[Caml-list] [ANN] First release of metapp

[Thierry Martinez <Thierry.Martinez@inria.fr>]

Dear users of OCaml,

I am happy to announce the first release of metapp, yet another
preprocessor for OCaml. Similarly to ppx_optcomp, metapp is a PPX
rewriter. But instead of introducing a specific DSL for preprocessor
directives, metapp provides a [%meta ...] extension, where the dots
... are arbitrary OCaml expressions that are substituted at compile-time
by the AST nodes they evaluate into. These expressions build AST nodes
either by (anti-)quoting some code directly, or by using compiler-libs
(Parsetree, Ast_helper, ...).

In particular, this preprocessor is easy to use for conditional
compilation, and is an alternative to cppo and ppx_optcomp.

let option_get o =
  [%meta if Sys.ocaml_version >= "4.08.0" then
     [%e Option.get o]
  else
     [%e match o with
     | None -> invalid_arg "option_get"
     | Some x -> x]]

In this example, the code between [%e ... ] is "anti-quoted": it is the
code that is inserted (conditionally) in the rewritten module. Of
course, the anti-quoted code can contain itself some [%meta ...]
code. [%meta ...] can even itself contain other levels of [%meta ...]
code for multi-stage programming.

An example of usage of metapp is the metaquot package, which implements
the same quoters as ppx_tools.metaquot: [%expr ...], [%type: ...],
etc. These quoters are implemented by meta-programming: the meta-code
introspects Parsetree.cmi from compiler-libs to generate the code
matching the current OCaml version.

Happy hacking,
-- 
Thierry.