How can I derive to_string methods for data types using ppx with jbuilder?
For instance, I'm trying to use @@deriving show to derive show_* methods for a data type. I have a simple main.ml file that looks like this:
open Core
type foo = Bar | Baz
[@@deriving show]
let () = printf "%s\n" (show_foo Bar)
I have a jbuild file in the same directory that looks like this:
(jbuild_version 1)
(executables
((names (main))
(libraries (core ppx_deriving))
(preprocess (pps (ppx_deriving.show)))))
(install
((section bin)
(files ((main.exe as my_foo)))))
When I run jbuilder build, I get the following error:
File "main.ml", line 6, characters 24-32:
Error: Unbound value show_foo
It seems like jbuilder doesn't run the ppx_deriving.show preprocessor, and doesn't generate the show_foo function.
Is my use of @@derive show correct? Do I need to add something to my jbuild file to get it to work correctly? Does ppx_deriving have a problem working with jbuilder? Should I be using a different ppx library? Should I be using a different build system? How do most OCamlers deal with these kind of build system problems?
edit: I am using OCaml version 4.04.2, jbuilder version 1.0+beta11, and ppx_deriving version 4.1.
As you found in the comments, this requires particular handling in ppx_deriving which was only merged in version 4.2.
The underlying reason is that jbuilder uses ppx_driver to apply ppx rewriters and not the -ppx flag.