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Migration from Camlp4 to PPX

Recent versions of OCaml (4.02 and newer) provide PPX, which is a new mechanism for extending the language. Eliom 5.0 utilizes PPX to provide a superset of OCaml, which is suitable for developing client-server Web applications.

This document provides a migration guide from our older Camlp4-based language to the new PPX-based language. While our Camlp4 language is still supported, future development will focus on PPX, and therefore users are advised to migrate.

For more extensive information on the PPX extension itself, refer to the corresponding manual page.

Source code conversion

The camlp4-to-ppx tool can be used to translate source code using Camlp4 syntax extensions to PPX. We provide a version of camlp4-to-ppx that handles all the Ocsigen extensions.

You need to run our version of camlp4-to-ppx on all the .eliom and .eliomi files in your project, e.g., with the following shell snippet:

for FILE in *.eliom *.eliomi; do
  camlp4-to-ppx $FILE > $FILE.ppx
  mv $FILE.ppx $FILE

Build system changes

Eliom provides wrappers around the standard OCaml and js_of_ocaml tools, e.g., eliomc, eliomopt, js_of_eliom, and eliomdep. Starting from Eliom 5.0, these tools accept a -ppx flag, which specifies that our PPX syntax extension is to be used. (The default is Camlp4 at this time.) The build configuration needs to be adapted accordingly.

Our tools do not automatically load PPX extensions for js_of_ocaml, Lwt, etc.; these need to be loaded explicitly, e.g., by loading Ocamlfind packages like js_of_ocaml.ppx and lwt_ppx (via the -package argument). Similarly, Ocamlfind packages need to be loaded for any ppx_deriving plugins that apply. Note that ppx_deriving needs to be installed manually, since it is only an optional dependency of Eliom and js_of_ocaml.

Camlp4 and PPX extensions cannot be used in the same file. Code relying on Camlp4 extensions with no PPX equivalents (as is the case for Macaque and PGOcaml) needs to be moved to separate files that are preprocessed through Camlp4.

Good style

Our PPX language is more flexible than our Camlp4 language. Specifically, the [%%client.start], [%%server.start], and [%%shared.start] can be used to specify the default location of all the signature or structure items that follow in the file, while locations can apply to single structure or signature items (overriding the default), e.g.,

let%client x = 0

The code produced by camlp4-to-ppx is a literal translation of the original code, and thus does not use the additional features. It is advised that future development utilizes the new constructs, for example by using one of [%%client.start], [%%server.start], and [%%shared.start] in the beginning of the file to specify a sane default for the functionality implemented, and to explicitly override with let%client, let%server, let%shared only when necessary.