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

Maybe sed?

https://www.commandlinefu.com/commands/view/10104/remove-comments-from-files

or awk?

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

You are looking for https://hexdocs.pm/elixir/Macro.html#to_string/2

Thats properly because the formatter use https://hexdocs.pm/elixir/Inspect.Algebra.html