[wg-camlp4] Status of extension_points extensions

Alain Frisch alain.frisch at lexifi.com
Wed Jun 5 09:28:37 BST 2013

On 06/04/2013 11:08 AM, Romain Bardou wrote:
> The syntax makes it very clear that extensions are not part of the
> language and are extensions. This is good. However, it also means that
> if the extension proves so useful that we want it part of the actual
> language, we are left with the following choices:
> - provide the extension as-is;
> - reprogram the extension with a dedicated syntax.

I don't think that many of the syntactic tools implemented currently 
with Camlp4 or later with -ppx/extension_points will be seriously 
considered for integration into OCaml.

If we look at existing Camlp4 extensions, very few made their way into 
the language.  I can actually only think of one example, pa_open_in => 
"let open ... in" (the syntax changed lightly).  And I don't see any 
obvious other candidates for integration into the language.

So I'm not sure the question you raise will be of huge practical importance.

> If we provide the extension as-is, the fact that the syntax says it is
> an extension suddenly is a problem because it is not actually an
> extension anymore if it is part of the language. Moreover the syntax may
> not be ideal. And if this approach is used, and several extensions are
> included, we may end up with programs which are cluttered with brackets,
> so to speak.

For "extension nodes", indeed, it would be very weird to have some of 
them supported natively.  For attributes, I'm less sure.  Java or C#, 
for instance, have notions of attributes/meta-data attached to methods 
and classes; some of them are interpreted by external tools, but some of 
them are given a built-in interpretation by the compiler.  I wouldn't be 
shocked if, at some point, the OCaml compilers started to interpret some 
attributes to drive optimizations or to tweak the OCaml FFI calling 
convention, for instance.  This needs to be discussed, though.

> If, on the other hand, we reprogram the extension, we are able to
> provide a nicer syntax. For instance, just write "abstract" instead of
> "[@@mli abstract]". But it means more work and we would not know what to
> do with programs that used the extension before: rewrite them with the
> new syntax? Keep using the extension, which will probably no longer be
> maintained if it is part of the language?

The extension will probably become straightforward, mapping directly to 
the new built-in construction; so maintaining it would be very simple.

> Until now, I saw extension_points as a very nice way to experiment with
> language extensions, and to provide custom, project-local extensions.

Extension_points can certainly help to experiment with new language 
features, but maybe not in the way you suggest.  In particular, most new 
interesting language features expect some special support in the 
type-checker and/or the backend.  Extension_points can help by allowing 
the people working on such an extension to focus on the interesting part 
(type-checker / back-end) without having to adapt the parser, the 
Parsetree and all the code relying on it (including Camlp4 if they want 
to be able to try their new extension on large code base which require 

Project-local extensions are also certainly a good candidate for using 
extension_points.  But the main goal of extension_points, in my mind, is 
to support "syntactic tools" which, most of the time, clearly don't 
deserve to be part of the core language, and where a slightly "foreign" 
syntax is not a big deal.  These tools can be code generators (such as 
type-conv/deriving, or bitstring/ulex), code analyzers which require 
some annotations (such as Bisect), or tools which need to extract 
(annotated) information for external use (such as ocamldoc, or an FFI 



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