What does the fail keyword do in Ruby?

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.