.. post:: 2016-02-15
   :tags: OCaml, ppx, camlp4
   :author: Rudi Grinberg

Scrap your Camlp4
=================

ppx has been out for a while but it seems like the community has been
taking its time transitioning away from camlp4. There's probably a
couple of reasons for that:

1) Only the very latest version of the compiler (4.02.3) works with the
   most common ppx's.

2) People aren't aware how easy it is to convert your code with JSC's
   excellent
   `camlp4-to-ppx <https://github.com/janestreet/camlp4-to-ppx>`__

The purpose of this post is to help out with the 2nd point a with a
lightning fast walk through of how to do use it:

First, let's get the code and install it:

::

    $ git clone https://github.com/janestreet/camlp4-to-ppx && cd camlp4-to-ppx && make

This should yield us a small executable that will convert a camlp4
source file to ppx:

::

    $ ./main.native
    Usage: main.native FILE

All that remains is gluing together a small driver to run this on your
project:

.. code-block:: bash

    #!/bin/bash

    # Make sure to set this correctly
    p4toppx="/path/to/camlp4-to-ppx/main.native"

    # If you're on OSX, install gnu sed with $ brew install gnu-sed
    # s/gsed/sed/ on the next line if you're on linux
    ppxify() { gsed -ni r<($p4toppx $1) $1; }

    # you can tweak how you select your source files
    files=$(git ls-files '*.ml*' | grep '\.mli\?$')
    for f in $files; do ppxify $f; done

Copy and tweak the script above and you can test it out:

::

    $ https://github.com/mirage/ocaml-cohttp
    $ cd ocaml-cohttp
    $ ./ppxify.sh

Now go on and upgrade your package to use ppx. Here's to not dealing
with camlp4 in 2016.