Can we run Erlog inside IEx?

This is mostly aimed at @rvirding but anyone who knows Erlang and Prolog well enough can answer, of course.

Here goes: I’d like to dip my toes in logic programming a bit. I don’t want to become an expert in Prolog the programming language, only the logic programming aspect of it.

Can Erlog (Prolog running in Erlang console) can actually run in IEx as well? If not, what exactly must be done so it can run in an Erlang console? Sorry if it’s a naive and dumb question, I don’t aim to become an expert in Erlang and I am looking for a quick-and-easy way.

Any help is appreciated.


Yes, erlog runs without problems in IEx. It is basically just a big library and with some interface modules.

  • The basic interface is in the :erlog module which you can call from programs or from a shell like IEx. The Erlog state is in a structure which you pass around.
  • There is also an Erlog gen_server in the module :erlog_server which you can startup. It manages the state and there is a set of interface functions to access it, basically the same interface as in :erlog.
  • You can also start up a simple prolog-like shell with :erlog_shell.

The interface is standard Erlang but this mainly because no-one has shown any interest in writing one for Elixir. It would not be difficult.


P.S. Saw your question in the irc but hadn’t got around to answering it.


Thank you! Does that mean that the Erlang modules you listed are a part of Erlang and don’t have to be installed on top of the vanilla Erlang?

For the record, I use Erlang/OTP 19.2.

EDIT: I can’t see erlog in Erlang’s website docs, that’s why I am asking if it’s a standalone package that needs to be installed extra. Also, how exactly do I exit the erlog_shell?

1 Like

No, erlog is a separate system which you will have to down load from github, , and connect into your application. This can be done with mix (I guess) but I can’t help you there as my knowledge of mix is not great.


How do I connect a downloaded GIT repo to my app in an Erlang environment? If I know that, I can probably infer how to do it with mix.

1 Like

If you are using rebar3 with erlang you can specify it as a dependency and it will be downloaded and put in a suitable place. Pretty much what mix does. If you are not using rebar3 then you need to download it by hand and put it somewhere that erlang can find it, the easiest is by either using the -pa <dir> flag when starting erlang or by setting ERL_LIBS environment variable, . With mix you can also give it as a dependency but you need to tell mix that it is erlang which it must compile and not elixir. i don’t know how you do that.


I’ll figure the mix part out. You helped me a lot, thank you!

1 Like

No worries, just ask if have more questions about running it. Seriously.


Definitely will, just don’t want to spam you with all of it before even having tried. :wink: I’ll be bumping the thread when I get around to actually trying erlog.

1 Like

2 posts were split to a new topic: How to run Erlog inside IEX

No, there should be no problems using erlog from Elixir. The only issue is that the argument order to the functions in some of the interface modules, e.g. erlog, is in the standard Erlang ordering with the state the last argument and not in the standard Elixir ordering. This will make them a little more difficult to use in pipes.

if there is interest I can fix a set of Elixir modules for erlog.

1 Like

Couldn’t then/2 help in this case?

Use the develop branch which contains the latest fixes and improvements. It sometimes takes some times before things make it into the master branch. You can usually rely on the develop as new developments and work are usually done in other branches.

There is now an elixir Erlog module with the same interface as the erlang erlog module but with the state as the first argument in the elixir way.

What does then/2 do?

Function application, but argument is passed as 1st argument to then/2. Useful for in-pipeline incantations, especially when the pipeline argument isn’t the 1st.

1 Like

There is now an elixir Erlog module with the same interface as the erlang erlog module but with the state as the first argument in the elixir way.

Would you happen to know where we could find it? I checked

I believe Robert is saying you need to install it as a GitHub dependency. ie: {:erlog, githhub: "rvirding/erlog", branch: "develop"}

1 Like

Yes, @kip beat me to it. :grinning: I have not put it in hex. I will get around to it eventually, but for now take it from github.

1 Like

Ah hah - no problem! Thank you.