OCaml libraries

Web

• Hyper - OCaml Web client, composable with Dream.
• h2 - HTTP/2 implementation written in pure OCaml.
• Opium - Sinatra like web toolkit for OCaml.
• OCaml On Ice - Web framework for OCaml in the style of Ruby on Rails.
• http/af - High performance, memory efficient, and scalable web server written in OCaml.
• Eliom - Framework for building client/server Web and mobile applications in OCaml.
• ocaml-wasmtime - OCaml WebAssembly runtime powered by wasmtime.
• Dream - Tidy, feature-complete Web framework. (Docs) (HN) (Example App)
• OCaml JavaScript bindings
• Tiny_httpd - Minimal HTTP server using good old threads + blocking IO, with small request router.
• ocaml-cohttp - OCaml library for HTTP clients and servers using Lwt or Async.
• Async_http - Simple HTTP server for async applications.
• Pollinate - Platform agnostic OCaml library for P2P communications using UDP and Bin_prot.

Test

• Alcotest - Lightweight and colorful test framework.
• Mutaml - Mutation Tester for OCaml.
• Crowbar - Property fuzzing for OCaml.
• qtest - Inline (Unit) Tests for OCaml.
• QCheck - Property-based testing for OCaml.
• Popper - Property-based testing at ease.
• expect-test - Framework for writing tests in OCaml, similar to Cram.

Science

• Owl - OCaml library for scientific and engineering computing. (HN) (Web)

ML

• zeta - Functional neural networks in ocaml. (HN)
• prbnmcn-clustering - Clustering with k-means, k-medoids & agglomerative clustering.

DB

• Caqti - Cooperative-threaded access to relational data.
• PostgreSQL-OCaml - PostgreSQL Bindings for OCaml.
• postgres-protocol - IO independent postgres protocol implementation.
• chamelon - Small, simple block-backed key-value store for MirageOS.
• ocaml_sql_query - Functional-style SQLite queries for OCaml.
• epiSQL - SQL schema parser and code generator.
• PGX - Pure OCaml PostgreSQL client library.
• Sequoia - Type-safe query builder for OCaml.
• rel - Relational database programming for OCaml.

Bindings

• Python 3.5+ bindings for OCaml

CLI

• Cmdliner - Declarative definition of command line interfaces for OCaml. (Web) (Cmdliner cheatsheet)
• Notty - Declarative terminal graphics for OCaml.
• Progress - User-definable progress bars for OCaml.

Error

• ppx_let_locs - Improving stack trace for let monadic.
• pp_loc - Provides support to additionally quote and highlight the input fragment the input that corresponds to a location (or a set of locations).
• ppx_assert - Assert-like extension nodes that raise useful errors on failure.

Graphics

• Vg - Declarative 2D vector graphics for OCaml.
• unicorn - Purely functional GUI algebra.
• ocaml-orx - OCaml bindings to the Orx game library.
• chapgame - Physics engine based on a priori collision detection approach.

Linux

• ocaml-uring - Bindings to io_uring for OCaml.

Crypto

• Cryptokit - Library of cryptographic primitives (ciphers, hashes, etc) for OCaml.

