# given that you have this module
defmodule MyApp.Example do
  def sum(x, y) do
    x + y
  end
end

# start a new iex session
$ iex -S mix

# then start :debugger
iex> :debugger.start()

# prepare your module for debugging
iex> :int.ni(MyApp.Example)

# set a break point at the line you want to capture
iex> :int.break(MyApp.Example, 3)

# and finally call your function
iex> MyApp.Example.sum(1,2)