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capnp-ocaml

This is an OCaml code generator plugin for the Cap'n Proto serialization framework.

This plugin is roughly feature-complete for the Cap'n Proto basic serialization.

For RPC support, see https://github.com/mirage/capnp-rpc.

Design Notes

The generated code follows the general Cap'n Proto convention of using the serialized message format as the in-memory data structure representation. This enables the "infinitely faster" performance characteristic, but it also means that the generated code is not directly compatible with OCaml records, arrays, etc.

The generated code is functorized over the underlying message format. This enables client code to operate equally well on messages stored as—​for example—​OCaml bytes buffers or OCaml Bigarray (perhaps most interesting for applications involving mmap()).

Generated Code

The Cap'n Proto types are mapped to OCaml types as follows:

Table 1. Type mapping
Cap'n Proto Type OCaml Type

Void

unit

Bool

bool

Int8

int

Int16

int

Int32

int32

Int64

int64

UInt8

int

UInt16

int

UInt32

Uint32.t (from stdint library)

UInt64

Uint64.t (from stdint library)

Float32

float

Float64

float

Text

string

Data

string

List<T>

('a, T, 'b) Capnp.Array.t

The Capnp.Array module is roughly API-compatible with OCaml arrays and provides similar performance characteristics.

A Cap'n Proto struct maps to an OCaml module. The generated module hierarchy reflects the hierarchy of structs specified in the schema file. For example, the Cap'n Proto schema

@0xbdb65202adbdc4a0;

struct Outer1 {
  struct Inner {
  }
}

struct Outer2 {
}

would yield a generated module with the signature

module type S = sig
  module Reader : sig
    module Outer1 : sig
      type t

      module Inner : sig
        type t

        (* Read-only accessors for Inner.t *)
        ...
      end

      (* Read-only accessors for Outer1.t *)
      ...
    end

    module Outer2 : sig
      type t

      (* Read-only accessors for Outer2.t *)
      ...
    end
    ...
  end

  module Builder : sig
    module Outer1 : sig
      type t

      module Inner : sig
        type t

        (* Read/write accessors for Inner.t *)
        ...
      end

      (* Read/write accessors for Outer1.t *)
      ...
    end

    module Outer2 : sig
      type t

      (* Read/write accessors for Outer2.t *)
      ...
    end
    ...
  end
end

The Reader and Builder submodules provide read-only and read/write message accessors, respectively. The accessors for struct fields are constructed as follows:

Primitive Fields

A struct field foo of primitive type will result in generation of the following accessors:

(* for field 'foo' of type Void *)
val foo_get : t -> unit
val foo_set : t -> unit -> unit

(* for field 'foo' of type Bool *)
val foo_get : t -> bool
val foo_set : t -> bool -> unit

(* for field 'foo' of type Float32 or Float64 *)
val foo_get : t -> float
val foo_set : t -> float -> unit

(* for field 'foo' of type Text or Data *)
val has_foo : t -> bool
val foo_get : t -> string
val foo_set : t -> string -> unit

(* for field 'foo' of type Int8 *)
val foo_get : t -> int
(* Raise [Invalid_argument] if out of Int8 range *)
val foo_set_exn : t -> int -> unit

(* for field 'foo' of type Int16 *)
val foo_get : t -> int
(* Raise [Invalid_argument] if out of Int16 range *)
val foo_set_exn : t -> int -> unit

(* for field 'foo' of type Int32 *)
val foo_get : t -> int32
(* Raise [Message.Out_of_int_range] if not representable as int *)
val foo_get_int_exn : t -> int
val foo_set : t -> int32 -> unit
(* Raise [Invalid_argument] if out of Int32 range *)
val foo_set_int_exn : t -> int -> unit

(* for field 'foo' of type Int64 *)
val foo_get : t -> int64
(* Raise [Message.Out_of_int_range] if not representable as int *)
val foo_get_int_exn : t -> int
val foo_set : t -> int64 -> unit
val foo_set_int : t -> int

