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Erlang Cheatsheet

A quick reference guide to the Erlang programming language, covering syntax, data types, concurrency, and OTP principles.

Erlang Basics

Syntax Fundamentals

Variable Assignment

Erlang uses single assignment. Variables start with an uppercase letter.

X = 10.

Atoms

Atoms are literal constants, starting with a lowercase letter.

status = ok.

Comments

Single-line comments start with %.

% This is a comment

Tuples

Tuples are compound data types.

Point = {10, 20}.

Lists

Lists are dynamic arrays.

Numbers = [1, 2, 3].

Strings

Strings are lists of character codes.

Name = "Erlang".

Basic Operators

Arithmetic

+, -, *, /, div, rem

Comparison

==, /=, <, >, =<, =>

Boolean

and, or, xor, not

List Operators

++, -- (append and subtract lists)

Concurrency

Processes

Spawning Processes

Use spawn to create a new process.

spawn(Module, Function, Args).

Sending Messages

Use ! to send messages to a process.

ReceiverPid ! {self(), Message}.

Receiving Messages

Use receive to handle incoming messages.

receive
  {Sender, Message} ->
    io:format("Received ~p from ~p~n", [Message, Sender])
end.

Process Identifiers (PIDs)

Returned by spawn, used to identify processes.

Message Handling

Messages are the primary means of communication between Erlang processes. They are asynchronous and can be any Erlang term.

The receive block selectively receives messages based on pattern matching. Messages that don’t match remain in the mailbox.

Use after to specify a timeout for the receive block.

receive
  Message ->
    ...
after 5000 ->
    io:format("Timeout~n")
end.

OTP Principles

Supervisors

Supervisors are processes that monitor and restart other processes (children) in case of failure. They ensure the system’s fault tolerance.

Common supervision strategies include one_for_one, rest_for_one, and one_for_all.

Example:

{simple_one_for_one, {local, my_supervisor},
    [{my_worker, {my_worker, start_link, []}, permanent, brutal_kill, worker, [my_worker]}]}.

Behaviours

gen_server

Generic server behaviour for stateful processes.

gen_statem

Generic state machine behaviour.

gen_event

Generic event handler behaviour.

supervisor

Behaviour for creating supervisor processes.

Applications

Applications are a collection of modules, processes, and other resources that form a reusable component. They provide a way to package and manage Erlang code.

An application resource file (.app) defines the application’s metadata, such as its name, description, and dependencies.

Common Built-in Functions (BIFs)

Process Related

self()

Returns the PID of the current process.

spawn(Module, Function, Args)

Spawns a new process.

exit(Reason)

Terminates the current process with the given reason.

erlang:monitor(process, Pid)

Sets up a monitor for the specified process.

Data Type Conversion

list_to_atom(List)

Converts a list to an atom.

atom_to_list(Atom)

Converts an atom to a list.

list_to_integer(List)

Converts a list to an integer.

integer_to_list(Integer)

Converts an integer to a list.

I/O

io:format(Format, Args)

Prints formatted output.

file:read_file(Filename)

Reads the contents of a file.
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