Can you show a full example of what you have at this point?
benwilson512
Author of Craft GraphQL APIs in Elixir with Absinthe
sadcad
Process.unregister(:standard_error)
{:ok, device} = StringIO.open("")
Process.register(device, :standard_error)
{result, _} = Code.eval_string(code)
This is what I have in my .exs
code is what I get from the user, so for example mix run script.exs 1+1
I usually get the result in json, but after adding the lines above I don’t get anything returned.
0
gregvaughn
I would expect the output to be in the device you registered. With a quick check of hexdocs I note StringIO.contents/1 will return a 2-tuple of input/output buffers. Have you tried that?
0
sadcad
I get eof. I think what you mentioned is true of I IO.write first, which I haven’t done..
Is there a way to actually encode the error to json, instead of working on hiding it ?
0
josevalim
Creator of Elixir
sadcad
The results to stdio and when there’s an error (using try-rescue) then I write to stderr. after adding the lines you gave me to the .exs I don’t get anything back when there’s any type of error. normal results still work fine.
0
josevalim
Creator of Elixir
Yes, nothing is written to stderr because you have replaced the stderr device by one that captures it. You can do the reverse work and register the :standard_error back to the original process. I will leave this as an exercise. 
2
josevalim
Creator of Elixir
lackac
@sadcad after reading this thread two times it seems to me that all information you need has been included here already.
One last piece of the puzzle that you might be missing still is that you can save the original device registered as :standard_error and use it later to either write to it directly or reregister it once you’re done with the eval and write the captured output after. Something like:
iex(2)> original_stderr = Process.whereis(:standard_error)
#PID<0.59.0>
iex(3)> Process.unregister(:standard_error)
true
iex(4)> {:ok, dev} = StringIO.open("")
{:ok, #PID<0.184.0>}
iex(5)> Process.register(dev, :standard_error)
true
iex(6)> IO.puts :stderr, "Oops!"
:ok
iex(7)> captured = StringIO.flush(dev)
"Oops!\n"
iex(8)> IO.write(original_stderr, captured)
Oops!
:ok
1
