What does the fail keyword do in Ruby?

I am learning Ruby and encountered the fail keyword. What does it mean?

if password.length < 8
   fail "Password too short"
end
unless  username
   fail "No user name set"
end

In Ruby, fail is synonymous with raise. The fail keyword is a method of the Kernel module which is included by the class Object. The fail method raises a runtime error just like the raise keyword.

The fail method has three overloads:

• fail: raises a RuntimeError without an error message.

• fail(string): raises a RuntimeError with the string argument as an error message:

   fail "Failed to open file"
  

• fail(exception [, string [, array]]): raises an exception of class exception (first argument) with an optional error message (second argument) and callback information (third argument).

  Example: Assume you define a function which should fail if given a bad argument. It is better to raise an ArgumentError and not a RuntimeError:

   fail ArgumentError, "Illegal String"
  

  Another Example: You can pass the whole backtrace to the fail method so you can access the trace inside the rescue block:

   fail ArgumentError, "Illegal String", caller
  

  caller is a Kernel method which returns the backtrace as an array of strings in the form file:line: in 'method'.

> With no arguments, raises the exception in $! or raises a RuntimeError > if $! is nil. With a single String argument, raises a RuntimeError > with the string as a message. Otherwise, the first parameter should be > the name of an Exception class (or an object that returns an Exception > object when sent an exception message). The optional second parameter > sets the message associated with the exception, and the third > parameter is an array of callback information. Exceptions are caught > by the rescue clause of begin...end blocks.

Source: Ruby Documentation on the Kernel Module.

Rubocop says about usage of both words; > 'Use fail instead of raise to signal exceptions.' > > 'Use raise instead of fail to rethrow exceptions.'

Here is an example.

def sample
  fail 'something wrong' unless success?
rescue => e
  logger.error e
  raise
end

fail == raise

In other words, fail is just a popular alias for raise error-raising method. Usage:

fail ArgumentError, "Don't argue with me!"

www.ruby-doc.org is your friend. When I googled rubydoc fail "Kernel" was the first hit. My advice is, when in doubt, go to the definitive source for definitional stuff like this.