• OCamlgraph – A generic graph library for OCaml.
• ods – A large collection of data structures and algorithms for OCaml.
• combine – OCaml library for combinatorics https://www.lri.fr/~filliatr/combine/.
• ODiff – Library of YIQ NTSC transmission image difference alghoritm implemented in OCaml and ReasonML.

• Batteries Included – A community-maintained foundation library for your OCaml projects.
• Cmdliner – Declarative definition of command line interfaces for OCaml.
• Core – Jane Street Capital's full-fledged standard library overlay. A portable subset of Core is also available: Core_kernel.
• React – React is an OCaml module for functional reactive programming (FRP). It provides support for programs with time-varying values, declarative events, and signals.
• Minicli – Minimalist library for command-line parsing.
• easy-format – Pretty-printing library for OCaml.
• ocaml-rpc – Light library to deal with RPCs in OCaml.
• ocaml-containers – A lightweight, modular standard library extension, string library, and interfaces to various libraries (bigarrays, Unix, etc.) BSD license.

• core_bench – Micro-benchmarking library for OCaml by Jane Street.
    - Getting Started with Core_bench
• benchmark – Benchmarking functions for measuring the run-time of functions using latency or throughput.

• More OCaml: Algorithms, Methods, and Diversions – In More OCaml, John Whitington takes a meandering tour of functional programming with OCaml, introducing various language features and describing some classic algorithms. The book ends with a large-scale example dealing with the production of PDF files. There are questions for each chapter, along with worked-out answers and hints.
• How to Think Like a (Functional) Programmer by Allen Downey and Nicholas Monje – How to Think Like a Computer Scientist is an introductory programming textbook based on the OCaml language. It is a modified version of Think Python by Allen Downey. It is intended for newcomers to programming and also those who know some programming but want to learn programming in the function-oriented paradigm, or those who simply want to learn OCaml.
• OCaml from the Very Beginning by J. Whitington - OCaml from the Very Beginning will appeal both to new programmers and experienced programmers eager to explore functional languages such as OCaml.
• Pearls of Functional Algorithm Design by Richard Bird - It summarizes 30 hard algorithmic problems in the function programming world. Although it is for Haskell, the algorithm problems are very interesting, and trying to solve them in OCaml also helps the thinking of functional programming. Partial solutions in OCaml are here.
• Real World OCaml by Y. Minsky, A. Madhavapeddy, and J. Hickey - Functional Programming for the masses.
• Unix System Programming in OCaml by X. Leroy and D. Rémy – Introduction to Unix Systems Programming, with an emphasis on communications between processes.
• Using, Understanding, and Unraveling OCaml – This book describes both the OCaml language and the theoretical grounds behind its powerful type system.

• OCaml Programming: Correct + Efficient + Beautiful - List of 200 bite-sized videos recorded by Michael R. Clarkson. It can be watched independently of the textbook titled the same and listed above in the Books section.

• pfff – pfff is a set of tools and APIs to perform some static analysis, dynamic analysis, code visualizations, code navigations, or style-preserving source-to-source transformations such as refactorings on source code.

• Languages and Compilers:

  • Caramel - Caramel is a functional language for building type-safe, scalable, and maintainable applications.

  • cDuce - cDuce is a modern XML-oriented functional language with innovative features.

  • Compcert C Compiler - It is a C Compiler supporting most of the ISO C90 and C99 / ANSI C features.

  • Eff Programming Language - Eff is a functional language with handlers of not only exceptions, but also of other computational effects such as state or I/O.

  • Hack Programming Language

  • Haxe Programming Language

  • Neko Programming Language - Originally the compiler was written in OCaml.

  • Mazeppa - A modern supercompiler for call-by-value functional languages.

  • Mezzo Programming Language - Mezzo is a programming language in the ML tradition, which places strong emphasis on the control of aliasing and access to mutable memory.

  • OCaml-Java - OCaml to Java bytecode compiler.

  • Opa Programming Language

  • Rhine – A Lisp on LLVM written in OCaml.

  • Rust Programming Language - Originally written in OCaml before bootstrapping.

  • Quick C-- Target Language - It is now a dead project. Github Repo. Alternative website.

