doc/talks/go_talk-20100323.html - go - Git at Google

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 <div class="slide titlepage">
 <br/>
 <br/>
 <img src="../go-logo-white.png" width="588px" height="217px">
 <br/>
 <h1 style="padding-right: 0pt; margin-right: 0pt; color: #0066cc; font-size: 250%; border-bottom: 0px;">The Go Programming Language</h1>
 <div style="color: #ffcc00;">
 <br/>
 <h3>Sydney University<br/><br/>March 23, 2010</h3>
 </div>
 </div>

 <div class="slide">
 	<h1>Go</h1>

 	<h2>New</h2>
 	<h2>Experimental</h2>
 	<h2>Concurrent</h2>
 	<h2>Garbage Collected</h2>
 	<h2>Systems Language</h2>
 </div>

 <div class="slide">
 	<h1>Hello, world</h1>
 <pre>
 package main

 import "fmt"

 func main() {
 	fmt.Printf("Hello, 世界\n")
 }
 </pre>
 </div>

 <div class="slide">
 	<h1>Hello, world 2.0</h1>

 	<h2>Serving <a href="http://localhost:8080/world">http://localhost:8080/world</a></h2>
 <pre>
 package main

 import (
 	"fmt"
 	"http"
 )

 func handler(c *http.Conn, r *http.Request) {
 	fmt.Fprintf(c, "Hello, %s.", r.URL.Path[1:])
 }

 func main() {
 	http.ListenAndServe(":8080",
 			http.HandlerFunc(handler))
 }
 </pre>
 </div>

 <div class="slide">
 	<h1>New</h1>

 	<h2>It's about two years old:</h2>
 	<ul>
 		<li>Design started in late 2007</li>
 		<li>Implementation starting to work mid-2008</li>
 		<li>Released as an open source project in November 2009</li>
 		<li>Development continues with an active community</li>
 	</ul>

 	<h2>Why invent a new language? Older languages weren't designed for concurrency, but modern software needs it:</h2>
 	<ul>
 		<li>Large scale, networked computing, such as Google web search</li>
 		<li>Multi-core hardware</li>
 	</ul>
 </div>

 <div class="slide">
 	<h1>New</h1>

 	<h2>Older languages are also frustrating on a day-to-day basis</h2>
 	<h2>Statically-typed languages (C, C++, Java) have issues:</h2>
 	<ul>
 		<li>Edit-Compile-Run cycle takes far too long</li>
 		<li>Type hierarchy can hurt as much as it helps</li>
 	</ul>
 <div style="text-align:center">
 <img src="java-typing.png" width="800px" height="90px"><br>
 </div>

 	<h2>Dynamic languages (Python, JavaScript) fix some issues but introduce others:</h2>
 	<ul>
 		<li>No compilation means slow code</li>
 		<li>Runtime errors that should be caught statically</li>
 	</ul>

 	<h2>Go has the lighter feel of a scripting language but is compiled</h2>
 </div>

 <div class="slide">
 	<h1>New</h1>

 	<h2>Large C++ programs (e.g. Firefox, OpenOffice, Chromium) have enormous build times:</h2>
 	<ul>
 		<li>XKCD's #1 Programmer Excuse for Legitimately Slacking Off: &quot;<a href="http://xkcd.com/303/">My Code's Compiling</a>&quot;</li>
 	</ul>

 	<h2>On a Mac (OS X 10.5.8, gcc 4.0.1):</h2>
 	<ul>
 		<li>C: <code>#include &lt;stdio.h&gt;</code> reads 360 lines from 9 files</li>
 		<li>C++: <code>#include &lt;iostream&gt;</code> reads 25,326 lines from 131 files</li>
 		<li>Objective-C: <code>#include &lt;Carbon/Carbon.h&gt;</code> reads 124,730 lines from 689 files</li>
 		<li>We haven't done any real work yet!</li>
 	</ul>

 	<h2>In Go: <code>import "fmt"</code> reads <i>one</i> file: 184 lines summarizing 7 packages</h2>
 </div>

 <div class="slide">
 	<h1>New</h1>

 	<h2>Compilation demo</h2>
 </div>

 <div class="slide">
 	<h1>Experimental</h1>

 	<h2>Go is still unproven</h2>
 	<h2>Language is still evolving</h2>
 	<h2>Package library is incomplete</h2>
 	<h2>Concurrent garbage collection is an active research problem</h2>
 	<h2>Reviving forgotten concepts:</h2>
 	<ul>
 		<li>Go's concurrency is strongly influenced by <i>Communicating Sequential Processes</i> (Hoare, 1978)</li>
 		<li>Go has types and interfaces, but no inheritance. It is arguably more object-oriented than previously mentioned languages, being closer to the original Smalltalk meaning (1970s)</li>
 	</ul>
 </div>

