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copied!<p>It's not that <code>()</code> must be added after (only) a <em>closure</em> in <code>defer</code>. The language specs for the <a href="http://golang.org/ref/spec#Defer_statements">defer statement</a> mandate that its 'Expression' <em>always</em> must be a function call.</p> <p>And why is it so? It's the same as with any other function, in 'defer' or not:</p> <p>Consider:</p> <pre><code>func f() int { return 42 } </code></pre> <p>and</p> <pre><code>a := f </code></pre> <p>vs</p> <pre><code>b := f() </code></pre> <p>The first expression RHS is a function value. In the second version the RHS is the value <em>returned by the function</em> - i.e. a function call.</p> <p>So is the semantics of:</p> <pre><code>defer f </code></pre> <p>vs</p> <pre><code>defer f() </code></pre> <p>except that the first version doesn't make sense in the context of 'defer', and so the specifications mention that it must be the second form (only).</p> <p>It's IMHO also easier to learn because of the orthogonality with the above discussed function call outside of the 'defer' statement.</p> <p>Also note that a function call is not only fn-expr followed by <code>()</code>, but an expression list is generally inside the parenthesis (including an empty list). There's a big difference between:</p> <pre><code>for i := range whatever { defer func() { fmt. Println(i) }() } </code></pre> <p>and </p> <pre><code>for i := range whatever { defer func(n int) { fmt. Println(n) }(i) } </code></pre> <p>The first version prints the value of 'i' in the moment when the closure <em>executes</em>, the second prints the value of 'i' in the moment when the defer statement <em>was</em> executed.</p>

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