View Source Debugging

There are a number of ways to debug code in Elixir. In this chapter we will cover some of the more common ways of doing so.

IO.inspect/2

What makes IO.inspect(item, opts \\ []) really useful in debugging is that it returns the item argument passed to it without affecting the behavior of the original code. Let's see an example.

(1..10)
|> IO.inspect()
|> Enum.map(fn x -> x * 2 end)
|> IO.inspect()
|> Enum.sum()
|> IO.inspect()

Prints:

1..10
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
110

As you can see IO.inspect/2 makes it possible to "spy" on values almost anywhere in your code without altering the result, making it very helpful inside of a pipeline like in the above case.

IO.inspect/2 also provides the ability to decorate the output with a label option. The label will be printed before the inspected item:

[1, 2, 3]
|> IO.inspect(label: "before")
|> Enum.map(&(&1 * 2))
|> IO.inspect(label: "after")
|> Enum.sum

Prints:

before: [1, 2, 3]
after: [2, 4, 6]

It is also very common to use IO.inspect/2 with binding/0, which returns all variable names and their values:

def some_fun(a, b, c) do
  IO.inspect binding()
  ...
end

When some_fun/3 is invoked with :foo, "bar", :baz it prints:

[a: :foo, b: "bar", c: :baz]

See IO.inspect/2 and Inspect.Opts respectively to learn more about the function and read about all supported options.

dbg/2

Elixir v1.14 introduced dbg/2. dbg is similar to IO.inspect/2 but specifically tailored for debugging. It prints the value passed to it and returns it (just like IO.inspect/2), but it also prints the code and location.

# In my_file.exs
feature = %{name: :dbg, inspiration: "Rust"}
dbg(feature)
dbg(Map.put(feature, :in_version, "1.14.0"))

The code above prints this:

[my_file.exs:2: (file)]
feature #=> %{inspiration: "Rust", name: :dbg}
[my_file.exs:3: (file)]
Map.put(feature, :in_version, "1.14.0") #=> %{in_version: "1.14.0", inspiration: "Rust", name: :dbg}

When talking about IO.inspect/2, we mentioned its usefulness when placed between steps of |> pipelines. dbg does it better: it understands Elixir code, so it will print values at every step of the pipeline.

# In dbg_pipes.exs
__ENV__.file
|> String.split("/", trim: true)
|> List.last()
|> File.exists?()
|> dbg()

This code prints:

[dbg_pipes.exs:5: (file)]
__ENV__.file #=> "/home/myuser/dbg_pipes.exs"
|> String.split("/", trim: true) #=> ["home", "myuser", "dbg_pipes.exs"]
|> List.last() #=> "dbg_pipes.exs"
|> File.exists?() #=> true

While dbg provides conveniences around Elixir constructs, you will need IEx if you want to execute code and set breakpoints while debugging.

Pry

When using IEx, you may pass --dbg pry as an option to "stop" the code execution where the dbg call is:

$ iex --dbg pry

Or to debug inside a of a project:

$ iex --dbg pry -S mix

Now any call to dbg will ask if you want to pry the existing code. If you accept, you'll be able to access all variables, as well as imports and aliases from the code, directly from IEx. This is called "prying". While the pry session is running, the code execution stops, until continue (or c) or next (or n) are called. Remember you can always run iex in the context of a project with iex -S mix TASK.

Breakpoints

dbg calls require us to change the code we intend to debug and has limited stepping functionality. Luckily IEx also provides a IEx.break!/2 function which allows you to set and manage breakpoints on any Elixir code without modifying its source:

Similar to dbg, once a breakpoint is reached, code execution stops until continue (or c) or next (or n) are invoked. Breakpoints can navigate line-by-line by default, however, they do not have access to aliases and imports when breakpoints are set on compiled modules.

The mix test task direct integration with breakpoints via the -b/--breakpoints flag. When the flag is used, a breakpoint is set at the beginning of every test that will run:

Here are some commands you can use in practice:

# Debug all failed tests
$ iex -S mix test --breakpoints --failed
# Debug the test at the given file:line
$ iex -S mix test -b path/to/file:line

Observer

For debugging complex systems, jumping at the code is not enough. It is necessary to have an understanding of the whole virtual machine, processes, applications, as well as set up tracing mechanisms. Luckily this can be achieved in Erlang with :observer. In your application:

$ iex
iex> :observer.start()

Missing dependencies

When running iex inside a project with iex -S mix, observer won't be available as a dependency. To do so, you will need to call the following functions before:

iex> Mix.ensure_application!(:wx)             # Not necessary on Erlang/OTP 27+
iex> Mix.ensure_application!(:runtime_tools)  # Not necessary on Erlang/OTP 27+
iex> Mix.ensure_application!(:observer)
iex> :observer.start()

If any of the calls above fail, here is what may have happened: some package managers default to installing a minimized Erlang without WX bindings for GUI support. In some package managers, you may be able to replace the headless Erlang with a more complete package (look for packages named erlang vs erlang-nox on Debian/Ubuntu/Arch). In others managers, you may need to install a separate erlang-wx (or similarly named) package.

The above will open another Graphical User Interface that provides many panes to fully understand and navigate the runtime and your project.

We explore the Observer in the context of an actual project in the Dynamic Supervisor chapter of the Mix & OTP guide. This is one of the debugging techniques the Phoenix framework used to achieve 2 million connections on a single machine.

If you are using the Phoenix web framework, it ships with the Phoenix LiveDashboard, a web dashboard for production nodes which provides similar features to Observer.

Finally, remember you can also get a mini-overview of the runtime info by calling runtime_info/0 directly in IEx.

Other tools and community

We have just scratched the surface of what the Erlang VM has to offer, for example:

Happy debugging!