title: What do parentheses () used on their own mean?
body: <p>I read in <a href="http://learnyouahaskell.com/" rel="noreferrer">learn you haskell</a> that</p>
  
  <blockquote>
    <p>Enum members are sequentially ordered types ... <strong>Types in this class:
    ()</strong>, Bool, Char ...</p>
  </blockquote>
  
  <p>Also it appears in some signatures:</p>
  
  <pre><code>putChar  ::    Char -&gt; IO ()
  </code></pre>
  
  <p>It is very difficult to find info about it in Google as the answers refer to problems of the "common parentheses" (use in function calls, precedence problems and the like).</p>
  
  <p>Therefore, what does the expression <code>()</code> means? Is it a type? What are variables of type <code>()</code>? What is used for and when is it needed?</p>