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ppx_type_conv - Support Library for type-driven code generators

What is type_conv?

The type_conv library factors out functionality needed by different preprocessors that generate code from type specifications. Example libraries currently depending on type_conv:

  • ppx_bin_prot
  • ppx_compare
  • ppx_fields_conv
  • ppx_sexp_conv
  • ppx_variants_conv

type_conv for users of ppx_deriving

type_conv based code generators are meant to be used with ppx_driver. However type_conv allows to export a compatible ppx_deriving plugin. By default, when not linked as part of a driver, packages using type_conv will just use ppx_deriving.

So for instance this will work as expected using ppx_deriving:

ocamlfind ocamlc -c -package ppx_sexp_conv

For end users, the main advantage of using type_conv based generators with ppx_driver is that it will catch typos and attributes misplacement. For instance:

# type t = int [@@derivin sexp]
Error: Attribute `derivin' was not used
Hint: Did you mean deriving?
# type t = int [@@deriving sxp]
Error: ppx_type_conv: 'sxp' is not a supported type type-conv generator
Hint: Did you mean sexp?
# type t = int [@deriving sexp]
Error: Attribute `deriving' was not used
Hint: `deriving' is available for type declarations, type extensions
and extension constructors but is used here in the context of a core type.
Did you put it at the wrong level?"

For instruction on how to use ppx_driver, refer to the ppx_driver's documentation.


This part is only relevant if you are using ppx_driver. If you are using ppx_deriving the syntax is the one of ppx_deriving.

type_conv interprets the [@@deriving ...] attributes on type declarations, exception declarations and extension constructor declarations:

type t = A | B [@@deriving sexp, bin_io]

sexp and bin_io are called generators. They are functions that generate code given the declaration. These functions are implemented by external libraries such as ppx_sexp_conv or ppx_bin_prot. type_conv itself provides no generator, it does only the dispatch.

Generators can take arguments. This is done using the following syntax:

type t = A | B [@@deriving foo ~arg:42]

For arguments that are just switches, it is common to use the following syntax:

type t = A | B [@@deriving foo ~bar]

Plugin as findlib libraries

You must essentially follow the same rule for ppx_type_conv plugins as for ppx_driver ones when writing the META file.

Contact Information and Contributing

In the case of bugs, feature requests, contributions and similar, please contact the maintainers:

Up-to-date information should be available at


Deprecated, use ocaml-ppx/ppxlib instead




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