A function is a block. It can take arguments, and it returns a something. Let’s see an example to understand how it works:
let multiply a b =
a * b;;
let n = multiply 3 4;; (* n = 12 *)
In this example, we create a function called multiply, with two arguments, a and b. Then, after the =, we write instructions to tell to our function what it does.
There is no return keyword in OCaml: the last thing on the stack is automatically returned by the function (and if the stack is empty then unit is returned). In this example, the last thing on the stack is a * b, so the product of both arguments is returned.
Finally, we can call a function by using its name, and passing arguments after the name (without paranthesis).