Function fixedUnion

fixedUnion<T extends Record<string, CodecLike<any, any, any>>>(
    codecLayout: T,
    fields?: Record<keyof T, number | null | undefined>,
): Codec<
    UnionEncodable<T, keyof T>
    | { inner: UnionEncodable<T> },
    UnionDecoded<T, keyof T>,
    ChildContext<T>,
>
FixedUnion is a fixed-size union type where all variants must be fixed-size types of identical byte length. The total byteLength is payloadByteLength + 4 (4-byte uint32 ItemId tag).

Compatibility note: fixedUnion is a CCC extension for scenarios requiring fixed memory layouts (such as embedding inside mol.struct, mol.array, or mol.vector as FixVec). While the standalone wire format (4-byte tag + variant payload) is binary-compatible with a union encoding, composing it into struct, array, or FixVec is not part of the standard upstream Molecule union schema specification.

Migration guide:
```
// Preserve legacy equal-length union fixed container layout (FixVec):
const Legacy = mol.vector(mol.fixedUnion(layout));

// Use canonical Molecule union vector (DynVec):
const Canonical = mol.vector(mol.union(layout));
```
Type Parameters
- T extends Record<string, CodecLike<any, any, any>>
Parameters
- codecLayout: T
  an object mapping variant names to fixed-size codecs of identical byteLength
- Optionalfields: Record<keyof T, number | null | undefined>
  optional mapping from variant names to custom numeric IDs
Returns Codec<
    UnionEncodable<T, keyof T>
    | { inner: UnionEncodable<T> },
    UnionDecoded<T, keyof T>,
    ChildContext<T>,
>
- Defined in molecule/codec.ts:700

Function fixedUnion

Type Parameters

Parameters

Returns Codec< UnionEncodable<T, keyof T> | { inner: UnionEncodable<T> }, UnionDecoded<T, keyof T>, ChildContext<T>,>

Settings

Returns Codec<
UnionEncodable<T, keyof T>
| { inner: UnionEncodable<T> },
UnionDecoded<T, keyof T>,
ChildContext<T>,
>