Exceptions in

About Exceptions

The Exception Behaviour defines how errors are raised and displayed.

It includes two optional callbacks:

• message/1 transforms the error-struct into a readable message.
• exception/1 creates the message from the 2nd argument in raise/2 allowing the message to be modified.

Defined errors:

• Share the name of the module they are implemented in.
• Are an extension of the struct-type.
• Have a :message field.
• Can be be used with raise/1 and raise/2 to raise the intended error

Defining an exception

To define an exception from an error module, use the defexception macro:

# Defines a minimal error, with the name `MyError`
defmodule MyError do
  defexception message: "error"
end

# Defines an error with a customized exception/1 function
defmodule MyCustomizedError do
  defexception message: "custom error"

  @impl true
  def exception(value) do
    case value do
      [] ->
        %MyCustomizedError{}

      _ ->
        %MyCustomizedError{message: "Alert: " <> value}
    end
  end
end

Using exceptions

Defined errors may be used like a built in error using either raise/1 or raise/2:

raise MyError
# => ** (MyError) error

raise MyError, "an error occurred"
# => ** (MyError) an error occurred

raise MyCustomizedError
# => ** (MyCustomizedError) "custom error"

raise MyCustomizedError, "a very bad error occurred"
# => ** (MyCustomizedError) Alert: a very bad error occurred