OCAML Course


OCaml is a functional language, from the family of Meta Languages (".ml", along with SML or CAML). Unfortunately, OCaml is impure, as it allows side effects. It's strongly advised to read the course about Functional programming before starting, so you know about immutable, purity, and things like this. We are currently in OCaml 4.12.

We are using a lot OCaml in my school because a lot of students learned it before, but it would be better to learn functional programming with a language like Haskell 🙄, as there are no side effects in it.


Like a lot of languages such as Python or Java (👀), you are able to write OCaml in a console (REPL). Otherwise, you can also write a file.ml and compile it like you would in C. You can use

  • ocaml: to open OCaml console
  • ocamlc file.ml: to compile a OCaml file and create a a.out (this is working like gcc in C).

As the installation is hard on Windows, I gathered tutorials here to install OCaml on Windows. On Linux, you should not have a problem. You can also use OCaml in your browser with TryOCamlPro (I through this was a good solution, but it made me write poor code because I was adding ;; or parenthesis in a lot of unneeded places).

Functional languages

You must declare a variable with let. You can't modify a variable, you will have to create a new one (=immutable ✨).

let x = 5
x = 6 (* NOT POSSIBLE *)
let x = 6 (* delete the old x, create a new one *)

In functional languages, there should be no side effects. Hence, if you are not storing something in a variable, the compiler won't evaluate this line, as you are not using the result of the function (=purity ✨). The variable starting by a _ are not stored by the compiler, so you can print a value like this

Printf.printf "%s\n" "Hello, World" (* SKIPPED !!! *)

(* all of them are equivalent *)
let _ = Printf.printf "%s\n" "Hello, World"
let _ = Format.printf "%s\n" "Hello, World"
let _ = Format.printf "%s@." "Hello, World" (* my teacher and mine *)

(* in OCaml, 1+2 = 3 parameters '1' '+' '2', so add parenthesis *)
let _ = Format.printf "%d@." (1+2)
(* same *)
let _ = Format.printf "%d@." (-1)

To be accurate, in OCaml everything is a value, so you can give - (minus) to a function, as for OCaml - is a function taking two integers and returning one integer. A function taking another function is argument is called a higher-order function ✨ (fonction d'ordre supérieur).


  • you need to add ;; at the end of every expression
  • you do not need to use printf/store expressions in variables

Basic concepts

You might challenge yourself with these exercises

Intermediary concepts

And here are some exercises

Advanced concepts

Data structures

We are testing different ways of storing of big amount of data, along with their efficiency.


French course