package ppx_bin_prot

ppx_bin_prot

Generation of binary serialization and deserialization functions from type definitions. There's more information about:

• The bin-prot format

• bin-prot-shape, which is useful for checking compatibility of the bin_prot representations of different types.

A note about signatures

In signatures, ppx_bin_prot tries to generate an include of a named interface, instead of a list of value bindings. That is:

type 'a t [@@deriving bin_io]

will generate:

include Binable.S1 with type 'a t := 'a t

instead of:

val bin_t : 'a Bin_prot.Type_class.t ‑> 'a t Bin_prot.Type_class.t
val bin_read_t : 'a Bin_prot.Read.reader ‑> 'a t Bin_prot.Read.reader
val __bin_read_t__ : 'a Bin_prot.Read.reader ‑> (int ‑> 'a t) Bin_prot.Read.reader
val bin_reader_t : 'a Bin_prot.Type_class.reader ‑> 'a t Bin_prot.Type_class.reader
val bin_size_t : 'a Bin_prot.Size.sizer ‑> 'a t Bin_prot.Size.sizer
val bin_write_t : 'a Bin_prot.Write.writer ‑> 'a t Bin_prot.Write.writer
val bin_writer_t : 'a Bin_prot.Type_class.writer ‑> 'a t Bin_prot.Type_class.writer
val bin_shape_t : Bin_prot.Shape.t ‑> Bin_prot.Shape.t

There are however a number of limitations:

• the type has to be named t

• the type can only have up to 3 parameters

• there shouldn't be any constraint on the type parameters

If these aren't met, then ppx_bin_prot will simply generate a list of value bindings.

Expression extensions

Several new expression forms are supported to derive the individual values defined by ppx_bin_prot. Each of the following extensions can be used, with arbitrary type expressions in place of t, to produce values of the corresponding types on the right:

[%bin_shape: t]      : Bin_prot.Shape.t
[%bin_digest: t]     : string
[%bin_size: t]       : t Bin_prot.Size.sizer
[%bin_write: t]      : t Bin_prot.Write.writer
[%bin_read: t]       : t Bin_prot.Read.reader
[%bin_writer: t]     : t Bin_prot.Type_class.writer
[%bin_reader: t]     : t Bin_prot.Type_class.reader
[%bin_type_class: t] : t Bin_prot.Type_class.t

Weird looking type errors

In some cases, a type can meet all the conditions listed above, in which case the rewriting will apply, but lead to a type error. This happens when the type t is an alias to a type which does have constraints on the parameters, for instance:

type 'a s constraint 'a = [> `read ]
val bin_s : ([> `read ] as 'a) Type_class.t0 -> 'a s Type_class.t0
val bin_read_s : ([> `read ] as 'a) Read.reader -> 'a s Read.reader
val bin_reader_s : ([> `read ] as 'a) Type_class.reader0 -> 'a s Type_class.reader0
val bin_size_s : ([> `read ] as 'a) Size.sizer -> 'a s Size.sizer
val bin_write_s : ([> `read ] as 'a) Write.writer -> 'a s Write.writer
val bin_writer_s : ([> `read ] as 'a) Type_class.writer0 -> 'a s Type_class.writer0
val bin_shape_s : Shape.t -> Shape.t

type 'a t = 'a s [@@deriving_inline bin_io]
include Binable.S1 with type 'a t := 'a t
[@@@end]

will give an error looking like:

Error: In this `with' constraint, the new definition of t
       does not match its original definition in the constrained signature:
       Type declarations do not match:
         type 'a t = 'a t constraint 'a = [> `read ]
       is not included in
         type 'a t
       File "binable.ml", line 34, characters 2-11: Expected declaration
       Their constraints differ.

To workaround that error, simply copy the constraint on the type which has the [@@deriving] annotation. This will force generating a list of value bindings.