PPX

PPX, commonly written ppx, denotes the PreProcessor eXtensions framework in OCaml that enables syntax and code transformations through rewriters that operate on the compiler's abstract syntax tree. Rather than writing in language-level syntax, developers implement small programs, or "rewriters," that receive OCaml AST nodes as input and emit transformed AST that the compiler subsequently compiles.

PPX rewriters are invoked during the build process, typically via the -ppx option provided by the OCaml compiler or by build systems such as Dune, which can automatically apply registered ppx rewriters.

The PPX ecosystem emerged to provide safe, modular, and reusable extensions, replacing earlier approaches such as Camlp4. PPX rewriters can implement features like automatic derivations, syntax sugar, or domain-specific code generation.

Popular PPX packages include ppx_deriving, which automatically generates boilerplate such as equality, comparison, or pretty-printing functions; others provide let or binding syntax, test infrastructure, or inline optimizations. The modern tooling centers on ppxlib, a library that simplifies creating rewriters, and a set of conventions and runtimes that package rewriters for use with Dune and OCaml's compiler toolchain.

See also OCaml, Dune, Camlp4, ppx_deriving, ppxlib.