Hello World is the first program written by developers to learn programming language.

It is easy and quick to code a language in Ocaml.

This post covers

  • Writing a simple hello world program
  • Print the string to the console
  • Compile and execute a program using the command line.

How to write a Hello World program in OCaml?

  • Open any editor
  • Create a file called HelloWorld.ml, where ml is an extension for OCaml code.
  • Add the below lines of code
(*  HelloWorld*)
(* Simple first OCaml example
** Hello world program print to console
 print_string "Hello world Welcome to OCaml programming\n"

Comments are single-line or block/multi-line comments.

Single lines are started with (*, followed by single-line text and ends with *) Multi-line comments span in multiple lines, starting with (* and ending with *) and each line of text starts with *.

The print_string function prints the string to standard output.

Unlike other programming languages, There is no main function in OCaml

How to compile and execute the program.

There are multiple ways to compile and execute a program.

The compiler compiles program code to bytecode or native code

  • ocamlopt tool compiles the OCaml to native code

  • ocamlc compiles the code to byte code

  • One way using the ocamlopt tool The ocamlopt tool compiles the ocaml code into native code.

ocamlopt -o hello HelloWorld.ml

It generates the following files

hello.cmx hello.o Next, Execute native binary code as given below.

 Hello world Welcome to OCaml programming.

The second way using ocamlc.

ocamlc is an ocaml compiler also called byte code compiler. It converts code into binary code.

ocamlc -o hello HelloWorld.ml

Next, run binary code using give below.

 Hello world Welcome to OCaml programming.


You can use the OCaml tool to run the program without compiling the code to binary form.

ocaml HelloWorld.ml