Other

• OCamlbuild - Generic build tool, that has built-in rules for building OCaml library and programs.
• ocaml-vdom - Elm architecture and (V)DOM for OCaml.
• Lwt - OCaml promises and concurrent I/O.
• higher - Higher-kinded programming in OCaml.
• GraphQL Servers in OCaml - Create GraphQL servers in OCaml.
• piaf - Client library for HTTP/1.X / HTTP/2 written entirely in OCaml.
• OCamlgraph - OCaml graph library.
• luv - Minimalist binding to libuv from OCaml and Reason.
• Nice Parser - Nice parsers in OCaml without the boilerplate.
• QCSTM - simple state-machine framework for OCaml based on QCheck.
• deriving - Library simplifying type-driven code generation on OCaml.
• OCamline - Command line interface for user input.
• OCaml Batteries Included - Community-maintained foundation library for your OCaml projects.
• incremental - Library for incremental computations.
• Landmarks - Simple Profiling Library for OCaml.
• pfff - Set of tools and APIs to perform static analysis, code visualizations, code navigations, or style-preserving source-to-source transformations such as refactorings on source code.
• ppx_stage - Staged metaprogramming in stock OCaml.
• Streaming - Fast, safe and composable streaming abstractions. (Announcement)
• cameleer - Deductive Verification Tool for OCaml Programs.
• Ppxlib - Base library and tools for ppx rewriters.
• jsoo-react - Bindings to React for js_of_ocaml, including JSX ppx. Allows to use React from OCaml.
• OByteLib - OCaml bytecode library tools. Useful to read, write and evaluate OCaml bytecode files.
• bimage - Composable image processing library for OCaml.
• Lwd - Lets you build values that changes over time.
• Digestif - Hash algorithms in C and OCaml.
• directories - OCaml library that provides configuration, cache and data paths.
• Repr - Run-time representations of OCaml types.
• OCaml-MPST - Communication library powered by Multiparty Session Types (abbreviated as MPST) in OCaml.
• eqaf - Constant time equal function.
• Pp - Pretty printing library.
• capnp-ocaml - OCaml code generator plugin for the Cap'n Proto serialization framework.
• TyXML - Build valid HTML and SVG documents.
• parany - Parallelize any kind of computation.
• Metrics - Infrastructure to collect metrics from OCaml applications.
• ocaml-mustache - Mustache.js logic-less templates in OCaml.
• ppx_optcomp - Optional compilation for OCaml.
• Session-ocaml - Implementation of binary session types in OCaml.
• brr - Browser programming toolkit for OCaml.
• Dyntype - Dynamic types for OCaml.
• RML - ML modules and functors as first-class citizens.
• VOCaL - Verified OCaml Library.
• Fmt - OCaml Format pretty-printer combinators.
• awa-ssh - Purely functional SSH library in OCaml.
• Fmlib - Functional Monadic Library for Ocaml.
• eioio - Effects-based direct-style IO for multicore OCaml. (0.1 release)
• RES - OCaml library for resizable arrays and strings.
• Preface - Opinionated library designed to facilitate the handling of recurring functional programming idioms in OCaml.
• OCaml Containers - Modular, clean and powerful extension of the OCaml standard library.
• vec - Fast, safe mutable dynamic arrays for OCaml.
• um-abt - OCaml library implementing abstract binding trees (ABTs).
• ocaml-stun - Pure OCaml implementation of the Session Traversal Utilities for NAT (STUN) protocol (STUN).
• ortac - OCaml Runtime Assertion Checking.
• Bloomf - Efficient Bloom filters for OCaml.
• Index - Scalable implementation of persistent indices in OCaml.
• ppx_effects - Syntax extensions for untyped effects in OCaml 5.
• kv-hash - Key Value store using a persistent hashtable approach.
• DAI - Demanded Abstract Interpretation.
• field_repr - Efficient run-time representations of OCaml record fields.
• CacheCache - Efficient caching library for OCaml.
• ocaml-extlib - Extended Standard Library for OCaml.
• MParser - Simple monadic parser combinator library for OCaml.
• futurama - Finite permutation library with an optimizing compiler.
• algebra - Definitions for common algebraic structures.
• ocaml-lp - Helps the modeling of Linear Programming (LP) and Mixed Integer Programming (MIP) in OCaml.
• ppx_regexp - Matching Regular Expressions with OCaml Patterns.
• Shuttle - Buffered channels for async.