 <div class="slide">
 	<h1>Concurrent</h1>

 	<h2>Unix philosophy: write <i>programs</i> that do one thing and do it well</h2>
 	<h2>Connect them with <i>pipes</i>:</h2>
 	<ul>
 		<li>How many lines of test code are there in the Go standard library?</li>
 		<li><code>find ~/go/src/pkg | grep _test.go$ | xargs wc -l</code></li>
 	</ul>

 	<h2>Unlike other languages, Go makes it easy to:</h2>
 	<ul>
 		<li>Launch <i>goroutines</i></li>
 		<li>Connect them with <i>channels</i></li>
 	</ul>
 </div>

 <div class="slide">
 	<h1>Concurrent</h1>

 	<h2>Start a new flow of control with the <code>go</code> keyword</h2>
 	<h2>Parallel computation is easy:</h2>
 <pre>
 func main() {
 	go expensiveComputation(x, y, z)
 	anotherExpensiveComputation(a, b, c)
 }
 </pre>

 	<h2>Roughly speaking, a goroutine is like a thread, but lighter weight:</h2>
 	<ul>
 		<li>Goroutines have segmented stacks, and typically smaller stacks</li>
 		<li>This requires compiler support. Goroutines can't just be a C++ library on top of a thread library</li>
 	</ul>
 </div>

 <div class="slide">
 	<h1>Concurrent</h1>

 	<h2>Consider web servers ("the C10k problem"):</h2>
 	<ul>
 		<li>"Thread per connection" approach is conceptually neat, but doesn't scale well in practice</li>
 		<li>What does scale well (event-driven callbacks, asynchronous APIs) are harder to understand, maintain, and debug</li>
 		<li>We think "goroutine per connection" can scale well, and is conceptually neat</li>
 	</ul>
 <pre>
 	for {
 		rw := socket.Accept()
 		conn := newConn(rw, handler)
 		go conn.serve()
 	}
 </pre>
 </div>

 <div class="slide">
 	<h1>Concurrent</h1>

 	<h2>Let's look again at our simple parallel computation:</h2>
 <pre>
 func main() {
 	go expensiveComputation(x, y, z)
 	anotherExpensiveComputation(a, b, c)
 }
 </pre>

 	<h2>This story is incomplete:</h2>
 	<ul>
 		<li>How do we know when the two computations are done?</li>
 		<li>What are their values?</li>
 	</ul>
 </div>

 <div class="slide">
 	<h1>Concurrent</h1>

 	<h2>Goroutines communicate with other goroutines via channels</h2>
 <pre>
 func computeAndSend(ch chan int, x, y, z int) {
 	ch &lt;- expensiveComputation(x, y, z)
 }

 func main() {
 	ch := make(chan int)
 	go computeAndSend(ch, x, y, z)
 	v2 := anotherExpensiveComputation(a, b, c)
 	v1 := &lt;-ch
 	fmt.Println(v1, v2)
 }
 </pre>

 </div>

 <div class="slide">
 	<h1>Concurrent</h1>

 	<h2>In traditional concurrent programs, you <i>communicate by sharing memory</i>. In Go, you <i>share memory by communicating</i>:</h2>
 	<ul>
 		<li>Communication (the <code>&lt;-</code> operator) is sharing and synchronization</li>
 	</ul>

 	<h2>Threads and locks are concurrency primitives; CSP is a concurrency model:</h2>
 	<ul>
 		<li>Analogy: &quot;Go To Statement Considered Harmful&quot; (Dijsktra, 1968)</li>
 		<li><code>goto</code> is a control flow primitive; structured programming (<code>if</code> statements, <code>for</code> loops, function calls) is a control flow model</li>
 	</ul>

 	<h2>Learning CSP changes the way you think about concurrent programming:</h2>
 	<ul>
 		<li>Every language has its grain. If your Go program uses mutexes, you're probably working against the grain</li>
 	</ul>
 </div>

 <div class="slide">
 	<h1>Garbage Collected</h1>

 	<h2>Automatic memory management makes writing (and maintaining) programs easier</h2>
 	<h2>Especially in a concurrent world:</h2>
 	<ul>
 		<li>Who &quot;owns&quot; a shared piece of memory, and is responsible for destroying it?</li>
 	</ul>

