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

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

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

Dive into Elixir v1.6 Code Formatter !

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

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")

But is it safe to format?

In Elixir

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

Why is it safe? What is same AST mean?

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 #

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

Let's experiment

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

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

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"]]}] ] }

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"]]}] ] }

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"]]}]

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

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

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