package ppx_deriving_variant_string

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OCaml PPX deriver that generates converters between variants and strings


Dune Dependency






OCaml PPX deriver that generates converters between regular or polymorphic variants and strings. Supports both OCaml and Reason casing.

Quick Start

Install: opam install ppx_deriving_variant_string.

In Reason syntax:

[@deriving (fromString, toString)]
type foo =
  | [@as "first"] First
  | Second;

let a = fooFromString("first"); /* Some(First) */
let b = fooFromString("Second"); /* Some(Second) */
let c = fooFromString("First"); /* None */
let d = fooToString(First); /* "first" */
let e = fooToString(Second); /* "Second" */

In OCaml syntax:

type foo =
  | First [@as "first"]
  | Second
[@@deriving of_string, to_string]

let a = foo_of_string "first" (* Some(First) *)
let b = foo_of_string "Second" (* Some(Second) *)
let c = foo_of_string "First" (* None *)
let d = foo_to_string First (* "first" *)
let e = foo_to_string Second (* "Second" *)

Name mangling

If the type where the PPX is applied is named t, the generated functions won't include any prefix and will be just toString, fromString (or to_string, of_string).

Why not ppx_deriving show?

The original ppx_deriving includes a plugin named show which has some overlap in functionality. However it was missing a few things:

  • it doesn't include functionality to convert from strings to variants

  • showshows a backtick for polymorphic variants

  • it does support as (it's called printer), which supports cases with variants with payloads, but it's a bit more heavyweight, as one has to pass a formatter instead of just a string, example

  • printer is only supported in polyvars for some reason, but not on regular variants, which was a feature we wanted to have

Why not Melange jsConverter?

We originally used Melange jsConverter, but we ran into limitations when making code compatible with universal libraries, that have to run both on the client and the server.


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