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A simple mixin to make defining custom exceptions in ruby classes easier
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define_exception - Custom exceptions with default messages

This gem provides a dry way of defining custom exceptions in your application with default messages so that you don't have to constantly define a message when using raise.


gem install define_exception

Typical Exception Definition

It is common in ruby applications to write a custom exception to indicate a special error has occurred in your application

class MyCustomException < RuntimeError; end #nodoc

This can then be invoked by raise with a message

raise( MyCustomException, 'Your custom error message here' ) unless ...

If you raise this exception multiple times but want the same error message to be presented to the user you have to repeat the above statement which is cumbersome. You could define a hash for common error messages but this divorces the exception from the message it is to deliver.

A Better Way

A better way of approaching the problem is to define the exception class with a default message while preserving the ability to override it using raise. This may sound trivial but it is not so straightforward when you subclass off of the ruby standard exception classes. This can be done easily with this gem using any of the following constructs:

class MyClass
  define_exception 'MyTestException', 'This is my default message'
  define_exception :AnotherTestException, 'This is the default message for another exception'
  define_exception :yet_another_exception, 'There is always more than one way'

Syntactically this has more feel like attr_accessor and is succinct. The first argument is either a string or symbol that defines the name of the exception. Usage of the underscore in the symbol name automatically gets converted to camel case for the exception name. The second argument is the string to define the default message. This allows simple error handling to occur repeatedly

raise MyTestException unless ...

The ability to override the default message is still possible as well

raise MyTestException, 'This is a one time error message'

Changing the Parent Class

By default the parent class is RuntimeError. For cases where you need a different parent class you can supply the class as an optional third argument

Class MyClass
  define_exception 'MyTestException', 'This is my default message', ArgumentError


See examples/


For examples on how to test code written using this gem using RSpec see test/define_exception_spec.rb

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