  • tis-interpreter - An interpreter for finding subtle bugs in programs written in standard C

  • Reason - Friendly syntax & toolchain for OCaml by Facebook.

  • RaML - Resource Aware ML (RaML) is a tool that automatically and statically computes resource-use bounds for OCaml programs.

  • Liquid ML - Shopify's Liquid Templating language for OCaml.

• Parser and Lexer Generators:

  • Opal – Self-contained monadic parser combinators for OCaml.
  • Sedlex is a modern, encoding-agnostic (read: Unicode-supporting) lexer generator (the ppx-based successor to ulex.)
  • Menhir – Menhir is a LR(1) parser generator for OCaml.
    • See ocaml-parsing for a clearer example of using Menhir and Sedlex to produce a useful parser,
    • ... and Obelisk, a neat project to produce readable LaTeX, HTML, or plain-text EBNF-style documentation for your grammar.
  • ocamllex/ocamlyacc – lex and yacc implementation for OCaml.
  • Angstrom - Parser combinators built for speed and memory efficiency

• Articles:

  • Kaleidoscope: Implementing a Language with LLVM in Objective Caml¶

Before OCaml 5.0, there were two libraries for concurrent programming: Lwt and Async. They provide very similar functionality but make radically different decisions with regards to error handling and internal implementation details (see the links below for more details). Real World OCaml uses Async, but a version of the code examples translated to Lwt is also available.

With the introduction of Effect Handlers in OCaml 5.0, a bunch of other libraries have been created for concurrent programming, replacing the monadic approaches of LWT and Async with direct-style ones.

• Libraries:
  • Eio — effects-based direct-style IO for multicore OCaml.
  • Miou — a simple scheduler for OCaml 5.
  • Lwt — A cooperative threads library for OCaml.
  • Async — A monadic concurrence library to go with the Core library.
• Articles:
  • The blog post that introduced Async
  • A user gives up on Async
  • Cooperative Concurrency in OCaml: A Core.Std.Async Example.

• Bindings
  • Dbm — A binding to the NDBM/GDBM Unix "databases".
  • Mongo.ml – An OCaml driver for Mongodb
  • PG'OCaml — A type-safe interface to PostgreSQL in pure OCaml.
    • ppx_pgsql – A syntax extension for embedded SQL queries using PG'OCaml.
  • PostgreSQL-OCaml — An interface to PostgreSQL through the C API (libpq).
  • SQLite3 — OCaml bindings to the SQLite3 database.
  • Sqlite3EZ — Thin wrapper for SQLite3 with a simplified interface.
  • ocaml-redis – Redis bindings for OCaml.
  • mariadb - Bindings to MariaDB/MySQL, supporting the nonblocking API
  • pgx – A pure OCaml PostgreSQL client library.
  • mysql_protocol – Implementation of MySQL Protocol with the Bitstring library.
• New Implementations
  • Irmin — A distributed database that follows the same design principles as Git.
  • Obigstore — A database with BigTable-like data model atop LevelDB.
  • RunOrg - It is a WIP database server written in OCaml.
  • dokeysto - dumb OCaml key-value store, string keys and string
    values. Optional on-the-fly LZ4 compression of values or tokyocabinet backend.
• Overlays
  • Sequoia - Sequoia is a type-safe query builder for MySQL/MariaDB and PostgreSQL
  • Macaque — Macaque is a library for safe and flexible database queries using comprehensions on top of PG'OCaml.
  • ORM — ORM for SQLite.
  • Caqti - Cooperative-threaded access to relational data
  • Caqti setence preparation, ppx_rapper
• Articles:
  • Implementing the Binary Memcached Protocol with Ocaml and Bitstring
  • Interfacing OCaml and PostgreSQL with Caqti
  • Finally, Type-Safe, Extensible and Efficient Language Integrated Query by Oleg and Co.
    The proposed approach is to describe SQL queries in type-safe manner and optimize them (using term rewriting or normalization-by evaluation) before sending to database engine. It potentially could optimize O(n^2) queries to O(n) ones.

• Try OCaml – Try OCaml in your web browser.

