Effective Go: append and a few words about ...

R=rsc, gri, iant
CC=golang-dev
https://golang.org/cl/2821041

doc/effective_go.html

@@ -1218,6 +1218,11 @@ func Append(slice, data[]byte) []byte {
 We must return the slice afterwards because, although <code>Append</code>
 can modify the elements of <code>slice</code>, the slice itself (the run-time data
 structure holding the pointer, length, and capacity) is passed by value.
+<p>
+The idea of appending to a slice is so useful it's captured by the
+<code>append</code> built-in function.  To understand that function's
+design, though, we need a little more information, so we'll return
+to it later.
 </p>
 
 
@@ -1465,6 +1470,10 @@ func Println(v ...interface{}) {
 }
 </pre>
 <p>
+We write <code>...</code> after <code>v</code> in the call to <code>Output</code> to tell the
+compiler to treat <code>v</code> as a list of arguments; otherwise it would just pass
+<code>v</code> as a single slice argument.
+<p>
 There's even more to printing than we've covered here.  See the <code>godoc</code> documentation
 for package <code>fmt</code> for the details.
 </p>
@@ -1484,6 +1493,47 @@ func Min(a ...int) int {
 }
 </pre>
 
+<h3 id="append">Append</h3>
+<p>
+Now we have the missing piece we needed to explain the design of
+the <code>append</code> built-in function.  The signature of <code>append</code>
+is different from our custom <code>Append</code> function above.
+Schematically, it's like this:
+<pre>
+func append(slice []<i>T</i>, elements...T) []<i>T</i>
+</pre>
+where <i>T</i> is a placeholder for any given type.  You can't
+actually write a function in Go where the type <code>T</code>
+is determined by the caller.
+That's why <code>append</code> is built in: it needs support from the
+compiler.
+<p>
+What <code>append</code> does is append the elements to the end of
+the slice and return the result.  The result needs to be returned
+because, as with our hand-written <code>Append</code>, the underlying
+array may change.  This simple example
+<pre>
+x := []int{1,2,3}
+x = append(x, 4, 5, 6)
+fmt.Println(x)
+</pre>
+prints <code>[1 2 3 4 5 6]</code>.  So <code>append</code> works a
+little like <code>Printf</code>, collecting an arbitrary number of
+arguments.
+<p>
+But what if we wanted to do what our <code>Append</code> does and
+append a slice to a slice?  Easy: use <code>...</code> at the call
+site, just as we did in the call to <code>Output</code> above.  This
+snippet produces identical output to the one above.
+<pre>
+x := []int{1,2,3}
+y := []int{4,5,6}
+x = append(x, y...)
+fmt.Println(x)
+</pre>
+Without that <code>...</code>, it wouldn't compile because the types
+would be wrong; <code>y</code> is not of type <code>int</code>.
+
 <h2 id="initialization">Initialization</h2>
 
 <p>