Using a debugger

Elixir:
Using a debugger

How to:

Elixir ships with a built-in graphical debugger called :debugger. To use it, you’ll need to start it and attach to your running process.

First, ensure you have :debugger started within an iex session:

iex> :debugger.start()
{:ok, #PID<0.108.0>}

Now, interpret the code module you want to debug:

iex> :int.ni(MyApp.MyModule)
{:module, MyApp.MyModule}

You can set a breakpoint:

iex> :int.break(MyApp.MyModule, line_number)
:ok

And then, run your function to hit the breakpoint and step through your code:

iex> MyApp.MyModule.my_function(arg1, arg2)
# Debugger will pause execution at the line with the breakpoint

Deep Dive

Before Elixir’s :debugger, Erlang provided the debugger that Elixir uses; it’s robust and great at handling concurrent processes, a sweet spot of Erlang VM (BEAM). Unlike some other debuggers, :debugger doesn’t allow modification of variables on the fly, due to the immutable nature of data in Elixir. As for alternatives, you have IEx.pry that lets you pause execution and jump into a REPL at any point in your code, which can be super handy.

While :debugger is good for a graphical interface, some might prefer the built-in :observer tool that also offers process inspection and system metrics, albeit not specifically targeted at stepping through code. Elixir’s community also contributes tools like visualixir and rexbug, expanding the ecosystem of debug tools beyond the defaults.

Elixir:Using a debugger

How to:

Deep Dive

See Also

Elixir:
Using a debugger