ppx_enum

ppx_enum is an OCaml preprocessor to derive enum-like modules from variant definitions.

Overview

Enums are bare variants that are intended to represent a flag that can have more values than just true and false.

ppx_enum makes it easier to work with enums, in particular handling the conversion to and from strings. This is useful when (de)serializing values (for example, when serializing to store in a database), and cuts down on repetitive boilerplate code.

Consider the following simple example:

type my_enum =
  | Foo
  | Bar
  | Baz
[@@deriving str_enum]

The use of [@@deriving str_enum] will generate the following functions:

let my_enum_to_string = function
  | Foo -> "Foo"
  | Bar -> "Bar"
  | Baz -> "Baz"

let my_enum_from_string = function
  | "Foo" -> Ok Foo
  | "Bar" -> Ok Bar
  | "Foo" -> Ok Foo
  | _ -> Error ...

let my_enum_from_string_exn = function
  | "Foo" -> Foo
  | "Bar" -> Bar
  | "Foo" -> Foo
  | _ -> invalid_arg ...

Naming of Generated Functions

Generally, the generated functions for type mytype will be mytype_to_string, mytype_from_string and mytype_from_string_exn.

The only exception is when using type t = ..., in which case to_string, from_string and from_string_exn will be used.

Installation and Usage

You can install ppx_enum using opam:

$ opam install ppx_enum

If you're building your library or app with dune, add the following field to your library,
executable or test stanza:

(preprocess (pps ppx_enum))

or simply add ppx_enum to your preprocess field if it's already there.

You can now add the enum plugin to [@@deriving ...] attributes on variant type definitions.

Customizing the Generated Functions

Custom Values for Specific Variants

It is possible to customize the string value that will be used to represent a specific variant by using an [@value] attribute. An example is worth 1000 words here:

type myenum =
  | Foo [@value "baz"]
  | Bar
[@deriving str_enum]

my_enum_to_string Foo  (* "baz" *)
my_enum_to_string Bar  (* "bar" *)

my_enum_from_string "foo"  (* Error ... *)
my_enum_from_string "bar"  (* Ok Bar *)
my_enum_from_string "baz"  (* Ok Foo *)

The attributes will accept any valid suffix of ppx_enum.str_enum.value, so the following will work:

type myenum =
  | Foo [@value "foo-1"]
  | Bar [@str_enum.value "bar-1"]
  | Baz [@ppx_enum.str_enum.value "baz-1"]
[@deriving str_enum]