That looks great, but why is it not shaped as a custom backend to Kernel.dbg/2?

Very cool feature! I would like to suggest few ideas:

1. Add the module name and arity to trace logs
2. Use :owl package to generate links that opens specific function source code
3. Add option to disable trace log prefix to ensure output is clear
4. In other case I would use :owl again to have borderless table with prefix as first column and log as the second one.

Look that in my case the output looks like:

TRACE t515: (factorial [5])
TRACE t643: | (factorial [4])
TRACE t771: | | (factorial [3])
TRACE t899: | | | (factorial [2])
TRACE t1027: | | | | (factorial [1])
TRACE t1155: | | | | | (factorial [0])
TRACE t1155: | | | | | | => 1
TRACE t1027: | | | | | => 1
TRACE t899: | | | | => 2
TRACE t771: | | | => 6
TRACE t643: | | => 24
TRACE t515: | => 120

t before number is rather useless I think, but it’s not important. Some integers have 4 digits and some only 3. Because of that the output could be more or less harder to read in some cases.

# version 1 (with option to disable prefix set to true - default?)
(Math.factorial/1 [5])
| (Math.factorial/1 [4])
| | (Math.factorial/1 [3])
| | | (Math.factorial/1 [2])
| | | | (Math.factorial/1 [1])
| | | | | (Math.factorial/1 [0])
| | | | | | => 1
| | | | | => 1
| | | | => 2
| | | => 6
| | => 24
| => 120

# version 2 (with option to disable prefix set to false)
TRACE t515:  (Math.factorial/1 [5])
TRACE t643:  | (Math.factorial/1 [4])
TRACE t771:  | | (Math.factorial/1 [3])
TRACE t899:  | | | (Math.factorial/1 [2])
TRACE t1027: | | | | (Math.factorial/1 [1])
TRACE t1155: | | | | | (Math.factorial/1 [0])
TRACE t1155: | | | | | | => 1
TRACE t1027: | | | | | => 1
TRACE t899:  | | | | => 2
TRACE t771:  | | | => 6
TRACE t643:  | | => 24
TRACE t515:  | => 120

Edit: Oh, looks like there is a bug with pipes:

iex> 5 |> Math.factorial() |> IO.puts() |> DoTrace.trace()
** (UndefinedFunctionError) function IO.puts/0 is undefined or private. Did you mean:

      * puts/1
      * puts/2

    (elixir 1.19.0-rc.0) IO.puts()
    iex:13: (file)

Well, if only Common LISP had the OTP runtime. The LISP folks practically beat most of the computer science game ages ago, including allow one to easily devise business-specific DSLs that compile to quite well optimised LISP.