Camlp4 is a software system for writing extensible parsers for
programming languages. It provides a set of OCaml libraries that are
used to define grammars as well as loadable syntax extensions of such
grammars. Camlp4 stands for Caml Preprocessor and Pretty-Printer and
one of its most important applications is the definition of
domain-specific extensions of the syntax of OCaml.

Camlp4 was part of the official OCaml distribution until its version
4.01.0. Since then it has been replaced by a simpler system which is
easier to maintain and to learn: ppx rewriters and extension points.