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OCaml Cheat Sheet

A concise reference for OCaml syntax, data structures, and common functions. Perfect for quick lookups and reminders.

Basics & Syntax

Basic Syntax

Variable Declaration

let variable_name = value;;
let variable_name = value in expression

Function Definition

let function_name argument1 argument2 = expression;;

Function Application

function_name argument1 argument2 (no parentheses needed)

Semicolon Usage

;; terminates top-level phrases in the interactive toplevel.
; sequences expressions, like expr1; expr2

Comments

(* This is a comment *)

If-Then-Else

if condition then expression1 else expression2

Basic Data Types

Integer

10, -5, 0

Float

3.14, -2.0, 0.0 (use +., -., *., /. for operations)

Boolean

true, false

Character

'a', 'Z', '\n'

String

"hello", "world" (immutable)

Unit

() (used for side-effects)

Operators

Arithmetic (int)

+, -, *, /, mod

Arithmetic (float)

+., -., *., /., **

Comparison

=, <>, <, >, <=, >= (structural equality)

Logical

&&, ||, not

String Concatenation

^

Data Structures

Tuples

Definition

let my_tuple = (1, "hello", true);;

Accessing Elements

let (a, b, c) = my_tuple in a;;
fst (1,2);; (* returns 1 )
snd (1,2);; ( returns 2 *)

Type

int * string * bool

Lists

Definition

let my_list = [1; 2; 3; 4];;
let empty_list = [];;

Cons Operator

1 :: [2; 3];; (* Result: [1; 2; 3] *)

Head and Tail

List.hd [1; 2; 3];; (* returns 1 )
List.tl [1; 2; 3];; ( returns [2;3] *)

Common Functions

List.length, List.map, List.filter, List.fold_left

Type

int list, string list

Arrays

Definition

let my_array = [| 1; 2; 3; 4 |];;

Accessing Elements

my_array.(0);; (Access first element)

Modifying Elements

my_array.(0) <- 5;;

Type

int array, string array

Records

Definition

 
type person = { name : string; age : int };;
let john = { name = "John"; age = 30 };;

Accessing Fields

john.name;; (* returns “John” *)

Modifying Fields (using with keyword)

let older_john = { john with age = john.age + 1 };;

Variants

Definition

 
type color = Red | Green | Blue;;
let my_color = Red;;

Parameterized Variants

 
type option = Some of 'a | None;;
let some_value = Some 10;;
let no_value = None;;

Recursive Variants

 
type int_tree = Leaf of int | Node of int_tree * int_tree;;
let my_tree = Node (Leaf 1, Leaf 2);;

Functions & Modules

Function Basics

Anonymous Functions

fun x -> x + 1;;

Currying

All functions in OCaml are curried by default.
let add x y = x + y;;
let add_five = add 5;; (* add_five is a function that adds 5 to its argument *)

Recursive Functions

let rec factorial n = if n <= 1 then 1 else n * factorial (n - 1);;

Higher-Order Functions

Map

List.map (fun x -> x * 2) [1; 2; 3];; (* Result: [2; 4; 6] *)

Filter

List.filter (fun x -> x mod 2 = 0) [1; 2; 3; 4];; (* Result: [2; 4] *)

Fold Left

List.fold_left (fun acc x -> acc + x) 0 [1; 2; 3];; (* Result: 6 *)

Fold Right

List.fold_right (fun x acc -> acc + x) [1; 2; 3] 0;; (* Result: 6 *)

Modules

Module Definition

 
module MyModule = struct
  let x = 10
  let add y = x + y
end;;

Module Usage

MyModule.x;;
MyModule.add 5;;

Module Types (Interfaces)

 
module type MY_MODULE_TYPE = sig
  val x : int
  val add : int -> int
end;;

Module Implementation

 
module MyModule : MY_MODULE_TYPE = struct
  let x = 10
  let add y = x + y
end;;

Functors

Functor Definition

 
module type Comparable = sig
  type t
  val compare : t -> t -> int
end;;

module MakeSet (C : Comparable) = struct
  (* Set implementation using C.t for elements *)
end;;

Functor Application

 
module IntComparable = struct
  type t = int
  let compare = Stdlib.compare
end;;

module IntSet = MakeSet (IntComparable);;

Advanced Features

Pattern Matching

Basic Matching

 
let rec describe_list lst =
  match lst with
  | [] -> "empty"
  | [x] -> "singleton"
  | _ -> "longer";;

Matching with Guards

 
let classify_number x =
  match x with
  | x when x > 0 -> "positive"
  | x when x < 0 -> "negative"
  | _ -> "zero";;

Matching on Tuples

 
let process_tuple t =
  match t with
  | (1, "hello") -> "Special tuple"
  | (a, b) -> "Generic tuple";;

Exceptions

Defining Exceptions

exception MyException of string;;

Raising Exceptions

raise (MyException "Something went wrong");;

Handling Exceptions

 
try
  (* Code that may raise an exception *)
with
  | MyException msg -> (* Handle the exception *)

References

Creating References

let my_ref = ref 0;;

Accessing Reference Value

!my_ref;;

Modifying Reference Value

my_ref := 5;;

Objects

Basic Object Definition

 
class point x_init y_init =
  object (self)
    val mutable x = x_init
    val mutable y = y_init
    method get_x = x
    method get_y = y
    method move dx dy = x <- x + dx; y <- y + dy
  end;;

Creating an Object

let my_point = new point 1 2;;

Using Object Methods

my_point#get_x;;
my_point#move 3 4;;
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