Code.eval_string("IO.puts('a')") will print the a. When you do IO.puts(result), that is printing the :ok.

that prints both..

It does not. Do it in iex and you will see what is happening.

iex(1)> {result, _} = Code.eval_string("IO.puts('a')")
a
{:ok, []}
iex(2)> IO.puts result
ok
:ok

The {:ok, []} from the first function call is the return value of calling Code.eval_string/3. In this case, you are binding the result variable to the atom :ok.

In the second line, you are printing out the atom :ok, which gets printed as ok. And then we see :ok as the return value of the IO.puts/2 call.

So how would I be able to write a to stderr for example ? obviously without changing the code in eval_string.

To my knowledge, you can’t. The default IO device for the IO.puts/2 function is :stdio. You would need the code written as Code.eval_string("IO.puts(:stderr, 'a')") in order to have it go to stderr.

Any other way to split them ? maybe using regex ?

What are you trying to do?

I write the result to stdout and if I get something like IO.puts() then I write it’s output to stderr.
Like this:

{result, _} = Code.eval_string(code)
if code has IO.puts() then write it to stderr end
wrtie result to stdout

I mean at a high level. This seems like a very unusual requirement, what are you trying to achieve at the big picture level?

I simply want to write in case of IO.puts() Your console output is: and otherwise I’d write Your returned result is:

That’s pretty much what I want to achieve.