Elm init hangs in mix task

I’m trying to set up some custom mix tasks for working with elm but when I call:

Mix.shell.cmd("elm init")

I get the prompt from elm asking whether to proceed in creating elm.json, but after I type ‘y’ and hit enter the process hangs.

When I break out, the message

thread blocked indefinitely in an MVar operation

is printed to the console. I searched for this string, and only get back results relating to Haskell so I’m guessing the elm compiler and mix/elixir aren’t playing nice together when user input is required.

The following command works fine (presumably because it doesn’t require any user input from the cmd line):

Mix.shell.cmd("elm make #{@elm_src}  --output=#{@elm_output_file} --debug")

I’ve also tried with System.cmd to no avail.

Any ideas on how to interact with Elm/Haskell from a mix task? Is it even possible?


I don’t know Elm, but would this work?

Mix.shell.cmd("elm init -y")

The BEAM in general doesn’t like it.

stdin of your application isn’t magically wired to stdin of the executed shell program.

You need to find a way to either enforce overwriting all, or to cancel in case of a conflict.

Or to write a Port on your own, that intercepts output and splices input in a way that you can remote control the Port depending on the users input.

If I had a choice, I’d try with enforcing/canceling approach…

Unfortunately Elm doesn’t accept flags on the init cmd, so that’s not an option.

Not too sure what you mean here. Would you be able to elaborate/explain further?


A well designed generator has flags to either assume hard confirmation of overwrites on conflict or to abort the process on conflicts. You already said elm init does not support this.

In my opinion this is worth a feature request on the elm side of things…

Wasn’t sure if you meant the flag option or something above my ‘pay grade’ :slight_smile: Thanks for clarifying.

That was my thought’s too so I’ll head over to Elm and add the request.

This itself is not a major issue, the generator only generates a simple json file which I could do myself, but other cmds such as install also require y/n input so was hoping it was going to possible from a mix task.

The Port option might be outside my current elixir/IO skills, but I’ll dip my toes into the water over the weekend and see what transpires.


It’s not that hard, I recommend it. The erlexec library (yes it’s erlang but works perfect in elixir) wraps up that work with a much better interface and error handling. :slight_smile:

But yeah, you’ll need to send the information (the y<enter> in this case) to elm’s stdin.

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