Feature-full driver for OCaml AST transformers
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Ppx_driver - driver for AST transformers

A driver is an executable created from a set of OCaml AST transformers linked together with a command line frontend.

The aim is to provide a tool that can be used to:

  • easily view the pre-processed version of a file, no need to construct a complex command line: ppx file.ml will do
  • use a single executable to run several transformations: no need to fork many times just for pre-processing
  • improved errors for misspelled/misplaced attributes and extension points

Using Ppx_driver based rewriters

The recommended way to use rewriters based on Ppx_driver is through jbuilder. All you need to is add this line to your (library ...) or (executables ...) stanza:

(preprocess (pps rewriter1 rewriter2 ...))

jbuilder will automatically build a static driver including all these rewriters.

If you are not using jbuilder, you can:

  • use the the ocamlbuild plugin provided with Ppx_driver
  • build a custom driver yourself using ocamlfind

These methods are described in the following sections.

Creating a new Ppx_driver based rewriter

If using jbuilder, you can just use the following jbuild file:

 ((name        my_ppx)
  (public_name my_ppx)
  (kind ppx_rewriter)
  (libraries (ppx_core ppx_driver))
  (ppx_runtime_libraries (<runtime dependencies if any>))
  (preprocess (pps (ppx_metaquot)))))

(kind ppx_driver) has two effects: 1. it links the library with -linkall. Since plugins register themselves with the Ppx_driver library by doing a toplevel side effect, you need to be sure they are linked in the static driver to be taken into accound 2. it instructs jbuilder to produce a special META file that is compatible with the various ways of using ppx rewriters, i.e. for people not using jbuilder or the ocamlbuild plugin

Building a custom driver using ocamlfind

To build a custom driver using ocamlfind, simply link all the AST transformers together with the ppx_driver.runner package at the end:

ocamlfind ocamlopt -predicates ppx_driver -o ppx -linkpkg \
  -package ppx_sexp_conv -package ppx_bin_prot \
  -package ppx_driver_runner

Normally, ppx_driver based rewriters should be build with the approriate -linkall option on individual libraries. If one is missing this option, the code rewriter might not get linked in. If this is the case, a workaround is to pass -linkall when linking the custom driver.

The driver as a command line tool

$ ppx -help
ppx [extra_args] [files]
  -as-ppx                 Run as a -ppx rewriter (must be the first argument)
  -o FILENAME             Output file (use '-' for stdout)
  -loc-filename STRING    File name to use in locations
  -no-optcomp             Do not use optcomp
  -dump-ast               Dump the marshaled ast to the output file instead of pretty-printing it
  -dparsetree             Print the parsetree (same as ocamlc -dparsetree)
  -impl FILE              Treat the input as a .ml file
  -intf FILE              Treat the input as a .mli file
  -no-check               Disable checks (unsafe)
  -debug-attribute-drop   Debug attribute dropping
  -print-transformations  Print linked-in code transformations, in the order they are applied
  -apply NAMES            Apply these transformations in order (comma-separated list)
  -help                   Display this list of options
  --help                  Display this list of options

When passed a file as argument, a ppx driver will pretty-print the code transformed by all its built-in AST transformers. This gives a convenient way of seeing the code generated for a given attribute/extension.

A driver can simply be used as the argument of the -pp option of the OCaml compiler, or as the argument of the -ppx option by passing -as-ppx as first argument:

$ ocamlc -c -pp ppx file.ml
$ ocamlc -c -ppx "ppx -as-ppx" file.ml

When used with -pp, the driver will also interpret #-directives using ppx_optcomp.

Using ppx_driver with ocamlbuild

Ppx_driver provides an ocamlbuild plugin to help building custom drivers on demand.


To use it you need to first tell ocamlbuild to use the plugin in myocamlbuild.ml. If you are using oasis, add this to your _oasis file:

AlphaFeatures:         ocamlbuild_more_args
XOCamlbuildPluginTags: package(ppx_driver.ocamlbuild)

If you are calling ocamlbuild directly, you need to call it this way:

$ ocamlbuild -plugin-tag "package(ppx_driver.ocamlbuild)" ...

