# My first day with OCaml

## Variables

So, to declare variables we can use let keyword. For example:

let x = 1;;

We can also use ;; to separate expressions. For example:

let x = 1;;
let y = 2;;

We can do something like this:

let x = 50 in x * x;;

Obs: It will return 2500. But x is not defined outside of in block. So, we can’t use x outside of in block. If you try to print out x value, you will have an error.

## Functions

We can define functions with let keyword. For example:

let square x = x * x;;

Anonymous functions are defined with fun keyword. Example:

map (fun x -> x * 2) [1; 2; 3];;

### Recursion

We can do recursion with let rec keyword. For example:

let rec range a b =
if a > b then []
else a :: range (a + 1) b;;

Obs: The difference between let and let rec is that if we have used let we couldn’t use range function inside of range function, we would have an error.

## Float x Int

OCaml needs you to explicitly convert int to float and vice versa. For example:

let x = 1;;
let y = 2.0;;
let z = float_of_int x +. y;;

Obs: The +. operator is used to sum float numbers. If you try to sum int numbers with +. operator, you will have an error. The same for the otherwise case.

## Pattern Matching

We can use the match and with keywords to do pattern matching. For example:

let rec factorial n =
match n with
| 0 | 1 -> 1
| _ -> n * factorial (n - 1);;

Obs: The _ is used to match any value. We could replace the last line with | x -> x * factorial (x - 1). It would be the same. The x is a variable that receives the value of n in the case. Of course x doesn’t exist outside of the the case.

We can also simplify the pattern matching by using the function keyword. For example:

let rec factorial = function
| 0 | 1 -> 1
| n -> n * factorial (n - 1);;

Obs: The function keyword introduces pattern matching directly. The parameter is not named, but we can use it in the cases (hmm).

## Lists

Lists are ordered collections of elements of the same type. For example:

# [];;
- : 'a list = []
# [1; 2; 3];;
- : int list = [1; 2; 3]
# [false; false; true];;
- : bool list = [false; false; true]
# [[1; 2]; [3; 4]; [5; 6]];;
- : int list list = [[1; 2]; [3; 4]; [5; 6]]

The :: (cons) operator adds an element to the beginning of a list. For example:

# 1 :: [];;
- : int list = [1]
# 2 :: [1];;
- : int list = [2; 1]

The @ (append) operator concatenates two lists. For example:

# [1; 2] @ [3; 4];;
- : int list = [1; 2; 3; 4]

Non-empty lists have a head (it’s first element) and a tail (the list of the rest of the elements). For example:

# let x = [1; 2; 3];;
val x : int list = [1; 2; 3]
# List.hd x;;
- : int = 1
# List.tl x;;
- : int list = [2; 3]

We can create functions that operates over lists using pattern matching. For example:

# let rec total l =
match l with
| [] -> 0
| h :: t -> h + total t;;
val total : int list -> int = <fun>
# total [1; 3; 5; 3; 1];;
- : int = 13

Obs: The h is the head and the t is the tail. We use the :: operator to desconstruct here.

Another example:

let rec length l =
match l with
| [] -> 0
| _ :: t -> 1 + length t;;

Obs: This function will not operate just a list of integers as the latest example, it’ll operates lists of any types. So the list in the parameter is of type 'a list (pronounced alpha list), and the function is called polymorphic since the type isn’t relevant.

Let’s write a map function that will take a function and a list as parameters and will apply the function to every element of the list and return results, just like a map:

let rec map f l =
match l with
| [] -> []
| h :: t -> f h :: map f t;;

Example of usage (using the total function we’ve created before):

# total;;
- : int list -> int = <fun>
# map total [[1; 2]; [3; 4]; [5; 6]];;
- : int list = [3; 7; 11]
# map total [1; 2; 4];;
Error: This expression has type int but an expression was expected of type
int list

Obs: The reason the second returns an error is because total returns int and map should return a list, that’s why in the first one we passed a list of lists, so for each list total returned an int and the map function returned and list of ints;

Example using anonymous function:

map (fun x -> x * 2) [1; 2; 3];;

### Partial Application

We can apply a function passing only some of it’s parameters. For example:

let add x y = x + y;;
- : int = 3

Let’s use it with our map function:

map (add 1) [1; 2; 3];;
- : int list = [2; 3; 4]

or let’s use partial application of map function:

# map (map (add 1)) [[1; 2]; [3; 4]; [5; 6]];;
- : int list list = [[2; 3]; [4; 5]; [6; 7]]
# map (map total) [[[1; 2]; [1; 2]]; [[1; 2]; [1; 2]]];;
- : int list list = [[3; 3]; [3; 3]]

## Tuples

Tuples are ordered collections of elements of different types. For example:

- let t = (1, "okay", '1');;
val t : int * string * char = (1, "okay", '1')

## Records

They’re similar to tuples, but instead of having elements in a fixed order, they have named elements:

type person =
{name: string;
age: int};;
let frank =
{name: "frank";
age: 21};;
let age = frank.age;;

## Exceptions

Use raise keyword to throw exceptions, example:

let f a b =
if b = 0 then raise (E2 "division by zero") else a / b;;

Built-in polymorphic option type:

type 'a option = None | Some of 'a;;

We may write:

let f a b =
if b = 0 then None else Some (a / b);;

## Imperative OCaml

We use ref keyword to create references:

let r = ref 0;;

We use := operator to assign a value to the reference:

r := 100;;

We use ! operator to de-reference so we can get the reference contents:

# !r;;
- : int = 100

## Loops

For loop:

# let table n =
for row = 1 to n do
for column = 1 to n do
print_string (string_of_int (row * column));
print_string " "
done;
print_newline ()
done;;
val table : int -> unit = <fun>

While loop:

# let smallest_power_of_two x =
let t = ref 1 in
while !t < x do
t := !t * 2
done;
!t;;
val smallest_power_of_two : int -> int = <fun>

## Arrays

OCaml does have arrays, like:

# let arr = [|1; 2; 3|];;
val arr : int array = [|1; 2; 3|]
# arr.(0);;
- : int = 1
# arr.(0) <- 0;;
- : unit = ()
# arr;;
- : int array = [|0; 2; 3|]

## Standard Libraries

Printf module:

let print_length s =
Printf.printf "%s has %i characters\n" s (String.length s);;