Re: [Caml-list] META file standards for ppx extensions

[Anil Madhavapeddy <anil@recoil.org>]

On 10 Apr 2015, at 17:42, Daniel Bünzli <daniel.buenzli@erratique.ch> wrote:
> 
> In general it's better if the base compilation tools do not try to do too much/output many files in one go so that build systems can 1) trivially infer their outputs 2) reasonably rely on fs modification times to equate them with concrete content modification 3) be really in charge of composing the build pipeline.  
> 
> It's all about having a *composable* toolchain vs. inflexible monolithic black boxes.  

From a longer term perspective, the disentangling of preprocessing from compilation also makes it easier to do multi-stage preprocessing -- for instance, to perform type-driven I/O generation separately from an Lwt extension. The former affects dependency calculation, and the latter doesn't and is usually quite a local transformation within a module.  Why should they be clumped together in the build system just because they happen to use the same ppx/camlp4 mechanism?

Those examples aren't multistage yet, but the lines between MetaCaml, Camlp4, and (a hypothetical) OCaml macro system are pretty blurred.  OCaml is one of the few languages with a really good story for a statically typed processing pipeline, and it would be a great pity if build systems become the blocker for experimenting with them easily...

-Anil