Kernel.defexception
defexception
, go back to Kernel module for more information.
Defines an exception.
Exceptions are structs backed by a module that implements
the Exception
behaviour. The Exception
behaviour requires
two functions to be implemented:
exception/1
- receives the arguments given toraise/2
and returns the exception struct. The default implementation accepts either a set of keyword arguments that is merged into the struct or a string to be used as the exception's message.message/1
- receives the exception struct and must return its message. Most commonly exceptions have a message field which by default is accessed by this function. However, if an exception does not have a message field, this function must be explicitly implemented.
Since exceptions are structs, the API supported by defstruct/1
is also available in defexception/1
.
Raising exceptions
The most common way to raise an exception is via raise/2
:
defmodule MyAppError do
defexception [:message]
end
value = [:hello]
raise MyAppError,
message: "did not get what was expected, got: #{inspect(value)}"
In many cases it is more convenient to pass the expected value to
raise/2
and generate the message in the Exception.exception/1
callback:
defmodule MyAppError do
defexception [:message]
@impl true
def exception(value) do
msg = "did not get what was expected, got: #{inspect(value)}"
%MyAppError{message: msg}
end
end
raise MyAppError, value
The example above shows the preferred strategy for customizing exception messages.