 	<h2>Large C++ programs usually end up with semi-automatic memory management anyway, via &quot;smart pointers&quot;</h2>
 	<h2>Mixing the two models can be problematic:</h2>
 	<ul>
 		<li>Browsers can leak memory easily; DOM elements are C++ objects, but JavaScript is garbage collected</li>
 	</ul>
 </div>

 <div class="slide">
 	<h1>Garbage Collected</h1>

 	<h2>Go is also a safer language:</h2>
 	<ul>
 		<li>Pointers but no pointer arithmetic</li>
 		<li>No dangling pointers</li>
 		<li>Variables are zero-initialized</li>
 		<li>Array access is bounds-checked</li>
 	</ul>

 	<h2>No buffer overflow exploits</h2>
 </div>

 <div class="slide">
 	<h1>Systems Language</h1>

 	<h2>This just means you could write decently large programs in Go:</h2>
 	<ul>
 		<li>Web servers</li>
 		<li>Web browsers</li>
 		<li>Web crawlers</li>
 		<li>Search indexers</li>
 		<li>Databases</li>
 		<li>Word processors</li>
 		<li>Integrated Development Environments (IDEs)</li>
 		<li>Operating systems</li>
 		<li>...</li>
 	</ul>
 </div>

 <div class="slide">
 	<h1>Systems Language</h1>

 	<h2>Garbage collection has a reputation for being &quot;slower&quot;</h2>
 	<h2>We're expecting Go to be slightly slower than optimized C, but faster than Java, depending on the task. Nonetheless:</h2>
 	<ul>
 		<li>Fast and buggy is worse than almost-as-fast and correct</li>
 		<li>It is easier to optimize a correct program than to correct an optimized program</li>
 		<li>Fundamentally, it's simply a trade-off we're willing to make</li>
 	</ul>

 	<h2>Memory layout can drastically affect performance. These two designs are equivalent in Go, but significantly different in Java:</h2>
 <pre>
 type Point struct { X, Y int }
 type Rect struct { P0, P1 Point }

 // or ...

 type Rect struct { X0, Y0, X1, Y1 int }
 </pre>
 </div>

 <div class="slide">
 	<h1>Systems Language</h1>

 	<h2>Quote from http://loadcode.blogspot.com/2009/12/go-vs-java.html</h2>

 <h2>
 &quot;[Git] is known to be very fast. It is written in C. A Java version
 JGit was made. It was considerably slower. Handling of memory and lack
 of unsigned types was some of the important reasons.
 </h2>

 <h2>Shawn O. Pearce wrote on the git mailinglist:</h2>
 <ul><li>&quot;JGit struggles with not
 having an efficient way to represent a SHA-1. C can just say &quot;unsigned
 char[20]&quot; and have it inline into the container's memory allocation. A
 byte[20] in Java will cost an *additional* 16 bytes of memory, and be
 slower to access because the bytes themselves are in a different area
 of memory from the container object. We try to work around it by
 converting from a byte[20] to 5 ints, but that costs us machine
 instructions&quot;
 </li></ul>

 <h2>
 Like C, Go does allow unsigned types and defining data structures
 containing other data structures as continuous blocks of memory.&quot;
 </h2>
 </div>

 <div class="slide">
 	<h1>Go</h1>

 	<h2>New</h2>
 	<h2>Experimental</h2>
 	<h2>Concurrent</h2>
 	<h2>Garbage Collected</h2>
 	<h2>Systems Language</h2>

 	<h2>And more:</h2>
 	<ul>
 		<li>I haven't talked about the type system, interfaces, slices, closures, selects, ...</li>
 		<li>Tutorial, documentation, mailing list, source code all online</li>
 	</ul>
 </div>

 <div class="slide titlepage">
 	<h1>Questions?</h1>
 	<br><br>
 	<center>
 	<img src="../gordon/bumper640x360.png" width="640px" height="360px">
 	</center>
 </div>

 </body></html>
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	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
	<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
	<head>
	<title>Go Tech Talk</title>
	<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
	<meta name="font-size-adjustment" content="-1" />
	<link rel="stylesheet" href="slidy.css"
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	<script src="slidy.js" type="text/javascript">
	</script>
	</head>
	<body>
	<!-- this defines the slide background -->

	<div class="background">

	<div class="header">
	<!-- sized and colored via CSS -->
	</div>

	<div class="footer"></div>
	</div>

	<div class="slide titlepage">
	<br/>
	<br/>
	<img src="../go-logo-white.png" width="588px" height="217px">
	<br/>
	<h1 style="padding-right: 0pt; margin-right: 0pt; color: #0066cc; font-size: 250%; border-bottom: 0px;">The Go Programming Language</h1>
	<div style="color: #ffcc00;">
	<br/>
	<h3>Sydney University<br/><br/>March 23, 2010</h3>
	</div>
	</div>

