Add union type and null to SSZ

specs/simple-serialize.md

@@ -8,12 +8,13 @@ This is a **work in progress** describing typing, serialization and Merkleizatio
 - [Typing](#typing)
     - [Basic types](#basic-types)
     - [Composite types](#composite-types)
+        - [Illegal empty composites](#illegal-empty-composites)
     - [Aliases](#aliases)
     - [Default values](#default-values)
 - [Serialization](#serialization)
     - [`"uintN"`](#uintn)
     - [`"bool"`](#bool)
-    - [Containers, vectors, lists](#containers-vectors-lists)
+    - [Vectors, containers, lists, unions](#vectors-containers-lists-unions)
 - [Deserialization](#deserialization)
 - [Merkleization](#merkleization)
 - [Self-signed containers](#self-signed-containers)
@@ -41,8 +42,14 @@ This is a **work in progress** describing typing, serialization and Merkleizatio
     * angle bracket notation `[type, N]`, e.g. `["uint64", N]`
 * **list**: ordered variable-length homogeneous collection of values
     * angle bracket notation `[type]`, e.g. `["uint64"]`
+* **union**: union type containing one of the given subtypes
+    * round bracket notation `(type1, type2, ...)`, e.g. `("uint64", "null")`
 
-We recursively define "variable-size" types to be lists and all types that contains a variable-size type. All other types are said to be "fixed-size".
+We recursively define "variable-size" types to be lists and unions and all types that contain a variable-size type. All other types are said to be "fixed-size".
+
+#### Illegal empty composites
+
+The empty container `{}` (except as the `"null"` type inside a union, see below) and the empty fixed length list `[type, 0]` are **not** legal types.
 
 ### Aliases
 
@@ -51,6 +58,9 @@ For convenience we alias:
 * `"byte"` to `"uint8"` (this is a basic type)
 * `"bytes"` to `["byte"]` (this is *not* a basic type)
 * `"bytesN"` to `["byte", N]` (this is *not* a basic type)
+* `"null"`: `{}`, i.e. the empty container
+
+The `"null"` type is only legal as a union sub-type.
 
 ### Default values
 
@@ -77,7 +87,13 @@ assert value in (True, False)
 return b"\x01" if value is True else b"\x00"
 ```
 
-### Containers, vectors, lists
+### `"null"`
+
+```python
+return b""
+```
+
+### Vectors, containers, lists, unions
 
 ```python
 # Reccursively serialize
@@ -97,6 +113,16 @@ fixed_parts = [part if part != None else variable_offsets[i] for i, part in enum
 return b"".join(fixed_parts + variable_parts)
 ```
 
+If `value` is an union type:
+
+Define value as an object that has properties `value.value` with the contained value, and `value.type_index` which indexes the type.
+
+```python
+serialized_bytes = serialize(value.value)
+serialized_type_index = value.type_index.to_bytes(BYTES_PER_LENGTH_PREFIX, "little")
+return serialized_type_index + serialized_bytes
+```
+
 ## Deserialization
 
 Because serialization is an injective function (i.e. two distinct objects of the same type will serialize to different values) any bytestring has at most one object it could deserialize to. Efficient algorithms for computing this object can be found in [the implementations](#implementations).
@@ -108,13 +134,15 @@ We first define helper functions:
 * `pack`: Given ordered objects of the same basic type, serialize them, pack them into `BYTES_PER_CHUNK`-byte chunks, right-pad the last chunk with zero bytes, and return the chunks.
 * `merkleize`: Given ordered `BYTES_PER_CHUNK`-byte chunks, if necessary append zero chunks so that the number of chunks is a power of two, Merkleize the chunks, and return the root.
 * `mix_in_length`: Given a Merkle root `root` and a length `length` (`"uint256"` little-endian serialization) return `hash(root + length)`.
+* `mix_in_type`: Given a Merkle root `root` and a type_index `type_index` (`"uint256"` little-endian serialization) return `hash(root + type_index)`.
 
 We now define Merkleization `hash_tree_root(value)` of an object `value` recursively:
 
 * `merkleize(pack(value))` if `value` is a basic object or a vector of basic objects
 * `mix_in_length(merkleize(pack(value)), len(value))` if `value` is a list of basic objects
 * `merkleize([hash_tree_root(element) for element in value])` if `value` is a vector of composite objects or a container
 * `mix_in_length(merkleize([hash_tree_root(element) for element in value]), len(value))` if `value` is a list of composite objects
+* `mix_in_type(merkleize(value.value), value.type_index)` if `value` is of union type
 
 ## Self-signed containers