This EIP introduces a new Simple Serialize (SSZ) type to represent Optional[T] values.
Optional values are currently only representable in SSZ using workarounds. Adding proper support provides these benefits:
Better readability: SSZ structures with optional values can be represented with idiomatic types of the underlying programming language, e.g., Optional[T] in Python, making them easier to interact with.
Compact serialization: SSZ serialization can rely on the binary nature of optional values; they either exist or they don't. This allows more compact serialization than using alternative approaches based on workarounds.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174.
Optional[T] is defined as a type that can represent:
TNoneThe default value of Optional[T] is None.
The deserialization of an Optional[T] depends on the input length:
None.0x01, the remainder of the scope is deserialized same as T.An Optional[T] is merkleized as a List[T, 1].
None, the list length is 0.1, and the first list element contains the underlying value.Union[None, T]?Union[None, T] leaves ambiguity about the intention whether the type may be extended in the future, i.e., Union[None, T, U].
Furthermore, SSZ Union types are currently not used in any final Ethereum specification and do not have a finalized design themselves. If the only use case is a workaround for lack of Optional[T], the simpler Optional[T] type is sufficient, and support for general unions could be delayed until really needed. Note that the design of Optional[T] could be used as basis for a more general Union.
List[T, 1]?The serialization is less compact for variable-length T, due to the extra offset table at the beginning of the list to indicate the list length.
Union[None, T] and List[T, 1] workarounds are not used at this time to represent Optional[T].
See EIP assets.
protolambda/remerkleablestatus-im/nim-ssz-serializationNone
Copyright and related rights waived via CC0.