Metaprogramming in Elixir: Writing Code That Writes Code

A practical introduction to macros, quote, unquote, compile-time code generation, and domain-specific languages

Most applications are written as instructions for a computer to execute.

Metaprogramming adds another layer: we write programs that inspect, transform, or generate other programs.

That description can make metaprogramming sound more mysterious than it is. In Elixir, it is built on one relatively simple idea:

Elixir code can be represented as ordinary Elixir data.

Once code becomes data, it can be inspected, modified, and returned to the compiler as new code.

This capability powers many constructs that Elixir developers use every day, including:

• if
• unless
• def
• defmodule
• use
• ExUnit assertions
• Ecto schemas and migrations
• Phoenix routing

This article explains how Elixir metaprogramming works, how to create macros, and when macros are actually the right tool.

What Is Metaprogramming?