This tutorial originally appeared here and is slightly modified in this form. It is intended to get you up and running quickly with OCaml.
Crash course on the OCaml ecosystem
These are some key notes that you should know.
opamis the package manager for OCaml. It is very advanced and supports many features. The most basic of which is
$ opam install <some_package>
For people on
OS X, you can get it on
brew and all the
distros should have
opam. Attention Ubuntu people: if you are using
an outdated version of Ubuntu (e.g. Wily, Vivid or Trusty), you should
do this instead because apt-get's version of opam on those is madly
$ add-apt-repository ppa:avsm/ppa $ apt-get update $ apt-get install ocaml opam
For Windows people, this seems to be a decent option or you could get a VM. Do note that opam doesn't work on this platform and for a beginner you might ending up wasting a lot of time with environment issues or libraries that assume Unix.
- Once you have
opaminstalled, you probably want to do:
$ opam switch 4.02.3
This will install the latest version of the compiler. When you did
opam switch you can see all the other available compilers as
well. You can even switch to a beta compiler like so:
opam switch 4.03.0+beta1
ocamlfindis a program that predates
opamand wraps the standard
ocamlopt. The former is a byte code compiler and the latter creates native code.
ocamlbuildis a tool that helps build
OCamlprograms, many people have strong opinions on it. You can find the manual for it here. It seems that people are starting to invest effort into it again.
oasisis a tool that helps abstract usage of 3, 4. I resisted it for a while and wrote Makefiles instead, don't do that, just use oasis. The oasis flow basically goes like this: (Be aware that oasis is really finicky and its error messages are useless), see the oasis minitutorial at the end of this post.
merlinis a OCaml program that is simply amazing it drives code completion for plugins available in
vim. Once you have merlin installed with
$ opam install merlin
then you can add a
.merlin file to your project so that
knows what packages to code complete for, a sample
looks like this:
B _build/src S src PKG cmdliner lwt FLG -w +a-4-40..42-44-45-48
Notice how I put the
B _build/src That sort of assumes you're using
_oasis and you made the a
src directory. I also provided you with
some nice compiler flags for extra warnings.
You'll need to add some code to vim or emacs to truly get the most out
merlin, you can even get
jedi style docstring popups like so:
The elisp that I use for my
init.el is listed after the
tutorial at the end of this post.
There are no full blown IDEs for OCaml, learn
Sublime Texthas a merlin plugin, if you're already familiar with Sublime Text then just stick with it, merlin is really what matters here.
utopis an enhanced repl, its better than the plain
ocamlrepl. Install it with
opam install utop
OCaml does have a standard library but it is deliberately minimal. It was only created
to serve the needs of the compiler programmers, ie its not like
Python's standard library which has everything under the sun + the
moon. There are a few standard library replacements, one is called
Core and its provided by Jane Street. Its the library used in the
Real World OCaml book/website. Another standard library
replacement is called
Batteries, this is more "community"
supported. There is a more recent contender called
Containers. For a
categorized list of contemporary and well liked/must have libraries,
checkout the awesome-ocaml
Speaking of Libraries...
This is "functional programming," so many of the real world libraries
you'll encounter will have Monadic interfaces, like
lwt or Core's
async, both are asynchronous threading libraries, use Monads and
>>= function. But you really shouldn't fret about what a
Monad is or represents, just follow the type signature and you'll be
fine. For a more detailed treatment of Monads in OCaml and a code
example to talk to the
Stripe API, see
Doing simple tasks (shameless plug)
I try using
OCaml for literally everything and that includes going
to hackathons, to make this less painful I wrote a library called
Podge which helps with simple stuff. I don't claim its a standard
library replacement, just a library for getting stuff done. These two
code samples assume the file is named
code.ml and can be run with
First install with opam:
$ opam install podge
- Reading output of a process
#require "podge" let () = Podge.Unix.read_process_output "ls -halt" |> List.iter print_endline
|> just means piping, its piping the output of
read_process_output into the input of the partially applied function
- Reading a file
#require "podge" let () = Podge.Unix.read_lines "code.ml" |> List.iter print_endline
Similar to 1, this reads all lines of file and gives it to the input
of the partially applied function
These are two simple code samples from
Podge, check out the
repo for other useful modules
like: (The README has code examples)
Websimple HTTP requests and getting data back as JSON
Xmlquerying simple XML documents
ANSITerminalfor creating colored shell output
StringExtwhich is all due to Rudi Grinberg
What can you do with it?
