The ``data_encoding`` library ============================= Throughout the Tezos protocol, data is serialized so that it can be used via RPC, written to disk, or placed in a block. This serialization/de-serialization is handled via the `data-encoding `_ library by providing a set primitive encodings and a variety of combinators. Examples/Tutorial ----------------- The following is a very brief introduction to the data-encoding library. .. note:: A more complete documentation, including a full tutorial with many examples and a reference of all available combinators, is available in `data-encoding's API documentation `__. Encoding an integer ~~~~~~~~~~~~~~~~~~~ Integers are defined as other concrete data types with a generic encoding type ``type 'a encoding``. This means that it is an encoding to/from type ``int``. There are a variety of ways to encode an integer, depending on what binary serialization you want to achieve: - ``Data_encoding.int8`` - ``Data_encoding.uint8`` - ``Data_encoding.int16`` - ``Data_encoding.uint16`` - ``Data_encoding.int31`` - ``Data_encoding.int32`` - ``Data_encoding.int64`` For example, an encoding that represents a 31 bit integer has type ``Data_encoding.int31 = int Data_encoding.encoding``. .. code-block:: ocaml let int31_encoding = Data_encoding.int31 Encoding an object ~~~~~~~~~~~~~~~~~~ Encoding a single integer is fairly uninteresting. The data-encoding library provides a number of combinators that can be used to build more complicated objects. Consider the type that represents an interval from the first number to the second: .. code-block:: ocaml type interval = int64 * int64 We can define an encoding for this type as: .. code-block:: ocaml let interval_encoding = Data_encoding.(obj2 (req "min" int64) (req "max" int64)) In the example above we construct a new value ``interval_encoding`` by combining two ``int64`` integers using the ``obj2`` (object with two fields) constructor. The library provides different constructors, i.e. for objects that have no data (``Data_encoding.empty``), constructors for object up to 10 fields, constructors for tuples, list, etc. These are serialized to binary by converting each internal object to binary and placing them in the order of the original object and to JSON as a JSON object with field names. Lists, arrays, and options ~~~~~~~~~~~~~~~~~~~~~~~~~~ List, arrays and options types can be built on top of ground data types. .. code-block:: ocaml type interval_list = interval list type interval_array = interval array type interval_option = interval option And the encoders for these types as .. code-block:: ocaml let interval_list_encoding = Data_encoding.list interval_encoding let interval_array_encoding = Data_encoding.array interval_encoding let interval_option_encoding = Data_encoding.option interval_encoding Union types ~~~~~~~~~~~ The Octez codebase makes heavy use of variant types. Consider the following variant type: .. code-block:: ocaml type variant = B of bool | S of string Encoding for this types can be expressed as: .. code-block:: ocaml let variant_encoding = let open Data_encoding in union ~tag_size:`Uint8 [ case ~title:"B" (Tag 0) bool (function B b -> Some b | _ -> None) (fun b -> B b) ; case ~title:"S" (Tag 1) string (function S s -> Some s | _ -> None) (fun s -> S s) ] This variant encoding is a bit more complicated. Let’s look at the parts of the encoding: - We include an optimization hint to the binary encoding to inform it of the number of elements we expect in the tag. In most cases, we can use :literal:`\`Uint8`, which allows you to have up to 256 possible cases (default). - We provide a function to wrap the datatype. The encoding works by repeatedly trying to decode the datatype using these functions until one returns ``Some payload``. This payload is then encoded using the dataencoding specified. - We specify a function from the encoded type to the actual datatype. Since the library does not provide an exhaustive check on these constructors, the user must be careful when constructing union types to avoid unfortunate runtime failures. How the Dataencoding module works --------------------------------- This section is 100% optional. You do not need to understand this section to use the library. The library uses GADTs to provide type-safe serialization/de-serialization. From there, a runtime representation of JSON objects is parsed into the type-safe version. First we define an untyped JSON AST: .. code-block:: ocaml type json = [ `O of (string * json) list | `Bool of bool | `Float of float | `A of json list | `Null | `String of string ] This is then parsed into a typed AST (we eliminate several cases for clarity): .. code-block:: ocaml type 'a desc = | Null : unit desc | Empty : unit desc | Bool : bool desc | Int64 : Int64.t desc | Float : float desc | Bytes : Kind.length -> Bytes.t desc | String : Kind.length -> string desc | String_enum : Kind.length * (string * 'a) list -> 'a desc | Array : 'a t -> 'a array desc | List : 'a t -> 'a list desc | Obj : 'a field -> 'a desc | Objs : Kind.t * 'a t * 'b t -> ('a * 'b) desc | Tup : 'a t -> 'a desc | Union : Kind.t * tag_size * 'a case list -> 'a desc | Mu : Kind.enum * string * ('a t -> 'a t) -> 'a desc | Conv : { proj : ('a -> 'b) ; inj : ('b -> 'a) ; encoding : 'b t ; schema : Json_schema.schema option } -> 'a desc | Describe : { title : string option ; description : string option ; encoding : 'a t } -> 'a desc | Def : { name : string ; encoding : 'a t } -> 'a desc - The first few constructors define all ground types. - The constructors for ``Bytes``, ``String`` and ``String_enum`` include a length field in order to provide safe binary serialization. - The constructors for ``Array`` and ``List`` are used by the combinators we saw earlier. - The ``Obj`` and ``Objs`` constructors create JSON objects. These are wrapped in the ``Conv`` constructor to remove nesting that results when these constructors are used naively. - The ``Mu`` constructor is used to create self-referential definitions. - The ``Conv`` constructor allows you to clean up a nested definition or compute another type from an existing one. - The ``Describe`` and ``Def`` constructors are used to add documentation The library also provides various wrappers and convenience functions to make constructing these objects easier. Reading the documentation in the `mli file `__ should orient you on how to use these functions.