The document summarizes the key stages of the Elixir compilation process: 1. Elixir code is parsed and tokenized into an Elixir Abstract Syntax Tree (AST). 2. All Elixir macros are expanded in the AST. 3. The expanded AST is translated into an Erlang AST, which is then fed into the Erlang compiler to produce BEAM bytecode that can be executed by the BEAM virtual machine.

1. A (shallow) dive into
the Elixir Compiler
JEFFREY CHAN
ALEMBIC
TWITTER: @JGMCHAN
GITHUB: HTTPS://GITHUB.COM/JGMCHAN
EMAIL: JGMCHAN@GMAIL.COM

2. Elixir and Erlang VM
(BEAM)
Elixir combines lovely readable syntax, modern
programming concepts with the power and
resiliency of the BEAM VM
-module(helloworld).
-export([start/0]).
start() ->
io:fwrite("Hello, world!n").
defmodule HelloWorld do
def start do
IO.puts("Hello, world!")
end
end

3. Compilation Goal
BEAM VM can load and execute BEAM
bytecode (binary)
Goal is to transform Elixir code into BEAM
bytecode (.BEAM) file

4. Elixir Compiler
Elixir Code
|> Parse and Tokenize # Produces Elixir AST
|> Expand all Macros # Expanded AST
|> Translate to Erlang AST
|> Get Erlang Compiler to do the work!

5. Compilation Flow

6. Let’s Start!
defmodule HelloWorld do
def start do
IO.puts("Hello, world!")
end
end

7. Parse and Tokenise
Takes Elixir code, parses
it (leex, yecc) into Elixir
AST

8. Abstract Syntax Tree
(AST)
Representation of the code as a tree structure
Independent of the language syntax
Compiler will apply meaning (semantics) to the AST during compilation

9. Elixir AST
Typically a 3 - element tuple or a list
Tuple: {operator/data, metadata, arg/data}
List: List of tuples or lists

10. Expand Macros

11. Expanded AST
All Elixir Macros are
expanded
“if” becomes “case”
“defmodule”, “def” are
expanded

12. Translate to Erlang
AST

13. Erlang AST
Similar to Elixir AST
Tuples or Lists

14. Fed into Erlang
compiler

15. Erlang Compiler
Translates Erlang AST to Core Erlang (internal form)
Some other BEAM languages translate to Core
Erlang rather than Erlang AST
e.g. Lisp Flavoured Erlang (LFE)
Most optimisations are done to Core Erlang
Outputs BEAM bytecode
- File (.BEAM)
- Binary

16. Demo

17. References
https://elixir-lang.org/blog/2012/04/24/a-peek-inside-elixir-s-
parallel-compiler/
https://medium.com/@fxn/how-does-elixir-compile-execute-
code-c1b36c9ec8cf
https://elixirforum.com/t/getting-each-stage-of-elixirs-
compilation-all-the-way-to-the-beam-bytecode/1873
https://vinaytech.wordpress.com/2008/10/04/abstract-syntax-
tree/
https://elixirforum.com/t/beam-optimization-for-functions-
with-static-return-type/1868