Gentle Introduction to Elixir Macros

Presented at June 2017 meetup of Elixir Sydney
https://www.meetup.com/sydney-ex/events/239476421/
and Elixir Camp 2 (April 21–24, 2017) in Camp Wombaroo, Mittagong, NSW

Transcript

1. Gentle Intro to Macros in Elixir by Qing Wu
2. Blake Education
3. Why Macros — Powerful — Common in open source projects — Elixir itself — Phoenix — Plug — Ecto
4. Macros
6. Abstract Syntax Tree (AST) Internal representation of code
7. AST examples iex(1)> quote do: 1+2 {:+, [context: Elixir, import: Kernel], [1, 2]} iex(2)> quote do: IO.puts "hello" {{:., [], [{:__aliases__, [alias: false], [:IO]}, :puts]}, [], ["hello"]}
8. AST block example iex(7)> quote do ...(7)> a = 3 ...(7)> b = 2 ...(7)> a * b ...(7)> end {:__block__, [], [{:=, [], [{:a, [], Elixir}, 3]}, {:=, [], [{:b, [], Elixir}, 2]}, {:*, [context: Elixir, import: Kernel], [{:a, [], Elixir}, {:b, [], Elixir}]}]}
9. How to define a macro defmacro macro_name(args) do ... end Macros accept AST and return AST
10. macro example in Ecto defmodule Weather do use Ecto.Schema # weather is the DB table schema "weather" do field :city, :string field :temp_lo, :integer field :temp_hi, :integer ... end end Source
11. Define our own macro defmodule MyIf do defmacro if1(condition, do: action) do quote do case unquote(condition) do true -> unquote(action) _ -> nil end end end end
12. Simple timer defmodule SimpleTimer do defmacro simple_timer(message, action) do quote do start_time = DateTime.utc_now IO.puts "starting to #{unquote(message)} at #{DateTime.to_iso8601 start_time}" unquote(action) end_time = DateTime.utc_now IO.puts "finished at #{DateTime.to_iso8601 end_time}" duration = DateTime.to_unix(end_time, :microsecond) - DateTime.to_unix(start_time, :microsecond) IO.puts "Duration: #{duration} microseconds" end end end
13. Macro Expansion
14. useful tools Macro.expand Macro.expand_once Macro.to_string
15. Macro use cases 1. delay evaluation 2. reduce boilerplate
16. Warning 1. Do NOT Write Macros 2. Use Macros Gratuitously
17. Summary 1. quote do ... end to get AST 2. unquote to inject code into AST 3. Macros accept AST and return AST
18. Further reading — The Erlangelist http://theerlangelist.com/article/ macros_1 — Mataprogramming Elixir by Chris McCord — Elixir/Phoenix/Plug/Ecto source