How to debug an Erlang program in VS Code

Question

I'm learning Erlang now. As far as I know, there's a tool called rebar3 that can generate a project boilerplate. Ok, so after installing it, I generate an empty project like this:

$ rebar3 new umbrella myproj

Sweet. Now I open VS Code with the Erlang extension installed. I add a launch.json file as it is said in the documentation:

{
  // Use IntelliSense to learn about possible attributes.
  // Hover to view descriptions of existing attributes.
  // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
  "version": "0.2.0",
  "configurations": [        
    {
      "name": "Launch erlang",
      "type": "erlang",
      "request": "launch",
      "cwd": "${workspaceRoot}",
      "preLaunchTask": "rebar3 compile"
    }
  ]
}

Then, I add another tasks.json file:

{
  // See https://go.microsoft.com/fwlink/?LinkId=733558
  // for the documentation about the tasks.json format
  "version": "2.0.0",
  "tasks": [
    {
      "label": "rebar3 compile",
      "type": "shell",
      "command": "rebar3 compile",
      "group": {
        "kind": "build",
        "isDefault": true
      },
      "problemMatcher": "$erlang"
    }
  ]
}

Then, I set a breakpoint, here in the myproj_app.erl file:

%%%-------------------------------------------------------------------
%% @doc myproj public API
%% @end
%%%-------------------------------------------------------------------

-module(myproj_app).

-behaviour(application).

-export([start/2, stop/1]).

start(_StartType, _StartArgs) ->
    io:format("Hello, world!~n"), %<-- BREAKPOINT
    myproj_sup:start_link().

stop(_State) ->
    io:format("Hello, world!~n"),
    ok.

%% internal functions

I press F5, the debugger starts but it never stops. Here's the output:

> Executing task: rebar3 compile <

===> Verifying dependencies...
===> Analyzing applications...
===> Compiling myproj

Terminal will be reused by tasks, press any key to close it.

Compiling arguments file  "/tmp/bp_2863283.erl"
Compile result: sucess 
Module bp_2863283 loaded

If I add the line:

"arguments": "-config dev -s sample"

To the launch.json file, it gives me an error and doesn't start. And I'm pretty sure those aren't the arguments I have to pass to whatever is in charge of launching my program. Here is the output:

{
"
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t

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t
 kernel pid",application_controller,"error in config file \"./dev.config\" (none): configuration file not found"}
c
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d

n
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t

s
t
a
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k
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l

p
id (application_controller) (error in config file "./dev.config" (none): configuration file not found)

Crash dump is being written to: erl_crash.dump...
d
o
n
e


erl exit code:1
erl exit with code 1

How do I configure VS Code to debug? What is lacking?

Answer 1

I am new to Erlang and hit the same issue... Evidently, starting a debug session does not start your program, but only start elang shell with debug flags, where you can than start any module:function(args) you are interesting to debug. Note that the debug console window has ">" prompt where you can invoke it. So in your specific example, I'd be using "myproj_app:start(1, 2)." where the args does not really matter as they are not used. Once entered

Answer 2

There is a built in debugger in OTP. Simply call debugger:start() wherever you want and once the app hits the debugger:start/0 the debugger window will pop up, allowing you to interpret specific modules and put breakpoint etc. For more details read the official docs. Also, observer (observer:start()) can also help you some times while debugging, so have both of them in mind.