title: What do people use the identity function for?
body: <p>For example, <code>Haskell</code> has the <code>id</code> function, Julia has the <code>identity</code> function, and many questions on SO deal with the identity function. (I suppose in Python you can do <code>lambda x:x</code>)</p>
  
  <p>I've been busting my brain trying to think of a use case for this function; and I've failed.</p>
  
  <p><em>What is the purpose of the identity function, and what are its common use-cases?</em></p>