How many times do you use some_array.first as a way to get at the one and only member of that array?

It always bugged me that this can be error prone - if the array has more than one component you’ll never know about it, and the .first method isn’t semantically describing what you are doing - grabbing the one and only member.

If you add this somewhere in lib/initializiers, you get the #onely! method

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class Array
  def only!
    raise "called Array onely! with array of length #{self.length}" if self.length > 1
    self.first
  end
end

which provides a more semantic description of what you’re trying to do:

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> [].onely!
 => nil
> [1].onely
 => 1
> [1,2,3].onely
RuntimeError: called Array#onely! with array of length 3

Update 4/15/2014: As pointed out by @adamwong246 - “only” (the original method name I used) is already in use by ActiveRecord. He suggests “onely!” which has a nice ring to it.