Glossary

Terms the pages use, with a short definition of each. On a page, the first mention of a term has a dotted underline: hover, focus or tap it for the definition. The definitions paraphrase the official Elixir docs, and each links to the page that explains the term properly.

Agent

A simple wrapper around state: a process you read and update by passing functions.Read the page

Application

Every Elixir project is an application, and so is Elixir itself (:elixir) and parts of it such as :ex_unit.Read the page

AST

Abstract Syntax Tree. Elixir represents code as nested tuples shaped like a tree, and calls them quoted expressions.Read the page

Binary

A bitstring whose number of bits is divisible by 8. Every binary is a bitstring, but not every bitstring is a binary.Read the page

Charlist

A list of code points, written ~c"like this", mostly used for interoperability with Erlang.Read the page

Child specification

A map that tells a supervisor how to start, shut down and restart one child. It is usually written as {module, options} or just the module name.Read the page

Comprehension

Syntactic sugar for looping over an enumerable, filtering and mapping, grouped into the for special form.Read the page

Doctest

An example in a documentation string, written with an iex> prompt and the expected result on the next line, checked as a test so documentation stays accurate.Read the page

ETS

Erlang Term Storage: the :ets and :dets modules store large data structures in memory or on disk.Read the page

Exit signal

The message a process sends along its links when it ends. A linked process that isn't trapping exits ends too, unless the reason is :normal.Read the page

GenServer

A "generic server": a process that holds state and offers synchronous (call) and asynchronous (cast) requests, plus code reloading and more.Read the page

Guard

An extra condition on a clause, written after when. A clause only matches if the pattern matches and the guard is true.Read the page

Link

A relationship between two processes for the case of failure: if one fails, the other receives an exit signal. Without a link, a failure in one process never crashes another.Read the page

Macro

Code that receives quoted expressions and transforms them into something else. Macros are harder to write than functions, and it is bad style to use them when they aren't needed.Read the page

Macro hygiene

Macros have "late resolution": a variable defined inside a quote won't conflict with one in the code where the macro is expanded.Read the page

Mailbox

Where the messages sent to a process wait. receive/1 searches it for a message that matches a pattern. Sending does not block: the sender puts the message in the mailbox and carries on.Read the page

Monitor

A one-way way to track when a process dies, without tying exit signals together. When the monitored process ends, the watcher gets a message, whatever the reason.Read the page

Node

A running Erlang VM that can connect to others. Processes are location transparent: sending a message works the same whether the recipient is on this node or another.Read the page

OTP

Open Telecom Platform: the set of libraries that ships with Erlang, used to build robust, fault-tolerant applications. It includes supervision trees and event managers.Read the page

PID

A process identifier. It is what spawn/1 returns and what send/2 takes to reach that process.Read the page

Pipe operator

|> takes the result of the expression on its left and passes it as the first argument of the call on its right, to highlight the data being transformed.Read the page

Process

Elixir's unit of concurrency: code always runs inside one. Processes are isolated, run concurrently and talk by passing messages. They are not operating system processes, and are lightweight enough to run in the hundreds of thousands.Read the page

Protocol

A mechanism for polymorphism: behaviour varies with the data type, dispatching on the type of the first argument. Anyone can implement a protocol, at any time.Read the page

Quoted expression

Elixir's name for the AST: code represented as nested tuples, which quote produces and unquote splices into.Read the page

Sigil

A custom textual representation. It starts with ~, then a letter and a delimiter, with optional modifiers after the last delimiter. You can define your own.Read the page

Struct

An extension built on top of maps that adds compile-time checks and default values.Read the page

Supervision tree

Supervisors whose children can be supervisors themselves, so the processes of an application form a tree.Read the page

Supervisor

A process that detects when its children die and starts new ones in their place. Processes are often linked to a supervisor for this.Read the page

Task

An asynchronous computation: spawn it now and read its result later.Read the page

Trapping exits

Setting Process.flag(:trap_exit, true) so exit signals arrive as ordinary messages instead of ending the process. Supervisors do this to survive their children.Read the page