OCaml Coding Information

We are using OCaml version 4.12.0.

Installing OCaml 4.12.0 and associated tools

We require that you use the opam packaging system for installing OCaml and its extensions. Once you get opam installed and working, everything else should be easy to install .. so the only hard part is the first step.

  • For Linux or Mac see The OPAM install page for install instructions.
  • For Mac users, the above requires Homebrew (a package manager for Linux-ish libraries) so here is a more detailed suggestion of some copy/paste that should work.
    • Mac without homebrew installed:/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)" will install Homebrew
    • Mac with Homebrew (make sure you first do a brew update before this): brew install gpatch; brew install opam
  • You will then need to run some terminal commands to set up the basics:
    1. opam init to initialize OPAM;
    2. opam switch create 4.12.0 (this will take awhile) to build OCaml version 4.12.0 (the initial install is usually a slightly outdated version; also, if you already had an OPAM install you need to opam update before this switch to make sure OPAM is aware of the latest version);
    3. eval $(opam env) to let your shell know where the OPAM files are (use eval `opam env` instead if you are using zsh on a Mac); and
    4. Also add the very same line, eval $(opam env), to your.profile/.bashrc shell init file as you would need to do that in every new terminal window otherwise. (for .zshrc on macs, add line eval `opam env` instead)
  • Windows Windows Windows.. the OCaml toolchain is unfortunately not good in straight Windows.
    • If you are running a recent Windows install, we recommend installing WSL 2 which once you have set up will allow you to follow the Linux Ubuntu install instructions to get opam.
    • Note that your Ubuntu needs the C compiler and tools for the opam install to work; the following Linux shell command will get you those: sudo apt install make m4 gcc unzip.
    • More WSL2 for OCaml tips here.
    • Option 2 is to set up a Linux VM on your Windows box, and then set up a Linux install of OCaml within the VM. There are many good tutorials on how to build a Linux VM, here is one of them. Once your virtual Linux box is set up, you can follow the opam Linux install instructions.

Required OPAM Standard packages

Once you have opam and ocaml 4.12.0 installed, run the following opam command to install additional necessary packages for the class:

opam install merlin utop ppx_deriving core bisect_ppx ounit2 async ppx_deriving_yojson ocaml-lsp-server ocamlformat ocamlformat-rpc

Lastly, in order for the OCaml top loop to start up with some of these libraries already loaded, edit the file ~/.ocamlinit to add the lines below (note opam probably already created this file, just make sure the lines below are in it). The lines in this file are input to the top loop when it first starts. topfind really should be built-in, it allows you to load libraries. The require command is one thing topfind adds, here it is loading the Core libraries to replace the standard ones coming with OCaml. We will be using Core as they are improved versions.

#use "topfind";;
#require "core.top";;
open Core;;

OCaml Documentation

ocaml.org is the central repository of OCaml information.

The OCaml Manual

The OCaml manual is here.

  • We will cover most of the topics in Part I Chapters 1 and 2 from the manual.
  • Manual Chapter 7 is the language reference where you can look up details if needed.
  • We will be covering a few topics in the language extensions chapter:
  • Part III of the manual documents the tools, we will not be using much of this because third parties have improved on many of the tools and we will instead use those improved versions. See below in the Tools list where we give “our” list of tools.
  • Part IV describes the standard libraries; as with the tools we will primarily use Jane Street’s Base/Core which replaces these with more modern versions so we will generally be ignoring this Part.

Base and Core

Core is a complete rewrite of the standard libraries that come built in to OCaml. Think of it as a “more modern” version of lists, sets, hash tables, etc, with lots of little improvements in many places. Core is an extension of Base which is in fact what we will mainly be using.

  • Core documentation is not particularly readable as Core extends Core_kernel which in turn extends Base and most times you probably just want the Base version so I would suggest starting there.
  • Base Documentation - start here for most common data structures.
  • Core_kernel Documentation - occasionally you may need some of the extensions on Base here, but not very often.
  • The Real World OCaml book gives tutorial introductions to many of the Core/Base features.
  • Important note: if you use a search engine to look up e.g. “OCaml Set” to see how the OCaml Set module is defined you will likely not get the Core version and it can be very confusing. Even if you add Base or Core as keywords to the search you will usually get an outdated version. So, bookmark the above!