(* for field 'foo' of type UInt8 *)
val foo_get : t -> int
(* Raise [Invalid_argument] if out of UInt8 range *)
val foo_set_exn : t -> int -> unit

(* for field 'foo' of type UInt16 *)
val foo_get : t -> int
(* Raise [Invalid_argument] if out of UInt16 range *)
val foo_set_exn : t -> int -> unit

(* for field 'foo' of type UInt32 *)
val foo_get : t -> Uint32.t
(* Raise [Message.Out_of_int_range] if not representable as int *)
val foo_get_int_exn : t -> int
val foo_set : t -> Uint32.t -> unit
(* Raise [Invalid_argument] if out of UInt32 range *)
val foo_set_int_exn : t -> int -> unit

(* for field 'foo' of type UInt64 *)
val foo_get : t -> Uint64.t
(* Raise [Message.Out_of_int_range] if not representable as int *)
val foo_get_int_exn : t -> int
val foo_set : t -> Uint64.t -> unit
(* Raise [Invalid_argument] if out of UInt64 range *)
val foo_set_int_exn : t -> int -> unit

_get accessors will be available in both the Reader and the Builder modules; _set accessors will be available only for Builder types.

Embedded Struct Fields

A struct field foo which is of struct type will result in generation of the following accessors:

(* Assuming that field foo has generated type Foo.t... *)

(** [has_foo s] returns [true] if field [foo] was set in structure [s]. *)
val has_foo : t -> bool

(** [foo_init s] initializes the value of field [foo] to the default value
    for its type.

    @return a reference to the content of field [foo] *)
val foo_init : t -> Foo.t

(** [foo_get s] gets a reference to the content of field [foo].  (For the
    Builder implementation, if the field was not previously initialized
    then as a side-effect this function will default-initialize the
    structure and cause [has_foo s] to return [true].)

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_get : t -> Foo.t

(** [foo_get_pipelined s] is a reference to the field [foo] in the
     (possibly not yet received) struct [s]. Only available in the Reader
     section. *)
val foo_get_pipelined : struct_t StructRef.t -> Foo.struct_t StructRef.t

(** [foo_set_reader s v] sets the content of field [foo] by making a deep
    copy of the Reader-typed structure.

    @return reference to the content of field [foo]

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_set_reader : t -> Reader.Foo.t -> Builder.Foo.t

(** [foo_set_builder s v] sets the content of field [foo] by making a deep
    copy of the Builder-typed structure.

    @return reference to the content of field [foo]

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_set_builder : t -> Builder.Foo.t -> Builder.Foo.t

List Fields

A struct field foo which is of list type will result in generation of the following accessors:

(* Assuming that field foo contains values of type Inner... *)

(** [has_foo s] returns [true] if field [foo] was set in structure [s]. *)
val has_foo : t -> bool

(** [foo_init s n] initializes field [foo] to a zero-initialized list of
    length [n] (i.e. primitive types are initialized as zero, struct types
    are initialized as the default value for the struct type).

    @return a reference to the content of field [foo] *)
val foo_init : t -> int -> (rw, Inner.t, 'a) Capnp.Array.t

(** [foo_get s] gets a reference to the content of field [foo].  (For the
    Builder implementation, if the field was not previously initialized
    then as a side-effect this function will default-initialize the
    list and cause [has_foo s] to return [true].)

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_get : t -> ('cap, Inner.t, 'arr) Capnp.Array.t

(** [foo_get_list s] creates an OCaml list containing the content of
    field [foo].

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_get_list : t -> Inner.t list

(** [foo_get_array s] creates an OCaml array containing the content of
    field [foo].

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_get_array : t -> Inner.t array

(** [foo_set s v] sets the content of field [foo] by creating a deep copy
    of list [v].  (This may result in reallocation of [foo], which may
    lead to poor performance.)

    @return a reference to the content of field [foo]

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_set : t -> ('cap, Inner.t, 'a) Capnp.Array.t ->
                (rw, Inner.t, 'b) Capnp.Array.t

(** [foo_set_list s v] sets the content of field [foo] from OCaml list [v].
    (This may result in reallocation of [foo], which may lead to poor
    performance.)

