Defining Our Own Exceptions

Exceptions in Elixir are basically records. We can define our own exceptions by creating a module. Inside it, we use defexception to define the various fields in the exception, along with their default values. Because we’re creating a module, we can also add functions. Often, these are used to format the exception’s fields into meaningful messages.

Say we’re writing a library to talk to a Microsoft Kinect controller. It might want to raise an exception on various kinds of communication errors. Some of these are permanent, but others are likely to be transient and can be retried.

Implementation

We’ll define our exception with its required message field and an additional can_retry field. We’ll also add a function that formats these two fields into a nice message.

defmodule KinectProtocolError do

  defexception message: "Kinect protocol error", 
               can_retry: false 

  def full_message(me) do
    "Kinect failed: #{me.message}, retriable: #{me.can_retry}"
  end

end

Users of our library could write code like this:

try do
    talk_to_kinect()
rescue
    error in [KinectProtocolError] ->
    IO.puts KinectProtocolError.full_message(error)
    if error.can_retry, do: schedule_retry()
end

If an exception gets raised, the code handles it and possibly retries:

Kinect failed: usb unplugged, retriable: true
Retrying in 10 seconds

Run the B.talk_to_kinect, B.schedule_retry and B.start commands to start executing the specific functions below.