Dive into Elixir v1.6 Code Formatter

Speaking about that Elixir code formatter is safe.

M3 Tech Talk #87 2018/02/16 #m3dev

Transcript

1. Deep Dive into
   Elixir v1.6
   Code Formatter
   In 7 Minutes
   

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2. Who am I?
   me
   |> name # Takayuki "Taka" Matsubara
   |> job # Software Engineer
   |> work_at # M3, Inc.
   |> like # Ruby, Elixir and OSS
   |> twitter # ma2ge
   |> github # ma2gedev
   |> oss # PowerAssertEx, bundle-star, breadcrumble,
   # chrono_logger, faraday-encoding
   

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3. PowerAssertEx ⭐ 153
   github.com/ma2gedev/power_assert_ex
   

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4. My Keyboard
   

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5. Deep Dive into
   Elixir v1.6
   Code Formatter
   In 7 Minutes
   

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6. Deep Dive into
   Elixir v1.6
   Code Formatter
   In 7 Minutes
   

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7. Dive into Elixir
   v1.6 Code
   Formatter !
   

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8. Code Formatter
   • like gofmt
   • https:/
   /golang.org/cmd/gofmt/
   

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9. Formatting code in
   Elixir
   • The mix format command formats your code
   # Elixir lang example
   ## before
   IO .
   puts ("Elixir is Nice")
   ## after `mix format`
   IO.puts("Elixir is Nice")
   

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10. But is it safe to
    format?
    

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11. In Elixir
    

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12. It's safe with --check-equivalent option
    $ mix format --check-equivalent
    check if the file after formatting has the same AST. If
    the ASTs are not equivalent, it is a bug in the code
    formatter. This option is recommended if you are
    automatically formatting files.
    — https:/
    /hexdocs.pm/mix/1.6.1/
    Mix.Tasks.Format.html
    

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13. Why is it safe?
    What is same AST
    mean?
    

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14. What is AST?
    • AST is Abstract Syntax Tree !
    • Elixir's internal representation of code
    • Elixir parses code into AST before executing your
    program
    • Same AST means that code behavior is also same
    Code ! -> AST " -> ... -> Execute #
    

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15. Detail of formatting process with --check-equivalent option
    1. generate formatted code from original code
    • ! -> "
    2. generate AST both the codes
    • ! -> #
    • " -> #
    3. then compare
    • # ≒ #
    

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16. Let's experiment
    

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17. Example code
    # before
    defmodule My do; def coooolest do; "one-liner"; end; end
    # after `mix format`
    defmodule My do
    def coooolest do
    "one-liner"
    end
    end
    

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18. :elixir.string_to_quoted!/4
    • this function generates AST from the code
    

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19. Original code's AST
    iex(1)> string1 = "defmodule My do; def coooolest do; \"one-liner\"; end; end"
    "defmodule My do; def coooolest do; \"one-liner\"; end; end"
    iex(2)> :elixir.string_to_quoted!(to_charlist(string1), 1, "nofile", [])
    {
    :defmodule,
    [line: 1],
    [
    {:__aliases__, [line: 1], [:My]},
    [do: {:def, [line: 1], [{:coooolest, [line: 1], nil}, [do: "one-liner"]]}]
    ]
    }
    

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20. Formatted code's AST
    iex(1)> string2 = """
    ...(1)> defmodule My do
    ...(1)> def coooolest do
    ...(1)> "one-liner"
    ...(1)> end
    ...(1)> end
    ...(1)> """
    "defmodule My do\n def coooolest do\n \"one-liner\"\n end\nend\n"
    iex(2)> :elixir.string_to_quoted!(to_charlist(string2), 1, "nofile", [])
    {
    :defmodule,
    [line: 1],
    [
    {:__aliases__, [line: 1], [:My]},
    [do: {:def, [line: 2], [{:coooolest, [line: 2], nil}, [do: "one-liner"]]}]
    ]
    }
    

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21. Compare both of ASTs
    • The diff shows almost same but [line: X] is
    indifferent !
    • But this is metadata so it does not affect behavior "
    # diff
    + [do: {:def, [line: 2], [{:coooolest, [line: 2], nil}, [do: "one-liner"]]}]
    - [do: {:def, [line: 1], [{:coooolest, [line: 1], nil}, [do: "one-liner"]]}]
    

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22. Code formatting is safe with --check-equivalent option
    $ mix format --check-equivalent
    

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23. Recap
    • Elixir's code formatter can compare formatted code
    with original one at AST level so its safely to use
    

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24. Thanks !
    M3 Tech Talk #87 2018/02/16 #m3dev
    @ma2ge
    

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