	<div class="slide">
	<h1>Go</h1>

	<h2>New</h2>
	<h2>Experimental</h2>
	<h2>Concurrent</h2>
	<h2>Garbage Collected</h2>
	<h2>Systems Language</h2>
	</div>

	<div class="slide">
	<h1>Hello, world</h1>
	<pre>
	package main

	import "fmt"

	func main() {
	fmt.Printf("Hello, 世界\n")
	}
	</pre>
	</div>

	<div class="slide">
	<h1>Hello, world 2.0</h1>

	<h2>Serving <a href="http://localhost:8080/world">http://localhost:8080/world</a></h2>
	<pre>
	package main

	import (
	"fmt"
	"http"
	)

	func handler(c http.Conn, r http.Request) {
	fmt.Fprintf(c, "Hello, %s.", r.URL.Path[1:])
	}

	func main() {
	http.ListenAndServe(":8080",
	http.HandlerFunc(handler))
	}
	</pre>
	</div>

	<div class="slide">
	<h1>New</h1>

	<h2>It's about two years old:</h2>
	<ul>
	<li>Design started in late 2007</li>
	<li>Implementation starting to work mid-2008</li>
	<li>Released as an open source project in November 2009</li>
	<li>Development continues with an active community</li>
	</ul>

	<h2>Why invent a new language? Older languages weren't designed for concurrency, but modern software needs it:</h2>
	<ul>
	<li>Large scale, networked computing, such as Google web search</li>
	<li>Multi-core hardware</li>
	</ul>
	</div>

	<div class="slide">
	<h1>New</h1>

	<h2>Older languages are also frustrating on a day-to-day basis</h2>
	<h2>Statically-typed languages (C, C++, Java) have issues:</h2>
	<ul>
	<li>Edit-Compile-Run cycle takes far too long</li>
	<li>Type hierarchy can hurt as much as it helps</li>
	</ul>
	<div style="text-align:center">
	<img src="java-typing.png" width="800px" height="90px"><br>
	</div>

	<h2>Dynamic languages (Python, JavaScript) fix some issues but introduce others:</h2>
	<ul>
	<li>No compilation means slow code</li>
	<li>Runtime errors that should be caught statically</li>
	</ul>

	<h2>Go has the lighter feel of a scripting language but is compiled</h2>
	</div>

	<div class="slide">
	<h1>New</h1>

	<h2>Large C++ programs (e.g. Firefox, OpenOffice, Chromium) have enormous build times:</h2>
	<ul>
	<li>XKCD's #1 Programmer Excuse for Legitimately Slacking Off: "<a href="http://xkcd.com/303/">My Code's Compiling</a>"</li>
	</ul>

	<h2>On a Mac (OS X 10.5.8, gcc 4.0.1):</h2>
	<ul>
	<li>C: <code>#include <stdio.h></code> reads 360 lines from 9 files</li>
	<li>C++: <code>#include <iostream></code> reads 25,326 lines from 131 files</li>
	<li>Objective-C: <code>#include <Carbon/Carbon.h></code> reads 124,730 lines from 689 files</li>
	<li>We haven't done any real work yet!</li>
	</ul>

	<h2>In Go: <code>import "fmt"</code> reads <i>one</i> file: 184 lines summarizing 7 packages</h2>
	</div>

	<div class="slide">
	<h1>New</h1>

	<h2>Compilation demo</h2>
	</div>

	<div class="slide">
	<h1>Experimental</h1>

	<h2>Go is still unproven</h2>
	<h2>Language is still evolving</h2>
	<h2>Package library is incomplete</h2>
	<h2>Concurrent garbage collection is an active research problem</h2>
	<h2>Reviving forgotten concepts:</h2>
	<ul>
	<li>Go's concurrency is strongly influenced by <i>Communicating Sequential Processes</i> (Hoare, 1978)</li>
	<li>Go has types and interfaces, but no inheritance. It is arguably more object-oriented than previously mentioned languages, being closer to the original Smalltalk meaning (1970s)</li>
	</ul>
	</div>

	<div class="slide">
	<h1>Concurrent</h1>

	<h2>Unix philosophy: write <i>programs</i> that do one thing and do it well</h2>
	<h2>Connect them with <i>pipes</i>:</h2>
	<ul>
	<li>How many lines of test code are there in the Go standard library?</li>
	<li><code>find ~/go/src/pkg \| grep _test.go$ \| xargs wc -l</code></li>
	</ul>

