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    <p><code>()</code> in a function type is not a type at all, nor is it a value. <code>() =&gt;</code> is simply syntax to indicate a function that takes zero arguments. See the BNF notation in section 3.2.9 of <a href="http://www.scala-lang.org/docu/files/ScalaReference.pdf" rel="noreferrer">the Scala specification.</a></p> <p>When you see <code>(Type1, Type2, Type3) =&gt; Type4</code>, the <code>()</code> surrounds all of the argument types that the function takes. <code>(Type1, Type2, Type3)</code> is not a tuple type here -- the parentheses are just syntax. So when you see <code>() =&gt; Type4</code>, you have an argument list that takes zero parameters. Single argument functions are special, in that you can omit the parentheses. </p> <p>Functions with no arguments, specified without the <code>()</code>, simply <code>=&gt; Type4</code> are by-name parameters (which look like values, but are implemented as functions that get called implicitly when you try to evaluate their value.) They're defined in section 4.6.1 of the spec.</p> <p>Parentheses can mean a bunch of other things in other contexts.</p> <ul> <li>Parentheses in a function type declaration anywhere other than right before the <code>=&gt;</code> indicate a tuple type, and these can't be empty. (Section 3.2.5)</li> <li><p>Empty parentheses <code>()</code> are the return value of functions whose return type is <code>Unit</code>. (Section 12.2.3) This actually becomes a real value in certain contexts, for example <code>println(println("foo"))</code> will print</p> <pre><code>foo &lt;--- the argument to the inner println () &lt;--- the argument to the outer println </code></pre></li> <li><p>Parentheses are also syntax for function arguments when you make a function call, something that you're probably already aware of. These can be empty. They're defined in section 6.6 of the spec.</p></li> <li>Parentheses are also syntax for function arguments when you define a method using <code>def</code>. These are described in section 4.6 of the spec.</li> <li>Parentheses are also the syntax for creating tuple values (Section 6.9). If they're not empty, you get a <code>TupleN</code> where <code>N</code> is the number of values inside of the parentheses. If they are empty, the type of this expression is <code>Unit</code>. (Sections 6.9, 12.2.3)</li> </ul>
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