package ppx_bin_prot

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Generation of bin_prot readers and writers from types


Dune Dependency






Part of the Jane Street's PPX rewriters collection.

Published: 23 May 2024



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

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

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 "", 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.

Local-accepting bin_io functions

This ppx includes the option to support local allocation, a nonstandard OCaml extension available at:

In both structures and signatures, [@@deriving bin_io ~localize] (and similarly for bin_write and bin_size) additionally generates definitions for bin_write_t__local and bin_size_t__local which accept locally allocated ts.

As well, the following extension points are available:

[%bin_size_local: t]       : t Bin_prot.Size.sizer_local
[%bin_write_local: t]      : t Bin_prot.Write.writer_local

No other values from this ppx currently support local allocations.

Dependencies (7)

  1. ppxlib >= "0.28.0"
  2. dune >= "3.11.0"
  3. ppxlib_jane >= "v0.17" & < "v0.18"
  4. ppx_here >= "v0.17" & < "v0.18"
  5. bin_prot >= "v0.17" & < "v0.18"
  6. base >= "v0.17" & < "v0.18"
  7. ocaml >= "5.1.0"

Dev Dependencies


Used by (5)

  1. key-parsers >= "0.8.1" & < "0.10.1"
  2. oci
  3. ppx_bap < "v0.14.0"
  4. ppx_jane >= "v0.17.0"
  5. sexp_grammar >= "v0.17.0"




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