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copied!<p>Scala distinguishes between the following things:</p> <ul> <li>Functions/methods with <em>no parameter lists</em> ("by-name parameter" if a function)</li> <li>Functions with <em>one empty parameter list</em></li> <li>Functions with <em>one parameter of type Unit</em></li> </ul> <p>None of these are equivalent, although as a convenience Scala allows you to elide empty parameter lists. (Incidentally, two empty parameter lists are also not the same.)</p> <p>So, even though <code>Unit</code> is written <code>()</code>, this is <em>not</em> the same as the function argument parens <code>()</code> for a function or method. Instead, think of <code>()</code> as a <code>Tuple0</code>.</p> <p>So, if you say <code>f: Unit => Int</code>, what you mean is "f takes one parameter, but it's a really boring parameter because it is <code>Unit</code>, which must always be the same boring <code>Tuple0</code> value <code>()</code>". What you're writing is really short for <code>f: (Unit) => Int</code>.</p> <p>If you say <code>f: () => Int</code>, then you mean that "f takes no parameters and produces an <code>Int</code>".</p> <p>If you say <code>f: => Int</code>, then you mean that "delay the execution of whatever statement produces an <code>Int</code> value until we use it in this code (and re-evaluate it each time)". Functionally, this ends up being basically the same as <code>f: () => Int</code> (and internally is converted into the same <code>Function0</code> class), but it has a different usage, presumably to allow for a more compact form of closures (you always omit the <code>=></code> in the calling code).</p>

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