Metaprogramming Programs that Write Programs 

InElixir Elixir 

ElixirConf Orlando! 

How to give the talk 

1. Set up the Topic 

How much? 

It depends™ 

What is “Elixir”? 

Basics + STDLIB 

of Elixir is written in Elixir 78% 

of Elixir is written in Elixir 78% 87% 

Compare to Ruby’s 44% 

Numbers? Meh 

if 

def 

|> 

Metaprogramming 

2. Introductions 

https://iamvery.com — @iamvery Jay Hayes 

No content

No content

No content

We’re hiring! 

3. Definitions 

Metaprogramming 

No content

No content

Code generation 

Definition, plz 

Metaprogramming 

programming 

Work on Data 

4 + 2 

4 + 2 data 

4 + 2 data func 

+(4, 2) 

Kernel. ( , ) 4 2 + 

4 2 + 

4 2 + # => 6 

Meta… 

The program is data 

Programming the Program 

Code generation 

4. Macros & Expressions 

Macros 

Macros are everywhere 

good macros are invisible 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end macros 

What is the data? 

What is the data? Program 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end 

defmodule ItTest do use ExUnit.Case test “works” do assert It.works? end end 

Program macro Program ✨ ✨ 

Program macro Program ✨ ✨ 

Program macro Program ✨ ✨ 

Program macro Program Expression Expression ✨ ✨ 

Quoted expression? 

Code Expressed as Data 

{:+, [], [1, 2]} 

Abstract Syntax Tree 

/ 1 + 2 3 

/ 1 + 2 3 / 1 + 2 3 

( ( )) / 1 + 2 3 

( ( )) / 1 + 2 3 func 

( ( )) / 1 + 2 3 func Args 

( ( )) / 1 + 2 3 func Args 

1 / ( 2 + 3 ) 

/ 1 + 2 3 

{: , …, [ , {: , …, [ , ]} / 1 + 2 3 

{func, meta, args} 

{func, meta, args} +(1, 2) 

{:+, [], [1, 2]} 

Back to macros… 

Macros are Functions 

Args are quoted 

Args are quoted 

(not evaluated) 

Expression.match?(:lol > :wat) # => false Expression.match?(:lol = :wat) # => true 

defmacro match?({:=, _, _}) do true end defmacro match?(_expr) do false end 

Compile time 

Expression.match?(:lol > :wat) Expression.match?(:lol = :wat) 

false true 

Must return quoted 

Must return quoted 

Code generation 

defmacro badmatch do {:=, [], [:lol, :wat]} end 

Expression.badmatch # (MatchError) 

:lol = :wat # (MatchError) 

Program macro Program Expression Expression ✨ ✨ 

5. Quoting 

{:+, [], [1, {:-, [], [2, 3]}]} 

Quote/2 

Easier to grok 

Return AST of Expression 

Return AST of Expression 

{func, meta, args} 

quote do :lol = :wat end 

{:=, [], [:lol, :wat]} 

quote do 1 / (2 + 3) end 

{:/, [], [1, {:+, [2, 3]} 

{:/, [], [1, {:+, [2, 3]} 

]} {:/, [], [1, {:+, [2, 3]} 

Why “quote”? 

Like strings… 

Sort of… 

“1+1” 

eval “1+1” 

eval “1+1” # => 2 

quote(do: 1+1) 

expr = quote(do: 1+1) Code.eval_quoted(expr) 

expr = quote(do: 1+1) Code.eval_quoted(expr) # => {2, []} 

Bonus:Syntax✔ 

Recursive 

Terminal Values 

Quoted Literals 

quote do: “foo” # “foo” quote do: :foo # :foo quote do: 1 # 1 quote do: [1] # [1] quote do: {1,2} # {1,2} quote do: [a: 1] # [a: 1] 

6. Unquoting 

Value Injection 

Unquote/1 

Strings? 