• learn-ocaml. Web app (written in OCaml) underlying the learn-ocaml-corpus. Can be customized to serve lectures (with Markdown slides), playgrounds (with a toplevel prelude), and interactive exercises (with OCaml tests). MIT License.

• learn-ocaml.el. Minor mode for Emacs that can display exercise topics and grade exercise solutions, after logging to a Learn-OCaml instance. MIT License.

• BetterOCaml – An efficient, intuitive, and cross-platform web IDE with your OCaml code interpreted and running in your browser!

• codingground – Compile and execute OCaml code online.

• Jupyter – An OCaml kernel for the Jupyter notebook.

• utop – Universal toplevel for OCaml with support for multiline edition, history, real-time and context-sensitive completion, colors, and more.

• ocamlbrowser – A source and compiled interface browser, written using LablTk. Included in the standard distribution for ocaml <= 4.01 and with labltk for ocaml >= 4.02.

• ghim – A command-line tool to manage Github Issues.

• OCaml Yeoman Generator – Yeoman generator to scaffold OCaml modules.

• puml2xml – A PlantUML (.puml) to XML (.xmi) converter.

• Foreign Function Interface:

  • ctypes – Library for binding to C libraries using pure OCaml.
  • ocaml-main-program-in-c – Example build system for making mixed C/Ocaml binaries where the main program is in C.
  • Modular foreign function bindings
  • Py.ml - OCaml bindings for Python.

• Editor Integration:

  • ocaml-lsp - An LSP language server for OCaml that integrates with any editor that understands LSP like VSCode, Vim and Emacs.
  • merlin – Context sensitive completion for OCaml in Vim and Emacs.
  • tuareg - OCaml mode for Emacs that can run the toplevel and the debugger within Emacs.
  • opam-switch-mode - Minor mode for Emacs that extends Tuareg and Merlin with menus to change or reset the opam switch in the ambient Emacs session.
  • merlin-eldoc – Emacs package to provide merlin's features through eldoc.
  • vscode-ocaml – extension that provides OCaml language support for VSCode
  • OCaml Debugger – extension that provides OCaml Debugger for VSCode
  • Sublime better ocaml – Better OCaml mode for Sublime Text.
    • Sublime text package
  • ocp-index – Easy access to the interface information of installed OCaml libraries. Provides standalone tools like ocp-browser and ocp-grep.
    • ocp-browser – Small ncurses-based API and documentation browser.
    • ocp-index-top – Toplevel directive for looking up documentation using ocp-index.
    • Sublime text package
  • ocp-indent – Indentation tool for OCaml, to be used from editors like Emacs and Vim.
    • Vim plugin.

• Code coverage:

  • Bisect_ppx

• 99 problems. 99% of the solutions are here.
• learn-ocaml-corpus. Corpus of beginner-to-advanced online exercises (including those from the OCaml MOOC) with automatic grading tests.
• OCaml at Exercism – Exercism is your place to engage in thoughtful conversations about code. Explore simplicity, idiomatic language features, and expressive, readable code. Solutions.

• Coq – Coq is a formal proof management system. It provides a formal language to write mathematical definitions, executable algorithms, and theorems, together with an environment for semi-interactive development of machine-checked proofs.
• Why3 – Why3 is a platform for deductive program verification. It provides a rich language for specification and programming, called WhyML, and relies on external theorem provers, both automated and interactive, to discharge verification conditions.
• Alt-Ergo – Alt-Ergo is an open-source SMT solver dedicated to the proof of mathematical formulas generated in the context of program verification.

• camlPDF – OCaml library for reading, writing and modifying PDF files.
• slacko – A neat interface for Slack in OCaml.

• 2D
  • archimedes — 2D plotting library.
  • cairo2 — Binding to Cairo, a 2D Vector Graphics Library. Integrates well with lablgtk.
  • Vg – Declarative 2D vector graphics for OCaml.
• 3D
  • glMLite — OpenGL bindings for OCaml. Provides an (experimental) functional API. (homepage)
  • lablgl — Interface to OpenGL. Integrates well with lablgtk.
  • tgls — Thin bindings OpenGL 3.{2,3},4.{0,1,2,3,4} and OpenGL ES {2,3}.

