DRAFT: I’m still learning Ocaml myself, so some of what I say here may be plain wrong or misguided. I’m relying on those more experienced with Ocaml to correct my mistakes! If you have plenty of Ocaml experience, please correct my mistakes. By all means, be brutal!

Welcome to the very first part of my (hopefully enlightening) series of tutorials on the Ocaml programming language! In this tutorial we’ll be learning the Ocaml flavour of everybody’s favourite program: Hello World.

Since those interested in Ocaml will likely be coming from such a background, please note that this tutorial is written under the assumption that readers will have experience with other, imperative languages such as Java or C . Now, on with the code:

let main () =
print_endline "Hello, World!" ;;

main () ;;

Save this file as tmhoo01.ml. You can run this program from the command-line using the following:

$ ocaml tmhoo01.ml

Or compile it to native code:

$ ocamlopt -o tmhoo01 tmhoo01.ml

Predictably, running this program displays “Hello, World!” in the console window. It’s not the output we’re interested in, however: it’s Ocaml’s weird ass syntax! Let’s take this line by line:

let main () =

This declares a function called “main”. let is an Ocaml keyword which defines named expressions (sometimes called let-bindings). In this case, the name of our expression/let-binding is “main”.

main takes a single parameter: the parentheses () represent what is known as the “unit value”. It is the only possible value for the unit type, and has a similar use and meaning to what void has in C/C . In this case, it means the function main accepts no other parameters. This is necessary because Ocaml functions must always be applied to one or more parameters: thus, when we have no parameters to pass we must resort to using (). If we do not accept this unit parameter at a minimum, the expression in main is evaluated at the next double semi-colon ;;. This is not our intention.

  print_endline "Hello, World!" ;;

The print_endline call does as you would expect. Similar to System.out.println or printf. print_endline returns the unit value. Implicitly, our main function has a unit type. This distinction may not completely make sense just yet, but the importance of this will make sense later once we start doing more work with different types.

The ;; keyword is used to separate multiple top-level constructs (e.g. let-bindings and class definitions). Later, you will use ; to separate expressions within other constructs.

main () ;;

As you would expect, this calls our main function.

So that’s our first Ocaml program dissected. Thanks for reading! If you have any questions or comments, please post them here: I’ll surely be revising this article based on your recommendations.

UPDATE 1: Correction to the descriptions of ; and ;;. Thanks Paul!
UPDATE 2: Removed my haughty claim that Ocaml references are more like C pointers than Java references. Cheers Chris!
UPDATE 3: Tried to otherwise simplify the tutorial. Less “blah blah blah”!
UPDATE 4: Remove introduction to references all together. Several people felt it was out of place to introduce a concept like references in the first tutorial. I happen to agree. :)