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Elixir in Elixir

Jay Hayes
September 02, 2016

Elixir in Elixir

Metaprogramming doesn't have to be scary. Elixir does it really well. Let me show you...

Jay Hayes

September 02, 2016
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  1. Elixir Elixir
    In

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  2. How much?

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  3. It depends

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  4. What is “Elixir”?

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  5. Basics + STDLIB

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  6. of
    Elixir is
    written in
    Elixir
    78%

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  7. Compare to Ruby’s 44%

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  8. Numbers? Meh

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  9. if

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  10. def

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  11. |>

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  12. Metaprogramming

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  13. Code generation

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  14. +BZ)BZFT
    IUUQTJBNWFSZDPNn!JBNWFSZ

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  17. 8FEFWFMPQ

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  18. 8FUFBDI

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  19. 8FXSJUF

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  20. “I’m speaking at
    ElixirConf!”

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  21. “That’s nice. Where is it?”

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  22. “Orlando.”

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  23. “DISNEY IS THERE!!! CAN WE GO?!”

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  24. “Ya, actually it’s at Disney.”

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  25. “OMG!!!
    YOU’RE SPEAKING AT DISNEY?!!”

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  26. “Haha, sort of” *interrupted*

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  27. *on phone*
    “YA! He’s speaking at Disney!!
    We’re GOING!!!” *squeal*

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  30. Metaprogramming

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  32. programming
    Meta

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  33. Working on Data

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  34. data
    quantities on which
    operations are performed
    — Dictionary.app

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  35. 4 + 2
    data

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  36. +(4, 2)
    data

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  37. Kernel.+(4, 2)

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  38. The program is data

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  39. Macros

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  40. Macros are everywhere

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  41. good macros are
    invisible

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

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  43. Where is the data?
    Program

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

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

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

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

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  48. Expression
    macro
    Expression


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  49. Expression?

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  50. “Quoted” Elixir code

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  51. {func, …, args}

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  52. Abstract Syntax Tree

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  53. +
    1 -
    2 3

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  54. + 1 - 2 3
    ( ( ))
    func
    Args

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  55. 1 + ( 2 - 3 )

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  56. Args are quoted

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

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

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

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  60. false
    # => false
    true
    # => true

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  61. Must return quoted

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

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  63. defmacro badmatch do
    quote do
    :lol = :wat
    end
    end

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  64. Expression.badmatch
    # (MatchError)

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  65. :lol = :wat
    # (MatchError)

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  66. Quote

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  67. {func, meta, args}

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

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  69. quote do
    :lol = :wat
    end

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  70. Sort of like strings…

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  71. “1+1”

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  72. quote(do: 1+1)

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  73. quote do
    1+1
    end

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  74. Return AST of Expression

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  75. 2 - 3
    quote do
    end

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  76. - 2 3
    {: , […], [ , ]}
    (- 2 3)

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  77. quote do
    1 + (2 - 3)
    end

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  78. {: , […], [ , {: , […], [ , ]}]}
    + 1 - 2 3
    (+ 1 (- 2 3))

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  79. Easier to grok

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  80. {:+, […], [1, {:-, […], [2, 3]}]}
    quote do
    1 + (2 - 3)
    end

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  81. Quoted Literals

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  82. 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]

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  83. Unquote

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  84. “Inject value in quote”

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  85. Like string interpolation

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  86. “Inject value into quoted string”

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

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

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  89. “Inject value into quoted Expression”

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

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

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

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  93. if

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

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  95. if(…, do: …, else: …)

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  96. if(…, [do: …, else: …])
    cases
    cond

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  97. Truthy & Falsey

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  98. false || nil
    Falsey

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  99. anything else
    Truthy

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

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  101. 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 cases
    cond
    cases

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

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

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  104. When to macro…

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  105. #1 Don’t

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  106. Write functions

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  107. defmacro add(a, b) do
    quote do
    unquote(a) + unquote(b)
    end
    end

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  108. def add(a, b) do
    a + b
    end

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  109. More direct

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  110. Less error-prone

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  111. #2 Constants

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  112. defmacro seconds_per_minute, do: 60
    defmacro minutes_per_hour, do: 60

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  113. #3 DSL

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

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  115. #4 Hide details

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  116. test “works” do
    assert It.works?
    end

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  117. #5 Reduce boilerplate

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  118. Language extension

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  119. defmodule Lolwat do
    def lol do
    :wat
    end
    end

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

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  121. :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, 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: []})

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  122. good macros are invisible

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  123. CJUMZ&MJYJS.FUB

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  124. !JBNWFSZ
    Questions?

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