Loads, warning shameless plugs ahead.
- I wrote a opam package that makes it easy to get an
iOSOCaml cross-compiler, see here.
- Compilers!, lots of compilers/compiler tools are written in OCaml: Facebook uses OCaml for pfff and flow and the first cut of Rust was written in OCaml.
- Financial world, Jane Street uses OCaml for basically everything (AFAIK)
- Systems Programming: ahrefs, my former employer Ahrefs uses OCaml for heavy systems programming.
- Kernels: Unikernels are hot right now, the most prominent one is the Mirage-OS project and its all OCaml.
- Shameless plug: I use OCaml as well for
js_of_ocaml, in fact I'm using it to write an Electron app with a node backend (All code is OCaml compiled into JS, then run on node/Electron), see here.
- Genomics/Bioinformatics: Hammer Lab in NYC uses OCaml for their genomics/sequencing work.
- My employer MixRank let me write
sshtunnel multiplexer for jailbroken iDevices called gandalf
- You can even replace your shell scripts or Python with it, aka run it using the interpreter, see the self contained example listed after the elisp code at the end.
Stick with it!
This style of coding might be new to you or maybe its your first
programming language, stick with it and continue.
OCaml offers many
awesome features and has many strengths including a very professional
and pragmatic community. Also, if you're in the Bay Area then please
come to weekly office hours hours
hosted at MixRank in San Francisco. Its open to all levels of
experience and I still have some Enter the Monad tshirts to give away
courtesy of Jane Street.
Create a directory.
Go to the directory and create a file named _oasis and directory named
Here is a template of the contents of the _oasis file
OASISFormat: 0.4 OCamlVersion: >= 4.02.3 Name: opam_package_name Version: 0.1 Maintainers: New OCaml programmer Homepage: http://my_coolsite.com Synopsis: Some short description Authors: Cool@me.com License: BSD-3-clause Plugins: META (0.4), DevFiles (0.4) AlphaFeatures: ocamlbuild_more_args Description: Some cool description # This is a comment and this below creates an binary program Executable <some_program_name> Path: src BuildTools:ocamlbuild install: true MainIs: main.ml CompiledObject: native BuildDepends: package_one, package_two # Another comment, this builds a library called pg Library pg Path: src # oasis will figure out the dependencies, # Just list the modules you want public, # Note that there's no .ml, just give the name Modules: Pg CompiledObject: byte BuildDepends: some_package
- Generate the Makefile, setup.ml, configure and other build crap.
$ oasis setup -setup-update dynamic
- Actually build your code, yes its just a call to make.
Assuming that you were building an executable, then you should see
foo.native or a
foo.byte in the root directory of the
- You can stop here, but you can go even further with
oasis2opam. Install it with:
opam install oasis2opam
then in your project's root directory, aka the directory with the _oasis file, do:
This creates the
opam directory and some meta data for the opam
packaging system. Your local package can now be a first class citizen
with opam just by doing this in the same project root directory:
$ opam pin add <your_package_name> . -y
Elisp for OCaml coding
Code for the Emacs init file.
