OCaml (very) Briefly

• Operators
• Variables and Functions

In a my previous post we installed OCaml. I was pedantic about it, so it wasn’t as easy as just using apt or dnf. Now that we have it installed and working, let’s take a look at OCaml and try to get comfortable with the basics.

Operators

First, let’s look at basic, elementary OCaml. Let’s start with the usual. The operators.

# 1 + 1;;
- : int = 2

# 1 - 1;;
- : int = 0

# 4 / 2;;
- : int = 2

# 4 * 2;;
- : int = 8

So far so good! We can add, substract, divide, and multiply integers. Let’s throw in some floats.

# 3.14 + 5;;
Error: This expression has type float/2 but an expression was expected of type
         int
       File "_none_", line 1:
         Definition of type float/2

Uh oh. We broke it. I do not really understand that error message. I do not know why the type is float divided by 2. It also tells me “Definition of type float/2” but doesn’t actually show me the definition. Maybe I’m misunderstanding what it is trying to tell me?

Anyway, we can fix this using two more mandatory periods.

# 3.14 +. 5.;;
- : float = 8.14

Note the period after the + sign and after 5. Let’s try some variables. The reason for these variables is due to OCaml being functional. Instead of overloading the plus sign to work for integers and floats, they instead have a seperate +. to work floats.

Variables and Functions

# let x = 1 + 2;;
val x : int = 3

Nice, so we created a variable x and set it’s value to 3. There are of course rules about what variables names can be. For example, they cannot start with a capital letter or a number. Every language has little rules like this, so I am going to skip that for now. Rules like that have never been a problem for me, so I am not worried about it at this point.

Let’s make some functions.

# let square x = x * x;;
val square : int -> int = <fun>

# square 3;;
- : int = 9

# let add x y = x + y;;
val add : int -> int -> int = <fun>

# add 3 4;;
- : int = 7

Some things of note. You do not have to have paraenthesis around your function arguments. Now that we have made some variables and functions, the return statement from the compiler has changed. It now reads val ___ instead of just a -. It also prints the types of the input and the output of the function. In our case it’s always int.