RuntimeError

Constantly perplexed/habitually annoyed.

» Ruby's Favorite Exception

Ruby uses RuntimeError a lot, so much so that it's the default exception class when calling raise without an Exception class:

raise "creature is too angry"

is equivalent to:

raise RuntimeError, "creature is too angry"

and raises the exception:

RuntimeError: creature is too angry"

It's common to use the simpler form of raise when you don't really care about the exception class and just want to stop the program from continuing. For example, you could use a simple raise to guard the input to a method:

def rawr(creature)
  raise "this creature can't rawr :(" unless creature.respond_to?(:rawr)
  creature.rawr
end

Without the guard, accidentally calling rawr with nil would raise a NoMethodError:

rawr(nil) # => NoMethodError: undefined method `rawr' for nil:NilClass

With the guard, we get a more helpful exception:

rawr(nil) # => RuntimeError: this creature can't rawr :(

Of course, RuntimeError can also be overused. Because it inherits directly from StandardError, you must go pretty broad to rescue RuntimeError, which isn't usually a good idea. If you are building an API that will be used by others, it's a good idea to raise your own exception classes.

Fun fact: Prior to version 2.5, Ruby raised RuntimeError when attempting to modify a frozen object. Ruby 2.5 introduced FrozenError, a more specific error class.