking at the only public repository using Valita, @Bubz43’s https://github.com/Bubz43/ep2e, it seems that no changes are needed there at least 🙂