T.derive

A value computed from other schemas' fabricated output, without wrapping them in a throwaway object.

T.derive({
  to: T.string,
  from: [T.always("hello"), T.string.as(() => "world")],
}).as((parts) => parts.join(":"));
// "hello:world"

from is a tuple of schemas, each fabricated independently — one private stream per slot, the same way T.tuple works. to names the result's type and shape: a schema, or a bare builder like T.string when you only need to declare the kind. Adapters reflect to, and a resolver that returns the wrong kind throws.

Overriding a derive-valued field with .override() checks the value against to. coverage and combinatorial enumerate the enumerable inputs in from (an enum, a boolean) and run resolve on each combination.

.as(resolve) is required. resolve receives the fabricated from values as a positional tuple, then { random, clock } — this node's own seeded stream, so any extra draw still replays under a salt.

For a field that reads its siblings on the same object, use .refine(({ compute }) => ...) instead — see Computed fields. T.derive is when the inputs are not (or need not be) fields of a surrounding object.