Declare

If your ArkType definitions are your source of truth, it's easy to infer out the type with an expression like type User = typeof User.infer.

But what if you need to define a Type matching a pre-existing external type?

The declare API allows just that, with autocomplete for object keys and clear, type-level errors for mismatches:

type Expected = { a: string; b?: number }

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

const Bad = type.declare<Expected>().type({
	a: "string",
	// will error if the inferred type is too wide *or* too narrow
	"b?": "1"
TypeScript: declared: number; inferred: 1;
})

side

If your Type contains morphs or default values, its input and output will be inferred differently.

You can see this represented when you hover a Type like string.numeric.parse and see (In: string) => To<number>.

By default, declare does not allow the type you define to include morphs (unless you explicitly add them to the declared generic argument).

Passing a side config to declare can change this behavior:

type Expected = { a: number; b?: number }

const Bad = type.declare<Expected>().type({
	a: "string.numeric.parse",
TypeScript: declared: number; inferred: (In: string) => To<number>;
	"b?": "number"
})

// passing a config object like { side: "in" | "out" } to validate that side
const T = type.declare<Expected, { side: "out" }>().type({
	a: "string.numeric.parse",
	"b?": "number"
})