• Camomile — A Unicode library for OCaml.
• ocaml-m17n — Multilingualization for OCaml source code. Allows using Unicode identifiers in OCaml source code.
• Uucd — Unicode character database decoder for OCaml.
• Uucp — Unicode character properties for OCaml.
• Uunf — Unicode text normalization for OCaml.
• Uuseg — Unicode text segmentation for OCaml.
• Uutf — Non-blocking streaming Unicode codec for OCaml.

• lablgtk — GTK2 and GTK3 bindings for OCaml with various higher-level facilities to define GUIs.
• lablqml – QML Qt5 bindings for OCaml.
• labltk — Interface to the Tcl/Tk GUI framework. In the standard distribution for ocaml <= 4.01.
• TSDL – Tsdl is an OCaml module providing thin bindings to the cross-platform SDL library.
• Lambda-Term – Lambda-Term is a cross-platform library for manipulating the terminal. It provides an abstraction for keys, mouse events, and colors, as well as a set of widgets to write curses-like applications.

• mikmatch – OCaml pattern-matching extended with regexps

• dolog – A dumb OCaml logger.
• Volt – A variant of the Bolt OCaml logging tool.

• Libraries
  • Ocaml-sklearn scikit-learn for OCaml.
  • Object detection convolutional neural network with OCaml (based on Owl).
    - Ocaml-NN - Fully functional monadic implementation of a Neural Network (FCNNs) in OCaml
• Articles
  • Deep Learning with OCaml (PyTorch bindings).
  • Transfer Learning with OCaml (PyTorch bindings).
  • Reinforcement Learning with OCaml (PyTorch bindings).

• ocaml-zmq – ZeroMQ bindings for OCaml with Async and Lwt wrappers.
• onanomsg – nanomsg bindings for OCaml.
• Kafka – OCaml bindings for Apache Kafka.
• AMQP – AMQP client library for Async and Lwt.
• MPI – Message Passing Interface bindings for OCaml.
• MQTT – OCaml implementation of the MQTT pubsub protocol.
• capnp-ocaml – OCaml code generator plugin for the Cap'n Proto serialization framework.

• Articles:
  • A Guide to Extension Points in OCaml
  • Extension Points, or how OCaml is becoming more like Lisp
  • Syntax extensions without Camlp4: let's do it!
  • Reading Camlp4 – Ambassador to the Computers
• Syntax Extensions:
  • ppx_import – Import is a syntax extension that allows to pull in types or signatures from other compiled interface files.
  • ppx_string_interpolate – A simple ppx filter to support string interpolation like [%str "value of foo is $(foo)"].
  • ppx_monad – Monad syntax extension for OCaml.
  • ppx_deriving_yojson – A Yojson codec generator for OCaml.
• Tools and Language Extensions:
  • MetaOCaml – an OCaml dialect for multi-stage programming.
  • 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.
  • camlp5 - Camlp5 is a preprocessor-pretty-printer of OCaml.
  • camlp4 - Camlp4 is part of the standard OCaml distribution and is different from Camlp5.

• prometheus – OCaml client library for Prometheus monitoring.

• Articles:
  • OCaml on iOS 7 Released
  • OCaml + Cordova = more secured, typed and hybrid mobile applications
• Bindings:
  • Cordova plugins – List of bindings to Cordova plugins. Get access to native device components like accelerometer, SMS, geolocation, etc in OCaml.

• HTTP Tools:
  • ocaml-cohttp – Very lightweight HTTP server using Lwt or Async.
  • ocurl – OCaml bindings to libcurl.
  • httpaf – A high performance, memory efficient, and scalable web server written in OCaml.
  • piaf - Client/server library for HTTP/1.X / HTTP/2 written entirely in OCaml.
• ocaml-dns – A pure OCaml implementation of the DNS protocol.
• fluent-logger – Fluentd logger for OCaml.

• Cornell University – Data Structures and Functional Programming.
• Princeton University - Functional programming in OCaml.