• Poll - Portable OCaml API to poll file descriptors for IO readiness. Supports kqueue and epoll for now.
• ART - Adaptive Radix Tree in OCaml.
• Avro - Runtime library and schema compiler for the Avro serialization format.
• CMON - Library for printing OCaml values with sharing.
• Equinoxe - Equinix API client library for OCaml.
• ocaml_plugin - Automatically build and dynlink OCaml source files.
• Lwt Pipe - Multi-consumer, multi-producers blocking queue and stream for Lwt.
• sedlex - Unicode-friendly lexer generator for OCaml.
• Compact - Memory-efficient data structures in OCaml.
• PaComb - Parsing library that compiles grammars to combinators using elimination of left recursion.
• PPrint - Pretty-Printing Toolbox.
• Multicont - Continuations with multi-shot semantics in OCaml.
• ExtUnix - OCaml bindings to various low-level system APIs not covered by Unix module.
• ocaml-arrow - OCaml bindings for Apache Arrow using the C++ api.
• ocaml-in-python - Effortless Python bindings for OCaml modules.
• Yojson - Low-level JSON parsing and pretty-printing library for OCaml.
• ATD - Static types for JSON APIs.
• Biocaml - OCaml Bioinformatics Library.
• Iter - Simple iterator abstract datatype, intended to iterate efficiently on collections while performing some transformations.
• sosa - Sane OCaml String API.
• ocaml-stdint - Various signed and unsigned integers for OCaml.
• Current_incr - Self-adjusting computations.
• hlist - Heterogeneously-typed lists for OCaml.
• ppx_import - Syntax extension that allows to pull in types or signatures from other compiled interface files.
• Prime - OCaml Library Supplements.
• ADT rewriting - Representing ADTs in a flat way and compiling a pattern match-like construction as a rewrite on the memory representation.
• AlphaRegex - Synthesizer for regular expressions.
• py.ml - OCaml bindings for Python.
• Affect - Composable concurrency primitives with OCaml effects handlers.
• Depyt - Type combinator library.
• ocaml-websocket - Websocket library for OCaml.
• ORaft - Library of Raft consensus algorithm implemented in OCaml.
• ocaml-mpsc - Lockless Multiple Producer Single Consumer Queue for ocaml-multicore.
• Charset - Fast char sets.
• OCaml gRPC
• lockfree - Lock-free data structures for multicore OCaml.
• Backward Lists - Defines backward lists that are isomorphic to lists.
• bistro - Library to build and execute typed scientific workflows.
• domainslib - Parallel Programming over Multicore OCaml.
• Conformist - Schema definition and validation with support for decoding to bridge the gap between runtime types and static types.
• ocaml-crunch - Convert a filesystem into a static OCaml module.
• Ocaml protoc plugin
• ocaml-rust - Safe OCaml-Rust Foreign Function Interface.
• shared-block-ring - Simple on-disk fixed length queue.
• ocaml-protoc - Protobuf Compiler for OCaml.
• Diskuv Box - Basic, cross-platform set of commands to manipulate and query the file system.
• raylib-ocaml - OCaml bindings for raylib, a simple and easy-to-use library to enjoy video games programming.
• ocplib-json-typed - Libraries for reliable manipulation JSON objects.
• ocaml-alg - Algebraic structures in OCaml.
• Kai - Library for handling compiler diagnostics and span-associated metadata.
• midi - Experimenting with midi in OCaml.
• gen_js_api - Easy OCaml bindings for JavaScript libraries.
• metapp - Meta-preprocessor for OCaml.
• Ego - EGraphs in OCaml.
• Bindoj - Generative datatype toolkit for OCaml centric workflows.

Links

• OPAM packages
• dune-deps - Show the internal dependencies in your OCaml/Reason/Dune project.
• dune-release - Tool to streamline the release of Dune packages in opam.
• opam-dune-lint - Ensure dune and opam dependencies are consistent.
• Sherlocode - Search across 17 million lines of OCaml available on opam using grep. (Code)
• Camlidl - Stub code generator for OCaml/C interface.
• Reusable Effect-Based Components
• opam-nix - Turn opam-based OCaml projects into Nix derivations.
• Esperanto - Build-once run-anywhere OCaml programs.
• opam-ci - OCurrent pipeline for testing submissions to opam-repository.
• opam2nix - Generate nix expressions from opam packages.
• Networking experiments with OCaml 5's effects
• metaquot - OCaml syntax extension for quoting code.
• Effectful OCaml with Objects and Variants