The FPSE OCaml Toolbox

Here are all the tools we will be using. You are required to have a build for which all these tools work, and the above opam one-liner should install them all.

  • opam is the package management system. See above for install and setup instructions.
  • ocamlc is the standalone compiler which we will be invoking via the dune build tool.
  • utop is the read/eval/print loop. It is a replacement for the original ocaml command, with many more features such as command history, replay, etc.
  • Base/Core was described above
  • ocamldoc is the documentation generator, turning code comments into documentation webpages similar to JavaDoc etc.
  • dune is the build tool (think make) that we will be using.
  • OUnit is the unit tester for OCaml. The opam package is called ounit2 for obscure reasons.
  • ppx_jane adds boilerplate functions to type definitions as well as many other macros. Unfortunately it is not documented, but [@@deriving equal, compare, sexp] for example will add equal and compare on a type, and to/from s-expression convertor functions.

The above tools will be our “bread and butter”, we will be using them on many assignments. There are also a few specialized tools used on some specific assignments.

  • Bisect will be used for code coverage.
  • base_quickcheck is a fuzz tester / automated test generator for OCaml.
  • Async is a non-preempting asychronous threads library.
  • Domains and Effects are another approach to coroutines and asynchronous programming
  • Multicore OCaml will be used for parallel programming.

Development Environments for OCaml

We recommend VSCode since it has OCaml-specific features such as syntax highlighting, auto-indent, and lint analysis to make the coding process much smoother.

Visual Studio Code

VSCode has very good OCaml support and is the “officially recommended editor”.

  • We have decided to switch the primary recommendation for OCaml support in VSCode to OCaml Platform. To install it, first run opam install ocaml-lsp-server from your shell. Then from the View menu select Extensions, then type in OCaml in the search box and this extension will show up: select OCaml Platform from the list.

  • (Deprecated this extension, the previous one is now preferred. If you have it installed just disable it within VSCode and install the above) Install the OCaml and Reason IDE extension to get syntax highlighting, type information, etc: from the View menu select Extensions, then type in OCaml in the search box and this extension will show up; install it.

  • You can easily run a utop shell from within VSCode, just open up a shell from the Terminal menu and type utop.

  • If you are on Windows and using WSL2, to run Visual Studio “in WSL2 space” so you get OCaml syntax highlighting and other nice features; see this blog post for how you can set it up.

Atom is unfortunately being slowly phased out after Microsoft bought Github. So, it is probably a good time to switch from Atom to VSCode if you have not already. To use Atom with OCaml install the atom and apm shell commands (see the Atom..Install Shell Commands menu option on Macs, or type shift-command-p(⇧⌘P) and then in the box type command Window: Install Shell Commands). With those commands installed, type into a terminal

    apm install language-ocaml linter ocaml-indent ocaml-merlin

to install the relevant OCaml packages. Here are some handy Atom keymaps for common operations these extensions support – add this to your .atom/keymap.cson file:

    'atom-text-editor[data-grammar="source ocaml"]':
      'ctrl-shift-t': 'ocaml-merlin:show-type'
      'alt-shift-r': 'ocaml-merlin:rename-variable'
      'ctrl-shift-l': 'linter:lint'
      'ctrl-alt-f': 'ocaml-indent:file'

linter:lint will refresh the lint data based on the latest compiled version of your code. In addition, control-space should auto-complete.

vim: If you use vim, my condolances as it is woefully behind the times in spite of many band-aids added over the years. Still, if you have been brainwashed to believe it is good, type shell commands opam install user-setup and opam user-setup install after doing the above default opam install to set up syntax highlighting, tab completion, displaying types, etc. See here for some dense documentation.

emacs: See vim. Confession: I still use emacs a bit but am trying to wean myself. 35-year-old habits die hard. Note you will need to also opam install tuareg to get emacs to work, and follow the instructions the install prints out.


  • The Real World OCaml book has a fairly good overlap with what we will cover, and can be used as a supplementary resource.
    • It documents many of the extensions we will be using, Base/Core libraries in particular
  • Cornell cs3110 book is the online text for a somewhat-related course at Cornell.

Coding Style

  • The FPSE Style Guide is the standard we will adhere to in the class; it follows general best practices for modern OCaml. It will be expected of your code from Assignment 2 on.

Example Worked Exercises

One of the best ways to learn to write elegant OCaml is to study well-written OCaml code.