;; OCaml code (add-hook 'tuareg-mode-hook (lambda () ;; Add opam emacs directory to the load-path (setq opam-share (substring (shell-command-to-string "opam config var share 2> /dev/null") 0 -1)) (add-to-list 'load-path (concat opam-share "/emacs/site-lisp")) ;; Load merlin-mode (require 'merlin) ;; Start merlin on ocaml files (add-hook 'tuareg-mode-hook 'merlin-mode t) (add-hook 'caml-mode-hook 'merlin-mode t) ;; Enable auto-complete (setq merlin-use-auto-complete-mode 'easy) ;; Use opam switch to lookup ocamlmerlin binary (setq merlin-command 'opam) (company-mode) (require 'ocp-indent) (autoload 'utop-minor-mode "utop" "Minor mode for utop" t) (autoload 'utop-setup-ocaml-buffer "utop" "Toplevel for OCaml" t) (autoload 'merlin-mode "merlin" "Merlin mode" t) (utop-minor-mode) (company-quickhelp-mode) ;; Important to note that setq-local is a macro and it needs to be ;; separate calls, not like setq (setq-local merlin-completion-with-doc t) (setq-local indent-tabs-mode nil) (setq-local show-trailing-whitespace t) (setq-local indent-line-function 'ocp-indent-line) (setq-local indent-region-function 'ocp-indent-region) (if (equal system-type 'darwin) (load-file "/Users/Edgar/.opam/working/share/emacs/site-lisp/ocp-indent.el") (load-file "/home/gar/.opam/working/share/emacs/site-lisp/ocp-indent.el")) (merlin-mode))) (add-hook 'utop-mode-hook (lambda () (set-process-query-on-exit-flag (get-process "utop") nil)))
OCaml as shell scripting, example assume gandalf
You can do
chmod +x on the script, then can just invoke it as a
regular program, no compiling necessary.
#!/usr/bin/env ocaml (* Need topfind to make require work, need require to use podge package *) #use "topfind" #require "podge" module U = Yojson.Basic.Util module A = Podge.ANSITerminal type cmd = Scp of int * string | Ssh of int * string type error_condition = Copy_error | Exec_error let connected_devices () = Podge.Unix.read_process_output "gandalf -s" |> Podge.List.drop ~n:5 |> String.concat "" |> Yojson.Basic.from_string |> U.to_list |> List.fold_left (fun accum item -> U.(member "Local Port" item |> to_int) :: accum)  let command = function | Scp (port, target) -> Printf.sprintf "scp -P %d %s root@localhost:~/" port target | Ssh (port, cmd) -> Printf.sprintf "ssh root@localhost -p %d \"%s\"" port cmd let usage () = "This deploy script assumes that it was started\n\ from the makefile with `make deploy` and that gandalf is running" |> print_endline; exit 1 let with_gandalf udid ~f = let f_name = Filename.temp_file "gandalf" "deployment" in let f_chan = open_out f_name in Printf.sprintf "%s:2000:22" udid |> output_string f_chan; close_out f_chan; let gandalf_pid = Unix.(create_process "gandalf" [|"gandalf"; "-m"; f_name|] stdin stdout stderr) in Unix.sleep 1; f (); Unix.kill gandalf_pid 5 let cmd_result ~error_msg = function | outcome when outcome <> 0 -> A.colored_message ~m_color:Podge.T.Red error_msg |> prerr_endline; exit 1 | _ -> () let () = if Array.length Sys.argv <> 3 then usage () else with_gandalf Sys.argv.(2) begin fun () -> let devices = connected_devices () in devices |> List.iter begin fun i_device -> let scp_cmd = Sys.(Scp (i_device, argv.(1))) |> command in Printf.sprintf "Deploying binary to remote device: %s" scp_cmd |> A.colored_message |> print_endline; Sys.command scp_cmd |> cmd_result ~error_msg:"Was unable to copy over the binary"; let ssh_cmd_sign = Sys.(Ssh (i_device, "ldid -S /var/root/" ^ argv.(1))) |> command in Printf.sprintf "Signing binary on remote device: %s" ssh_cmd_sign |> A.colored_message |> print_endline; Sys.command ssh_cmd_sign |> cmd_result ~error_msg:(Printf.sprintf "Signing binary on remote device: %s" ssh_cmd_sign); let ssh_cmd = Sys.(Ssh (i_device, "/var/root/" ^ argv.(1))) |> command in Printf.sprintf "Executing binary on remote device: %s" ssh_cmd |> print_endline; Sys.command ssh_cmd |> cmd_result ~error_msg:"Was unable to execute the binary"; A.colored_message "Deployed and tested successfully" |> print_endline end end