Like string interpolation 

Sort of… 

Inject value into quoted string 

name = “Sue” “hi name” # => “hi name” 

name = “Sue” “hi #{name}” # => “hi Sue” 

Inject value into quoted Expression 

num = 42 expr = quote do: 1 + num # => {:+, [], [1, {:num, [], :Elixir}]} 

num = 42 expr = quote do: 1 + num # => {:+, [], [1, {:num, [], :Elixir}]} Code.eval_quoted(expr) # ERROR! `num` is undefined 

num = 42 expr = quote do: 1 + unquote(num) # => {:+, [], [1, 42]} 

num = 42 expr = quote do: 1 + unquote(num) # => {:+, [], [1, 42]} Code.eval_quoted(expr) # => {43, []} 

num = 42 expr = “1 + #{num}” # => “1 + 42” eval expr # => 43 

Inject value into quoted Expression 

“It’s already quoted” 

Expression Injection 

one = quote do: 1 + 1 # => {:+, [], [1, 1]} two = quote do: 2 + 2 # => {:+, [], [2, 2]} sum = quote do: unquote(one) + unquote(two) # => {:+, [], [ # {:+, [], [1, 1]}, # {:+, [], [2, 2]} # ]} 

Code.eval_quoted(sum) # => {6, []} 

7. Write a Macro 

if 

if foo > 0 do IO.puts(“Pass!”) else IO.puts(“Fail.”) end 

if(…, do: …, else: …) 

if(…, [do: …, else: …]) 

if(…, [do: …, else: …]) cond 

if(…, [do: …, else: …]) cases cond 

Truthy & Falsey 

false || nil Falsey 

Anything else Truthy 

case … do x when x in [false, nil] -> … _ -> … end 

case … do x when x in [false, nil] -> … _ -> … end cond 

case … do x when x in [false, nil] -> … _ -> … end cond cases 

defmacro if(c, do: tc, else: fc) do quote do case unquote(c) do x when x in [false, nil] -> unquote(fc) _ -> unquote(tc) end end end 

defmacro if(c, do: tc, else: fc) do quote do case unquote(c) do x when x in [false, nil] -> unquote(fc) _ -> unquote(tc) end end end cond 

defmacro if(c, do: tc, else: fc) do quote do case unquote(c) do x when x in [false, nil] -> unquote(fc) _ -> unquote(tc) end end end cases 

defmacro if(c, do: tc, else: fc) do quote do case unquote(c) do x when x in [false, nil] -> unquote(fc) _ -> unquote(tc) end end end 

defmacro if(c, do: tc, else: fc) do quote do case unquote(c) do x when x in [false, nil] -> unquote(fc) _ -> unquote(tc) end end end cond 

defmacro if(c, do: tc, else: fc) do quote do case unquote(c) do x when x in [false, nil] -> unquote(fc) _ -> unquote(tc) end end end cases 

defmacro if(c, do: tc, else: fc) do quote do case unquote(c) do x when x in [false, nil] -> unquote(fc) _ -> unquote(tc) end end end 

if do else end IO.puts(“Pass!”) IO.puts(“Fail.”) foo > 0 

case do x when x in [false, nil] -> _ -> end IO.puts(“Pass!”) IO.puts(“Fail.”) foo > 0 

8. Wrap Up with Guidance 

When to macro… 

#1 Don’t 

Write functions 

import String defmacro adds(a, b) do quote do to_integer(unquote(a)) + to_integer(unquote(b)) end end 

import String defmacro adds(a, b) do quote do to_integer(unquote(a)) + to_integer(unquote(b)) end end 

import String def adds(a, b) do to_integer(a) + to_integer(b) end 

Prefer direct solutions 

Decision based on args 

Transformation 

Less error-prone 

#2 Constants 

defmodule Conversions do defmacro seconds_per_minute, do: 60 defmacro minutes_per_hour, do: 60 end 

h/t module attrs @second_per_minute 

#3 DSL 

html do body do div class: “content” do p “Engaging content.” end end end 

scope “/”, HelloWeb do pipe_through :browser get “/”, PageController, :index end 

#4 Hide details 

test “works” do assert It.works? end 

#5 Reduce boilerplate 

Language extension 

defmodule Lolwat do def lol do :wat end end 

:elixir_module.compile(Lolwat, {:def, [context: Elixir, import: Kernel], [{:lol, [context: Elixir], Elixir}, [do: :wat]]}, [], __ENV__) 

