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Metaprogramming and PPX

PPX Syntax Extensions

ppx is the syntax extension format supported currently by OCaml. It replaces older techniques such as Camlp4 by limiting the scope of language extensions and dedicating to them a specific syntax.

  • ppxlib: The modern solution for writing PPX extensions. Without this library, writing PPX extensions is fragile and breaks with OCaml version changes. ppxlib merges several older projects together to provide a complete platform for writing efficient, resilient PPX extensions.


PPX Extensions

  • ppx_enum: A nice, simple ppx using ppxlib that serves as a good example for potential ppx writers.
  • ppx_deriving: Type-based framework for ppx extensions. Contains built-in plugins for show, eq, ord, enum, iter, map, fold, and make.
    • NOTE: ppx_deriving currently doesn’t make use of ppxlib, making it hard to update for future OCaml releases. As powerful as ppx_deriving is, we recommend trying to find a ppxlib-based extension instead, which will then be a lot more robust going forward.
  • ppx_show: A show deriver, creating functions for printing out values. Uses ppxlib, which makes it forward-compatible.
  • ppx_visitors: Automatically use the visitor object-oriented pattern on a data structure, extending it with behaviors rather than needing to specify each variant’s behavior.
  • ppx_import: Import is a syntax extension that allows to pull in types or signatures from other compiled interface files. This can be handy when not wanting to repeat a type in both the .ml and .mli file, for example.
  • ppx_override: Override allows you to import a module or its types, and then easily change and modify aspects, such as using ppx_deriving on that module’s types.
  • ppx_optcomp: Conditional compilation like #ifdef for OCaml.
  • ppx_string_interpolate: A simple ppx filter to support string interpolation like [%str "value of foo is $(foo)"].
  • ppx_monad: Monadic syntax extension.
  • ppx_let: A monadic syntax extension from Jane Street.
  • ocaml-monadic: Another monadic syntax extension.
  • ppx_regex: Contains 2 ppx parsers to OCaml regex libraries:
    • ppx_regexp: maps to use Re (untyped regex)
    • ppx_tyre: maps to use Tyre for typed regex.
  • ppx_expect: Cram-like tests for OCaml. See Testing.
  • Bisect_ppx: Code coverage for OCaml. See Code Tools.
  • ppx_pgsql: A syntax extension for embedded SQL queries using PG’OCaml. See Databases.
  • ppx_cstubs: Write C functions directly in your OCaml code. See FFI.
  • ppx-tyxml: Convert html/xml syntax to tyxml function calls.


  • cppo: A simple C++-like preprocessor for OCaml files.
  • MetaOCaml: An OCaml dialect for multi-stage programming.
  • metapp: A ppx extension that provides metaquoting facilities similar to MetaOCaml.
  • Fan: Fan is a compile-time metaprogramming system for OCaml, originally inspired from Camlp4. It’s a combination of OCaml and Lispy Macros. It shares the same concrete syntax with OCaml.
  • camlp4: Camlp4 is an older way of modifying OCaml syntax and applying metaprogramming. It is generally discouraged nowadays – use ppx instead.
  • camlp5: Another variant of metaprogramming that is discouraged nowadays.