Debugging

Source: Elixir guide, Debugging.

Tools run from lightest to heaviest:

Reach for the lightest tool that answers the question.

IO.inspect/2

IO.inspect/2 prints its argument and returns it unchanged, so you can drop it anywhere, especially between pipeline steps:

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

The dashed lines are output. The main path is untouched.

Add label: to tell prints apart (IO.inspect(label: "before")). Combine with binding/0, which returns every variable and its value:

def some_function(a, b, c) do
  IO.inspect(binding())   #=> [a: :foo, b: "bar", c: :baz]
end

dbg/2

Since Elixir 1.14. Like IO.inspect/2, but it also prints the code and its location, and in a pipeline it prints every step:

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

[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

Pry

iex --dbg pry (or iex --dbg pry -S mix in a project) makes each dbg call ask whether to stop. Accept and you get IEx with all variables, imports and aliases. Execution resumes on continue/c or advances on next/n.

Breakpoints

dbg needs code changes. IEx.break!/2 sets breakpoints on any code without editing it. It steps line by line but has no access to aliases and imports on compiled modules. mix test integrates them: --breakpoints sets one at the start of every test to run.

$ iex -S mix test --breakpoints --failed
$ iex -S mix test -b path/to/file:line

Observer

For understanding the whole system, use :observer.start(). It opens a GUI over the runtime: processes, applications, tracing.

iex> :observer.start()

Inside a project (iex -S mix), first run Mix.ensure_application!(:observer). If it fails, your Erlang was probably installed without WX (GUI) support, so install the complete package. Phoenix apps get the LiveDashboard for production nodes, and runtime_info/0 prints a quick runtime overview in IEx.

Beyond

:crashdump_viewer reads crash dumps. The VM also integrates with OS tracers (LTTng, DTrace, SystemTap), has microstate accounting (:msacc), and Mix ships profile.cprof and profile.fprof tasks. For advanced cases the guide recommends the free book Erlang in Anger.