:elixir_module.compile(Lolwat, {:def, [context: Elixir, import: Kernel], [{:lol, [context: Elixir], Elixir}, [do: :wat]]}, [], %{__struct__: Macro.Env, aliases: [], context: nil, context_modules: [Mex], export_vars: nil, file: "iex", function: nil, functions: [Mex: [expand_all: 2], "IEx.Helpers": [c: 1, c: 2, cd: 1, clear: 0, flush: 0, h: 0, i: 1, l: 1, ls: 0, ls: 1, nl: 1, nl: 2, pid: 1, pid: 3, pwd: 0, r: 1, recompile: 0, respawn: 0, v: 0, v: 1], Kernel: [!=: 2, !==: 2, *: 2, +: 1, +: 2, ++: 2, -: 1, -: 2, --: 2, /: 2, <: 2, <=: 2, ==: 2, ===: 2, =~: 2, >: 2, >=: 2, abs: 1, apply: 2, apply: 3, binary_part: 3, bit_size: 1, byte_size: 1, div: 2, elem: 2, exit: 1, function_exported?: 3, get_and_update_in: 3, get_in: 2, hd: 1, inspect: 1, inspect: 2, is_atom: 1, is_binary: 1, is_bitstring: 1, is_boolean: 1, is_float: 1, is_function: 1, is_function: 2, is_integer: 1, is_list: 1, is_map: 1, is_number: 1, is_pid: 1, is_port: 1, is_reference: 1, is_tuple: 1, length: 1, macro_exported?: 3, make_ref: 0, map_size: 1, max: 2, min: 2, node: 0, node: 1, not: 1, pop_in: 2, put_elem: 3, put_in: 3, rem: 2, round: 1, self: 0, send: 2, spawn: 1, spawn: 3, spawn_link: 1, spawn_link: 3, spawn_monitor: 1, spawn_monitor: 3, struct: 1, struct: 2, struct!: 1, struct!: 2, throw: 1, tl: 1, trunc: 1, tuple_size: 1, update_in: 3]], lexical_tracker: nil, line: 2, macro_aliases: [], macros: [Mex: [mex: 1], "IEx.Helpers": [b: 1, h: 1, import_file: 1, import_file: 2, import_file_if_available: 1, import_if_available: 1, import_if_available: 2, s: 1, t: 1], Kernel: [!: 1, &&: 2, ..: 2, <>: 2, @: 1, alias!: 1, and: 2, binding: 0, binding: 1, def: 1, def: 2, defdelegate: 2, defexception: 1, defimpl: 2, defimpl: 3, defmacro: 1, defmacro: 2, defmacrop: 1, defmacrop: 2, defmodule: 2, defoverridable: 1, defp: 1, defp: 2, defprotocol: 2, defstruct: 1, destructure: 2, get_and_update_in: 2, if: 2, in: 2, is_nil: 1, match?: 2, or: 2, pop_in: 1, put_in: 2, raise: 1, raise: 2, reraise: 2, reraise: 3, sigil_C: 2, sigil_D: 2, sigil_N: 2, sigil_R: 2, sigil_S: 2, sigil_T: 2, sigil_W: 2, sigil_c: 2, sigil_r: 2, sigil_s: 2, sigil_w: 2, to_char_list: 1, to_charlist: 1, to_string: 1, unless: 2, update_in: 2, use: 1, use: 2, var!: 1, var!: 2, |>: 2, ||: 2]], module: nil, requires: [IEx.Helpers, Kernel, Kernel.Typespec, Mex], vars: []}) 

No content

No content

9. Self Promotion 

bit.ly/ElixirMeta2 

Questions? @iamvery 

With love,