    @return a reference to the content of field [foo]

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_set_list : t -> Inner.t list -> (rw, Inner.t, 'b) Capnp.Array.t

(** [foo_set_array s v] sets the content of field [foo] from OCaml array [v].
    (This may result in reallocation of [foo], which may lead to poor
    performance.)

    @return a reference to the content of field [foo]

    @raise Message.Invalid_message if the message is ill-formatted *)
val foo_set_array : t -> Inner.t array -> (rw, Inner.t, 'b) Capnp.Array.t

Union Fields

Cap'n Proto has first-class support for union (sum) types. These are mapped to OCaml variants in a straightforward way. To retrieve a union value, use the generated get function which will return a variant specifying which of the possible fields is present. To set a union value, use the generated set_foo (or init_foo) functions which simultaneously set (or init) the field value and set the union discriminant.

Variant constructors are generated simply by capitalizing the first letters of the associated union fields. In addition, to allow forward compatibility the constructor Undefined of int is added to the variant type definition. This constructor value is returned whenever an unknown union discriminant is decoded.

Enum Fields

Enums map to OCaml variants in the way one would expect. Enum fields within structs will lead to generation of foo_get and foo_set accessors which work just like the accessors for other primitive types.

Additional Operations on Structs

In addition to field accessors, modules associated with structs also contain the following functions:

(* Assuming that the struct is called Bar... *)

(** [of_message m] parses message [m] to retrieve the root struct.

    @return a reference to the content of the root struct

    @raise Message.Invalid_message if the message is ill-formatted *)
val of_message : 'cap message_t -> t

(** [of_builder b] converts a read/write reference to the struct into
    a read-only interface.  (Found only in the Reader module.) *)
val of_builder : Builder.Bar.t -> Reader.Bar.t

(** [to_reader b] converts a read/write reference to the struct into
    a read-only interface.  (Found only in the Builder module.) *)
val to_reader : Builder.Bar.t -> Reader.Bar.t

(** [init_root ?message_size ()] constructs a new message and
    initializes an instance of this struct type as the root struct
    of the message.  The optional [message_size] can be used to set
    the initial message size.

    @return a reference to the content of the root struct *)
val init_root : ?message_size:int -> unit -> Bar.t

(** [init_pointer p] initializes an instance of this struct type inside
    [p]'s message and updates [p] to point to the new struct. This is
    useful when dealing with AnyPointer fields.

    @return a reference to the content of the struct *)
val init_pointer : pointer_t -> t

(** [to_message s] retrieves the underlying message which is used as
    the backing store for struct [s]. *)
val to_message : t -> rw message_t

Interfaces

A struct field foo which is of interface type will result in generation of the following accessors:

(* Assuming that field foo contains interfaces of type Foo... *)

(** The caller is responsible for freeing the result. *)
val foo_get : t -> Foo.t Capability.t option

(** [foo_get_pipelined t] is a capability that can be used to invoke methods
    on the object in the [foo] field of [t], which might not have arrived yet.
    The caller is responsible for freeing the result. *)
val foo_get_pipelined : struct_t StructRef.t -> Foo.t Capability.t

(** [foo_set t c] sets the field to capability [c], increasing [c]'s ref-count
    (i.e. the caller is still responsible for freeing [c]). *)
val foo_set : t -> Foo.t Capability.t option -> unit

Each interface Foo generates a module Foo. There will be one submodule for each method, with Params and Results submodules for any implicit structs needed for the method arguments and results:

module Reader : sig
  module Foo : sig
    type t = [`Foo_c36d76740ee15e68]
    module MyMethod : sig
      module Params : sig
        type struct_t = [`MyMethod_b104a8c98610c556]
        type t = struct_t reader_t
        val arg1_get : t -> string
        [...]
      end
      module Results : sig
        type struct_t = [`MyMethod_c6367b042fce8e87]
        type t = struct_t reader_t
        val result_get : t -> string
        [...]
      end
    end
  end
end
module Builder : sig
  module Foo : sig
    type t = [`Foo_c36d76740ee15e68]
    module MyMethod : sig
      module Params : sig
	type struct_t = [`MyMethod_b104a8c98610c556]
	type t = struct_t builder_t
	val arg1_set : t -> string -> unit
        [...]
      end
      module Results : sig
	type struct_t = [`MyMethod_c6367b042fce8e87]
	type t = struct_t builder_t
	val result_set : t -> string -> unit
        [...]
      end
    end
  end
end