• Distribution:

  • OPAM – A flexible Git-friendly package manager with multiple compiler support.
  • ocamlfind — Local OCaml library manager. Used by most of the OCaml ecosystem.
  • OCaml for Windows - opam repository and experimental build for Windows (deprecated since 2021).
  • Diskuv OCaml - Diskuv OCaml distribution for Windows.
  • makorel – Release OPAM packages easily.
  • esy - package.json workflow for native development with Reason/OCaml.

• Build Tools:

  • dune – A composable and opinionated build system for OCaml (former jbuilder)
  • Oasis - A tool to integrate a configure, build and install system in your OCaml project. It helps to create standard entry points in your build system and allows external tools to analyse your project easily.
    • oasis2opam — Tool to convert OASIS metadata to OPAM package descriptions.
  • obuild – Simple package build system for ocaml.
  • ocaml-makefile — Easy to use Makefile for small to medium-sized OCaml-projects.
  • topkg — OPAM-aware packaging system using ocamlbuild.
  • Bazel - OCaml rules for Bazel, Google's multi-language and platform build tool.

(Note: Sorted from the easier to use to the more flexible.)

• Libraries:

  • Parmap — Provides easy-to-use parallel map and fold functions.
  • ForkWork — A simple library for forking child processes to perform work on multiple cores.
  • Functory — A distributed computing library which facilitates distributed execution of parallelizable computations in a seamless fashion.
  • Rpc.Parallel — A library for spawning processes on a cluster of machines, and passing typed messages between them.
  • Ocamlnet — An enhanced system platform library. Contains the netmulticore library to compute tasks on as many cores of the machine as needed.
  • Nproc – Process pool implementation for OCaml.
  • Parany – Parallelize computation over independent items, even if there is an infinite number of them.
  • Sklml – Functional parallel skeleton compiler and programming system for OCaml programs.
  • SPOC - Libraries and syntax extensions to offload intensive computations to parallel accelerators (multicore CPUs, GPUs and other accelerators compatible with GPGPU frameworks).

• Articles:

  • What is the state of OCaml's parallelization abilities?
  • Parallel programming in multicore OCaml
  • Parallelism programming from the officiel OCaml manual
  • Awesome multicore OCaml. A compilation of resources

• Reason's native Console.log
• ppx_deriving ’s [@@deriving show].
• refl , a ppx_deriving-like.
• lrt , another ppx_deriving-like.
• tpf , again a ppx_deriving-like.
• typerep , probably a ppx_deriving-like with ppx_typerep_conv.
• repr, which appears to have the user build the type representation manually from combinators in addition to also having the user pass it where needed.
• data-encoding, also fully manual.
• cmon , fully manual.
• dyn in Dune. It appears to also be fully manual.

Science and Technical Computing

• biocaml – OCaml Bioinformatics Library http://biocaml.org.
• bistro – OCaml library for building bioinformatics pipelines.
• onumerical – Numerical library for OCaml.
• tensorflow-ocaml – OCaml bindings for TensorFlow.

• Re – a pure OCaml regular expressions library with combinators, supporting several formats (glob, posix, str, etc.).
• ocaml-pcre – bindings to the PCRE library (perl-compatible regular expressions)
• Humane-re – Humane-re attempts to provide an easy interface for 90% of your regex needs. Courtesy of ocaml-re.

• ocaml-tls – TLS in pure OCaml.
• cryptokit – The Cryptokit library for OCaml provides a variety of cryptographic primitives that can be used to implement cryptographic protocols in security-sensitive applications.
• nocrypto – A small cryptographic library behind the ocaml-tls project. It is built to be straightforward to use, adhere to functional programming principles, and able to run in a Xen-based unikernel.

Note: The differences between nocrypto and cryptokit cryptographic libraries are described in the following blog post: OCaml-TLS: building the nocrypto library core.

• OCaml-RDF – OCaml library to manipulate RDF graphs and execute Sparql queries.

• atdgen — A serialization compiler for multiple languages (OCaml, Java, Python, Scala, Typescript) with a Binou or JSON format
• bencode — Bencode (.torrent file format) reader/writer.
• biniou – Extensible binary data format, like JSON but faster.
• cbor — OCaml native CBOR decoder/encoder.
• jsonm — Non-blocking streaming JSON codec for OCaml.
• xmlm — A streaming codec to decode and encode the XML data format.
• yojson — An optimized parsing and printing library for the JSON format.
• sexplib – A S-expression parser and printer

• Mirage OS – Mirage is a programming framework for constructing secure, high-performance network applications across a variety of cloud computing and mobile platforms.

