Maybe sed?

or awk?

awk '!/^ *#/ && NF' file

I’m already using sed but I’d like to handle it with AST so that it doesn’t remove markdown header from multistring @doc, for example

Here is a command I usually run after mix new

sed -i '/^ *#/d' \
  .gitignore \
  config/*.exs \
  lib/**/*.ex \
  mix.exs \
  test/**/*.ex \
  test/**/*.exs

mix format

You are looking for Macro — Elixir v1.20.2

You can use Macro.to_string/1/2:

iex(1)> """
...(1)> defmodule M do
...(1)>   # a function
...(1)>   def foo, do: :foo
...(1)> end
...(1)> """
"defmodule M do\n  # a function\n  def foo, do: :foo\nend\n"
iex(2)> Code.string_to_quoted(v(1))
{:ok,
 {:defmodule, [line: 1],
  [
    {:__aliases__, [line: 1], [:M]},
    [do: {:def, [line: 3], [{:foo, [line: 3], nil}, [do: :foo]]}]
  ]}}
iex(3)> v(2) |> elem(1) |> Macro.to_string
"defmodule(M) do\n  def(foo) do\n    :foo\n  end\nend"
iex(4)> IO.puts(v(3))
defmodule(M) do
  def(foo) do
    :foo
  end
end
:ok

Thanks - but I found it does not return same format as mix format for .exs

remove-comment.exs file

# top-level comment

defmodule RemoveComment do
  # in-module comment

  def run(code_string) do
    # in-func comment
    code_string
    |> Code.string_to_quoted!()
    |> Macro.to_string()
  end
end

"./remove-comment.exs"
|> File.read!()
|> RemoveComment.run()
|> IO.puts()

$ elixir --version
Erlang/OTP 22 [erts-10.4.4] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Elixir 1.9.0 (compiled with Erlang/OTP 22)

$ elixir remove-comment.exs
(
  defmodule(RemoveComment) do
    def(run(code_string)) do
      code_string |> Code.string_to_quoted!() |> Macro.to_string()
    end
  end
  "./remove-comment.exs" |> File.read!() |> RemoveComment.run() |> IO.puts()
)

Thats properly because the formatter use Inspect.Algebra — Elixir v1.20.2