Ppx_deriving_protobuf exception when serializing a language front end?

Learning
1

I’m new to OCaml but pretty solid with Haskell, and so to learn some OCaml, I’m trying to write a front end for a J-like language, loosely following this tutorial. I’ve run into an error I can’t diagnose.

I’m trying to serialize the front end with protobufs into an intermediate representation; the grammar looks like

type adverb =
        | TAdverb of string [@key 1]
        [@@deriving protobuf {protoc= "../../frontend_ir.proto"}]

type conjunction =
        | TConjunction of string [@key 1]
        [@@deriving protobuf {protoc= "../../frontend_ir.proto"}]

type nounexpr =
        | TId of string [@key 1]
        | TIArray of int list [@key 2]
        | TFArray of float list [@key 3]
        [@@deriving protobuf {protoc= "../../frontend_ir.proto"}]

type verb =
        | TSimpleVerb of string [@key 1]
        | TModVerb of verb * adverb [@key 2]
        | THook of verb * verb [@key 3]
        | TFork of verb * verb * verb [@key 4]
        | TVerbalNounSuffix of nounexpr * conjunction * verb [@key 5]
        | TVerbalNounPrefix of verb * conjunction * nounexpr [@key 6]
        | TConj of verb * conjunction * verb [@key 7]
        [@@deriving protobuf {protoc= "../../frontend_ir.proto"}]

type expr =
        | TAssign of string * expr * expr [@key 1]
        | TLiteralNoun of nounexpr [@key 2]
        | TMonadic of verb * expr [@key 3]
        | TDyadic of expr * verb * expr [@key 4]
        [@@deriving protobuf {protoc= "../../frontend_ir.proto"}]

The accompanying dune file looks like this:

(library
 (name ir_gen)
 (libraries core core_unix core_unix.sys_unix core_unix.command_unix fmt ast)
 (flags
  (:standard -w -39))
 (preprocess
  (pps ppx_deriving_protobuf bisect_ppx --conditional))
 (lint
  (pps ppx_js_style -check-doc-comments -annotated-ignores -styler -pretty -dated-deprecation)))

When I do dune build, I get the following error:

7 |   (pps ppx_deriving_protobuf bisect_ppx --conditional))
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Fatal error: exception File "ppx_deriving_protobuf.cppo.ml", line 1146, characters 28-34: Assertion failed

The only reference to a similar error that I can find is a bug from 7 years ago.

Any advice about how to understand this error better? If I could look at the file ppx_deriving_protobuf.cppo.ml maybe I could figure out what’s wrong, but I think it’s part of the build and I don’t know how to access it.

2

The brief look gives an impression, that issue is in protobuf itself, and not your code… But I may be wrong.

3

Thanks. I’ll try raising the issue over on the ppx_deriving_protobuf GitHub, but it doesn’t seem that the repo is very active.

Ironically, I was trying to post the link to the bug here in Discourse, but it was giving me HTTP 422 errors. It’s errors all the way down…