• ocaml-fat – Read and write FAT-format filesystems from OCaml.

• ocaml-git – Pure OCaml low-level git bindings.

• ocaml-vchan – Pure OCaml implementation of the "vchan" shared-memory communication protocol.

• Embedded systems

  • OMicroB - A virtual machine designed to run OCaml bytecode on AVR (Arduino for instance) micro-controlers.
  • OCaPIC - An OCaml virtual machine for PIC18 micro-controlers.
  • ocaml-esp32 - A compiler for ESP32 SoC.

• Alcotest – A lightweight and colourful test framework.
• OUnit – OUnit is a unit test framework for OCaml. It allows one to easily create unit-tests for OCaml code. It is based on HUnit, a unit testing framework for Haskell.
• QCheck — QCheck is a property testing library inspired from Haskell's QuickCheck
• iTeML (formerly known as qtest) — supports inline pragma's to generate tests.
• Kaputt — comprehensive testing framework.
• Pa_test — General inline testing macro's.
• expect-test — A framework for writing tests in OCaml, similar to Cram, developed by JaneStreet.

• Frameworks:

  • Opium – Sinatra like web toolkit for OCaml.
  • Ocsigen Eliom – Eliom is a full-featured multi-tier framework, for developing multi-platform Web and mobile apps as 100% OCaml distributed applications. It can also be used for more traditional Web or mobile apps: Web sites, single page applications, REST API, etc.
  • Dream - Tidy Web framework for OCaml and ReasonML
  • webmachine – A REST toolkit for OCaml. OCaml webmachine is a layer on top of cohttp that implements a state-machine-based HTTP request processor. It's particularly well-suited for writing RESTful APIs. As the name suggests, this is an OCaml port of the webmachine project.
  • incr_dom - A library for building dynamic webapps, using Js_of_ocaml
  • fmlib_browser - a library which helps to write web applications which run in the browser in a pure functional style.
  • ocaml-vdom - Elm architecture and (V)DOM for OCaml

• Tools:

  • COW – Caml on the Web (COW) is a set of parsers and syntax extensions to let you manipulate HTML, CSS, XML, JSON and Markdown directly from OCaml code.
  • Ocamlnet
    has many relevant web libraries —
    Nethtml
    html parser,
    Netasn1
    for ASN.1 parsing,
    Netencoding
    for Base64, Quoted Printable, URL encoding and HTML escaping,
    Netmime
    for MIME processing, etc. See the list of
    modules
    in Ocamlnet's manual.
  • tyxml — Library to build valid (according to the W3C spec) Html and Svg trees.
  • js_of_ocaml – Js_of_ocaml is a compiler of OCaml bytecode to Javascript. It makes it possible to run Ocaml programs in a Web browser.
    • commonjs_of_ocaml - Easily import and export CommonJS modules from a js_of_ocaml project.
  • ReScript - ReScript is a robustly typed language that compiles to efficient and human-readable JavaScript.
  • ocaml-uri – RFC3986 URI parsing library.
  • Goji – An OCaml bindings generator for JavaScript libraries.
  • Syndic – RSS and Atom feed parsing
  • ocaml-mustache – mustache.js logic-less templates in OCaml.
  • atdjs – atd code generator (serialization) for OCaml/js_of_ocaml.
  • jingoo – OCaml template engine almost compatible with jinja2.
  • dispatch – Path-based dispatching for client- and server-side applications.
  • Lambda Soup - Functional HTML scraping and manipulation with CSS selectors, à la Python's Beautiful Soup.
  • Markup.ml - Error-recovering streaming HTML5 and XML parsers, serializers.
  • gen_js_api - gen_js_api aims at simplifying the creation of OCaml bindings for Javascript libraries.
  • routes - Typed routes for OCaml/ReasonML web applications.

• Open Source Projects:

  • Cumulus – Hacker news like website with the OCaml framework Ocsigen

Inspired by awesome projects line. Discover more awesomeness :sparkles:.