Saturday, December 9, 2017

How to migrate your ppx to OCaml migrate parsetree

OCaml migrate parse tree

OCaml migrate parse tree

Earlier this year, this blog post [2] explored the implementation of a small preprocessor extension (ppx).

The code of the above article worked well enough at the time but as written, exhibits a problem : new releases of the OCaml compiler are generally accompanied by evolutions of the OCaml parse tree. The effect of this is, a ppx written against a specific version of the compiler will "break" in the presence of later releases of the compiler. As pointed out in [3], the use of ppx's in the OCaml eco-system these days is ubiquitous. If each new release of the OCaml compiler required sychronized updates of each and every ppx in opam, getting new releases of the compiler out would soon become a near impossibilty.

Mitigation of the above problem is provided by the ocaml-migrate-parsetree library. The library provides the means to convert parsetrees from one OCaml version to another. This allows the ppx rewriter to write against a specific version of the parsetree and lets the library take care of rolling parsetrees backwards and forwards in versions as necessary. In this way, the resulting ppx is "forward compatible" with newer OCaml versions without requiring ppx code updates.

To get the ppx_id_of code from the earlier blog post usable with ocaml-migrate-parsetree required a couple of small tweaks to make it OCaml 4.02.0 compatible. The changes from the original code were slight and not of significant enough interest to be worth presenting here. What is worth looking at is what it then took to switch the code to use ocaml-migrate-parsetree. The answer is : very little!

open Migrate_parsetree
open OCaml_402.Ast

open Ast_mapper
open Ast_helper
open Asttypes
open Parsetree
open Longident

(* The original ppx as written before goes here!
   .                    .                   .
   .                    .                   .
   .                    .                   .

let () = Driver.register ~name:"id_of" (module OCaml_402) id_of_mapper
The complete code for this article is available online here and as a bonus, includes a minimal jbuilder build system demonstrating just how well the OCaml tool-chain comes together these days.

[1] "A Guide to Extension Points in OCaml" -- Whitequark (blog post 2014)
[2] "Preprocessor extensions for code generation" -- Shayne Fletcher (blog post 2017)
[3] "Extension Points - 3 Years Later" -- Rudi Grinberg (blog post 2017)