Once you have done that, you need to enable it in myocamlbuild.ml:

let () =
  Ocamlbuild_plugin.dispatch (fun hook ->
    Ppx_driver_ocamlbuild.dispatch hook;
    <other dispatch functions>


The plugin provides a new parametric tag: ppx-driver. The tag tags as argument + separated list of rewriters (as findlib package names) followed by any command line arguments.

For instance to use ppx_sexp_conv and ppx_bin_prot put this in your tags file:

<**/*>: predicate(custom_ppx)
<src/*.{ml,mli}>: ppx-driver(ppx_sexp_conv+ppx_bin_prot)

The first line is to instruct ocamlfind not to automatically add implicit -ppx argument. Without this, you might still get individual -ppx for both ppx_sexp_conv and ppx_bin_prot in addition to the main driver that already contains them both, meaning your code would be transformed more than it should...


For the future, there is an ongoing work to make this even simpler. This requires some changes to ocamlbuild, see this pull request for details.

ppx_driver rewriters as findlib libraries

Note: if using jbuilder, you do not need to read this as jbuilder already does all the right things for you.

In normal operation, Ppx_driver rewriters are packaged as findlib libraries. When using jbuilder everything is simple as preprocessors and normal dependencies are separated. However historically, people have been specifying both preprocessors and normal library dependencies together. Even worse, many build system still don't use a static driver and call out to multiple ppx commands to preprocess a single file, which slow downs compilation a lot.

In order for all these different methods to work properly, you need a peculiar META file. The rules are explained below.

It is recommended to split the findlib package into two: 1. one for the main library, which almost assume it is just a normal library 2. another sub-package one for:

  • allowing to mix preprocessors and normal dependencies
  • the method of calling one executable per rewriter

In the rest we'll assume we are writing a META file for a ppx_foo rewriter, that itself uses the ppx_driver, ppx_core and re libraries, and produce code using ppx_foo.runtime-lib.

We want the META file to support all of these: 1. mix normal dependencies and preprocessors, using one executable per rewriter:

   ocamlfind ocamlc -package ppx_foo -c toto.ml
  1. mix normal dependencies and preprocessors, using a single ppx driver:

    $ ocamlfind ocamlc -package ppx_foo -predicates custom_ppx \
      -ppx ./custom-driver.exe -c toto.ml
  2. build a custom driver:

    $ ocamlfind ocamlc -linkpkg -package ppx_foo -predicates ppx_driver \
      -o custom-driver.exe
  3. build systems properly specifying preprocessors as such, separated from normal dependencies, as jbuilder does

Since preprocessors and normal dependencies are always specified separately in jbuild files, jbuilder just always set the ppx_driver predicates.

In the end the META file should look like this:

# Standard package, expect it assumes that the "ppx_driver" predicate
# is set
version                     = "42.0"
description                 = "interprets [%foo ...] extensions"
requires(ppx_driver)        = "ppx_core ppx_driver re"
archives(ppx_driver,byte)   = "ppx_foo.cma"
archives(ppx_driver,native) = "ppx_foo.cmxa"
plugin(ppx_driver,byte)     = "ppx_foo.cma"
plugin(ppx_driver,native)   = "ppx_foo.cmxs"

# This is what jbuilder uses to find out the runtime dependencies of
# a preprocessor
ppx_runtime_deps = "ppx_foo.runtime-lib"

# This line makes things transparent for people mixing preprocessors
# and normal dependencies
requires(-ppx_driver) = "ppx_foo.deprecated-ppx-method"

package "deprecated-ppx-method" (
  description = "glue package for the deprecated method of using ppx"
  requires    = "ppx_foo.runtime-lib"
  ppx(-ppx_driver,-custom_ppx) = "./as-ppx.exe"

package "runtime-lib" ( ... )

You can check that this META works for all the 4 methods described above.