metapp is a PPX rewriter that 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 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]]

metapp can be used with dune by using the preprocess field.

(executable
  ...
  (preprocess (pps metapp.ppx))
  ...)

Inside [%meta ...] code, the [%e ...] extension quotes expressions
(of type [Parsetree.expression]). There are other quotations
available: the full list is given below.

|Quotation |Type |
|-----------------------------|--------------------------|
|[%e ...] or [%expr ...] |Parsetree.expression |
|[%p? ...] or [%pat? ...] |Parsetree.pattern |
|[%t: ...] or [%type: ...]|Parsetree.core_type |
|[%sig: ...] |Parsetree.signature |
|[%sigi: ...] |Parsetree.signature_item|
|[%str ...] |Parsetree.structure |
|[%stri ...] |Parsetree.structure_item|

Quoted expressions can in turn contain further [%meta ...] code.
Moreover, [%meta ...] code can itself contain other levels of
[%meta ...] code, for multi-stage programming.

In addition to this syntax extension, the Metapp module
provided by the metapp package provides convenient functions
for AST constructions. In particular, this module provides an
OCaml-version-independent interface. Moveover, this module provides a
common signature ValueS for constructing and transforming
expressions (module Exp), patterns (module Pat) or both at the
same time (module Value).

The Metapp module also provides a filter mapper, which handles
[@if <bool>] attributes à la ppx_optcomp. The [@if <bool>]
attribute can appear mostly everywhere syntax elements are enumerated,
including tuples, function applications, arrays, etc.

[%%meta Metapp.include_structure (
  Metapp.filter.structure Metapp.filter [%str
    type t =
    | A of int
    | B of int * int
        [@if [%meta Metapp.Exp.of_bool (Sys.ocaml_version >= "4.04.0")]]
    ...

    match (v: t) with
    | A x -> something x
    | B (y,z)
      [@if [%meta Metapp.Exp.of_bool (Sys.ocaml_version >= "4.04.0")]] ->
        something' y z
    ... ])]

Global definitions for meta-code can be included with [%%metadef ...].
By default, the meta-code is compiled with the compiler-libs package.
Other packages can be loaded with [%%metapackage ...].
More generally, flags can be passed to the compiler to compile meta-code
with [%%metaflag ...] (there is another convenient notation for
adding interface directories: [%%metadir ...]).
[%%metaload ...] loads a particular compilation unit.
For instance, [%%metapackage metapp] links the meta-code with the
metapp package in order to use the Metapp module.
All these notations can be applied to multiple arguments at once by using
comma as separator.

Note that dynamic package loading is broken in PPX with dune
(#3214). When packages are loaded with [%%metapackage ...], a
workaround (see discussion) is used to load the packages
correctly, but only with OCaml >=4.08. If you need to use
[%%metapackage ...] with a prior version of OCaml, you still can
statically link the packages you need by listing them in the
(preprocess (pps ...)) list. You will still have to import them with
[%%metapackage ...] to let the compiler see their interface when
compiling the meta-code.