Preprocessing in OCaml (using Ppxlib)

#ocaml #extension #points #reference #guide

Extension points are a functionality provided by the OCaml ecosystem to allow developers to define custom semantics for OCaml syntax in specific delimited contexts. While this means that extension-point based libraries can not introduce arbitrary new syntax (restricting some of the expressivity of such tools), it's been widely accepted as a comprimise between extending the semantics of the language while preventing the prevalence of esoteric syntax in OCaml codebases.

This much is mentioned on the Ppxlib github repository, but there's notably little documentation on how to actually get started on writing such libraries.

This is where this document comes in.

I have spent the past few days scouring through the sparse ppxlib documentation online and have managed to unify this into a single guide for getting started with OCaml extension points.

This guide will take you through the entire process - all the way from setting up a syntax extension library in dune, to techniques for producing useful and constructive error messages.

Let's get started.

Setting up project

In order to get started with an extension point, create a library dune project with the following stanza:

 (name <name>)
 (wrapped false)
 (kind ppx_rewriter)
 (libraries ppxlib)
 (preprocess (pps ppxlib.metaquot ppx_deriving.std)))

Note: you can add this stanza to an existing dune file, but in that case you will need a (library ... (modules <modules>) ..) field to split modules between your libraries (a module can only be included in a single library).

Obviously, make sure to install the ppxlib library if you don't already have it.

Basic Template

The minimal set of code you'll need to get started is as follows:

open Ppxlib

let name = "extension"

let expand ~loc ~path:_ expr =
  match expr with
  | _ -> ignore (Location.raise_errorf ~loc "not implemented"); expr

let ext =
    Ast_pattern.(single_expr_payload __)

let () = Driver.register_transformation name ~extensions:[ext]

This snippet works as follows:

defines an ast extension that matches an arbitrary expression (specified by the 3rd argument) and then executes expand on the ast. Further constraints on the things it matches can also be made - for example Ast_pattern.(single_expr_payyload (estring __)) will match only strings.
registers the extension under then name "extension" with the compiler
in this context is a function that should take an ast and return a transformed ast.

This template sets up an extension that can be invoked in the following ways:

match%extension value with ....
[%extension "value", ....]
let%extension x = value in ...

AST structure and Documentation

When defining the expand function, the typical strategy is to have your code match some kind of ast structure and then return a modified version of the structure.

But what is the structure of the AST, and how are expressions represented in this structure?

I've found two particularly useful strategies for answering these questions:

  • the dumpast tool
  • ppxlib "full" documentation


If you install the ppx_tools library from opam, it provides a tool dumpast that allows you to see the corresponding ast for a given expression.

I typically run it from the command line as follows:

ocamlfind ppx_tools/dumpast -e '[1;2;3;4]'

This prints out the ast for the expression [1;2;3;4].

ppxlib documentation

The Ppxlib library documentation is actually invaluable for working out how ast expressions should look. Unfortunately, rather annoyingly, the only documentation the Ppxlib developers have chosen to precompile and export is the documentation for "how-to-use-this-library" rather than the standard ocamldoc.

Thus, I'd highly recommend downloading the project and running ocamldoc to build the full documentation. Alternatively, I've found that this binary analysis project documentation has also helpfully included a compiled version of the ppxlib documentation, which can be helpful if you're lazy.

Generating AST values

If you look at the earlier dune stanza I recommended, the code for the extension point is preprocessed using ppxlib metaquot library - this library automates some of the process of constructing ast expressions, and I think may be more future compatible than manually constructing the ast terms yourself.

The general strategy for using the metaquot library is as follows.

  • [%expr ...] converts an arbitrary static ocaml expression into its corresponding ocaml ast structure - i.e [%expr [1;2;3;4]] would expand to the full ast for the expression [1;2;3;4].
  • within [%expr _], [%e _] allows "unquoting" and inserting an arbitrary expression into the static ast constructed by [%expr _]. The contents of [%e _] should be an ast object.
[%expr (Some [%e  ....])]

For example, the following code is how I might recursively convert some kind of ast into a list:

[%expr ([%e c1], [%e c2]) :: [%e expand ~loc ... ]]

Notice how I use [%expr] to automate constructing the tedious static parts of the AST, and then use [%e] at strategic points to insert the custom expressions I want.

More information on metaquot can be found on the ppxlib library documentation.

Error messages

Occasionally, users may invoke your extension in invalid ways, in which case your extension should fail in a sensible way.

We could throw a custom exception, but this can't be handled by the compiler, and will just result in the entire compilation process crashing with a huge backtrace. Merlin usually reports this kind of failure by claiming that the extension is unknown.

Instead, to obtain an error that provides a nicer message to the end user, use the Location.raise_errorf function to throw an error that can be handled by the compiler - typically I execute it as follows:

ignore(Location.raise_errorf ~loc "this is an %s message" "error"); ....


This concludes this guide on OCaml extension points. We've gone through the entire process of building extension points - from setting up the project to developing and returning errors.

There isn't much documentation online on this topic, so hopefully this has provided a suitable starting point to begin developing OCaml extension points, and I look forward to seeing your extensions.