The generated file will also contain Client and Service top-level modules:

  module Client : sig
    module Foo : sig
      type t = [`Foo_c36d76740ee15e68]
      val interface_id : Uint64.t
      module MyMethod : sig
        module Params = Builder.Foo.MyMethod.Params
        module Results = Reader.Foo.MyMethod.Results
        val method_id : (t, Params.t, Results.t) Capnp.RPC.MethodID.t
      end
    end
  end

  module Service : sig
    module Foo : sig
      type t = [`Foo_c36d76740ee15e68]
      val interface_id : Uint64.t
      module MyMethod : sig
        module Params = Reader.Foo.MyMethod.Params
        module Results = Builder.Foo.MyMethod.Results
      end
      class virtual service : object
        inherit MessageWrapper.Untyped.generic_service
        method virtual my_method_impl :
          (MyMethod.Params.t, MyMethod.Results.t) MessageWrapper.Service.method_t
      end
      val local : #service -> t MessageWrapper.Capability.t
    end
  end

The Client module is for use by clients. Each method links to a builder for the parameters and a reader for the results (in the Service section they are the other way around). The client section also includes the method’s globally-unique ID. This is just a (Uint64.t * int) pair, but its type gives the type of the interface and of the request and response structs. Consult your RPC library’s documentation for information about how to call the method.

To implement a service, inherit from the generated virtual service class and implement the virtual methods. Use the local function to export your service as a capability.

Inheritance is not currently supported.

Generating Code

You will need to install the Cap'n Proto compiler. Once the Cap'n Proto compiler and capnp-ocaml are both installed, you should be able to use capnp compile -o ocaml yourSchemaFile.capnp in order to generate yourSchemaFile.mli and yourSchemaFile.ml. These modules will link against the capnp library.

Instantiating the Modules

The modules generated by capnp-ocaml are functors which take the underlying message type as input.

In principle, messages can be stored using any underlying data structure that satisfies the Capnp.MessageStorage.S signature. At present, capnp-ocaml contains one implementation: Capnp.BytesStorage provides message storage in the form of native OCaml bytes buffers, and Capnp.BytesMessage provides a Message implementation based on BytesStorage. This module makes it easy to retrieve messages in a format suitable for use with file I/O, socket I/O, etc.

To instantiate your code using BytesMessage, you could use the following pattern:

module YSF = YourSchemaFile.Make(Capnp.BytesMessage)

let root_struct = YSF.Builder.Foo.init_root () in
(* ... *)

Performance

For certain applications, the overhead associated with OCaml functors may be problematic. The functors may be eliminated by compiling with an flambda build of the OCaml compiler (e.g. opam switch 4.04.1+flambda) and using the @inlined annotation, like this:

module YSF = YourSchemaFile.Make[@inlined](Capnp.BytesMessage)

You can use the ocamlopt -inlining-report option to check that the code has been inlined. It may also be a good idea to compile with -O3 if you care about speed.

I Need to See an Example

There are some simple examples in the examples directory.

The tests and benchmark subdirectories may also be helpful to look at.

The RPC tutorial also contains many examples.

Installation

capnp-ocaml requires OCaml >= 4.02.

You should be able to install capnp-ocaml with OPAM using using opam install capnp.

If you prefer to compile manually, you will need jbuilder, Findlib, and OCaml packages core_kernel, extunix, uint, ocplib-endian, and res. Run jbuilder build to build both the compiler and the runtime library, and then use jbuilder install to copy them into appropriate places within your filesystem.

Contact

pelzlpj at gmail dot com