As you may know, the Camlp4 project has been relatively inactive for the past few years. With the help of @gasche, @dra27 and a few other contributors, we have been keeping the project alive so that existing OCaml projects using Camlp4 could continue to build with new OCaml compilers.
At this point, we expect that the most used features of Camlp4 are covered by ppx, and in particular all the effort is focused towards the latter.
We feel like the time has come to officially abandon Camlp4. In order to help distributions and other package managers deal with this fact, we will soon release a 4.08 compatible version of Camlp4. This will in particular help getting OCaml 4.08 in Debian. However, this will be the very last release of Camlp4. After that, the project will be abandonned and will no longer receive updates.
Of course, anyone interested in taking over the project is very welcome to do so. Please get in touch if you are interested and we will happily arrange for the transfer of ownership.
Migrating away from Camlp4
There are a lot of code bases out there still using Camlp4. If you need to bring an old project using Camlp4 to the modern age, I recommend reading this blog post which describes in details how the huge Jane Street code base was migrated from Camlp4 to ppx. In particular, the camlp4-to-ppx tool mentioned in the blog post is availabe on github.
One of the pain point of migrating away from Camlp4 is the stream parser syntax. In the past, the OCaml compiler used to have a syntax for stream parsers. When Camlp4 was merged into OCaml, the stream parser syntax moved from the OCaml parser to Camlp4.
Nowadays, Camlp4 is the only tool that understands the stream parser syntax. Stream parsers are not considered to be a great design and we encourage users to explore alternatives such as lazy lists or parser combinator libraries.
However, may you need to upgrade a large code base using Camlp4 and stream parsers, it would be quite easy to develop a ppx syntax for stream parsers and automatically upgrade the syntax using camlp4-to-ppx. If you would like to explore this possibility and need pointers, do not hesitate to get in touch!