Setting breaks in IEx for debugging

With the release of Elixir 1.5 comes some handy new IEx helpers, one of which is the ability to add break points throughout your code.

You can add a break to any function using break!/2 or break!/4:

defmodule MyModule do
  def hello(name) do
    "hello " <> name
  end
end

iex(1)> break!(MyModule.hello/ 1)
or
iex(1)> break!(MyModule, :hello, 1)

Both break/2 and break/4 accept an additional argument for how many stops you want to make. Useful for recursive functions where you may want to stop multiple times.

To see what breaks you have use breaks/0

iex(1)> breaks()
 ID   Module.function/arity   Pending stops
---- ----------------------- ---------------
 1    MyModule.hello/1        1

Now when you call the function, you'll be placed into the a debugger and you can inspect whats being passed in:

iex(4)> MyModule.hello("world")
Break reached: MyModule.hello/1 (lib/my_module.ex:2)
    1: defmodule MyModule do
    2:   def hello(name) do
    3:     "hello " <> name
    4:   end
pry(1)> name
"world"

To exit the break and start a new shell process use respawn/0

pry(2)> respawn
Interactive Elixir (1.5.0) - press Ctrl+C to exit (type h() ENTER for help)
"hello world"
iex(1)>

Today I Learned

A Hashrocket project Follow on Twitter

Setting breaks in IEx for debugging