How to get the AST of an Elixir program

Getting the AST (Abstract Syntax Tree) representation of an Elixir source is pretty simple.

Let’s say we want to get the AST of this file:

lib/hello.ex
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defmodule Hello do

  def hi(name) do
    IO.puts "Hello " <> name
  end

end

We can do it right away from iex:

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$ iex
Erlang/OTP 18 [erts-7.0] [source] [64-bit] [smp:8:8] [async-threads:10] [kernel-poll:false]

Interactive Elixir (1.0.5) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> {:ok, ast} = Code.string_to_quoted(File.read!("lib/hello.ex"))
{:ok,
 {:defmodule, [line: 1],
  [{:__aliases__, [counter: 0, line: 1], [:Hello]},
   [do: {:def, [line: 3],
     [{:hi, [line: 3], [{:name, [line: 3], nil}]},
      [do: {{:., [line: 4],
         [{:__aliases__, [counter: 0, line: 4], [:IO]}, :puts]}, [line: 4],
        [{:<>, [line: 4], ["Hello ", {:name, [line: 4], nil}]}]}]]}]]}}

In our case, the ast variable will contain the full AST of the source code.

In case you want to get the AST of a single line, it’s even simpler:

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iex(1)> name = "John"
"John"

iex(2)> IO.puts "Hello " <> name
Hello John
:ok

iex(3)> ast = quote do: IO.puts "Hello " <> name
{{:., [], [{:__aliases__, [alias: false], [:IO]}, :puts]}, [],
 [{:<>, [context: Elixir, import: Kernel], ["Hello ", {:name, [], Elixir}]}]}

For more context, I recommend reading the introduction to meta-programming in Elixir on Elixir’s official site.

In case you’re interested in parsing Elixir, Tokenizing and parsing in Elixir with yecc and leex by Andrea Leopardi is a very recommended reading.

Have fun with Elixir!

Posted by Simone Vittori

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