	<h2>Unlike other languages, Go makes it easy to:</h2>
	<ul>
	<li>Launch <i>goroutines</i></li>
	<li>Connect them with <i>channels</i></li>
	</ul>
	</div>

	<div class="slide">
	<h1>Concurrent</h1>

	<h2>Start a new flow of control with the <code>go</code> keyword</h2>
	<h2>Parallel computation is easy:</h2>
	<pre>
	func main() {
	go expensiveComputation(x, y, z)
	anotherExpensiveComputation(a, b, c)
	}
	</pre>

	<h2>Roughly speaking, a goroutine is like a thread, but lighter weight:</h2>
	<ul>
	<li>Goroutines have segmented stacks, and typically smaller stacks</li>
	<li>This requires compiler support. Goroutines can't just be a C++ library on top of a thread library</li>
	</ul>
	</div>

	<div class="slide">
	<h1>Concurrent</h1>

	<h2>Consider web servers ("the C10k problem"):</h2>
	<ul>
	<li>"Thread per connection" approach is conceptually neat, but doesn't scale well in practice</li>
	<li>What does scale well (event-driven callbacks, asynchronous APIs) are harder to understand, maintain, and debug</li>
	<li>We think "goroutine per connection" can scale well, and is conceptually neat</li>
	</ul>
	<pre>
	for {
	rw := socket.Accept()
	conn := newConn(rw, handler)
	go conn.serve()
	}
	</pre>
	</div>

	<div class="slide">
	<h1>Concurrent</h1>

	<h2>Let's look again at our simple parallel computation:</h2>
	<pre>
	func main() {
	go expensiveComputation(x, y, z)
	anotherExpensiveComputation(a, b, c)
	}
	</pre>

	<h2>This story is incomplete:</h2>
	<ul>
	<li>How do we know when the two computations are done?</li>
	<li>What are their values?</li>
	</ul>
	</div>

	<div class="slide">
	<h1>Concurrent</h1>

	<h2>Goroutines communicate with other goroutines via channels</h2>
	<pre>
	func computeAndSend(ch chan int, x, y, z int) {
	ch <- expensiveComputation(x, y, z)
	}

	func main() {
	ch := make(chan int)
	go computeAndSend(ch, x, y, z)
	v2 := anotherExpensiveComputation(a, b, c)
	v1 := <-ch
	fmt.Println(v1, v2)
	}
	</pre>

	</div>

	<div class="slide">
	<h1>Concurrent</h1>

	<h2>In traditional concurrent programs, you <i>communicate by sharing memory</i>. In Go, you <i>share memory by communicating</i>:</h2>
	<ul>
	<li>Communication (the <code><-</code> operator) is sharing and synchronization</li>
	</ul>

	<h2>Threads and locks are concurrency primitives; CSP is a concurrency model:</h2>
	<ul>
	<li>Analogy: "Go To Statement Considered Harmful" (Dijsktra, 1968)</li>
	<li><code>goto</code> is a control flow primitive; structured programming (<code>if</code> statements, <code>for</code> loops, function calls) is a control flow model</li>
	</ul>

	<h2>Learning CSP changes the way you think about concurrent programming:</h2>
	<ul>
	<li>Every language has its grain. If your Go program uses mutexes, you're probably working against the grain</li>
	</ul>
	</div>

	<div class="slide">
	<h1>Garbage Collected</h1>

	<h2>Automatic memory management makes writing (and maintaining) programs easier</h2>
	<h2>Especially in a concurrent world:</h2>
	<ul>
	<li>Who "owns" a shared piece of memory, and is responsible for destroying it?</li>
	</ul>

	<h2>Large C++ programs usually end up with semi-automatic memory management anyway, via "smart pointers"</h2>
	<h2>Mixing the two models can be problematic:</h2>
	<ul>
	<li>Browsers can leak memory easily; DOM elements are C++ objects, but JavaScript is garbage collected</li>
	</ul>
	</div>

	<div class="slide">
	<h1>Garbage Collected</h1>

	<h2>Go is also a safer language:</h2>
	<ul>
	<li>Pointers but no pointer arithmetic</li>
	<li>No dangling pointers</li>
	<li>Variables are zero-initialized</li>
	<li>Array access is bounds-checked</li>
	</ul>

	<h2>No buffer overflow exploits</h2>
	</div>

	<div class="slide">
	<h1>Systems Language</h1>

	<h2>This just means you could write decently large programs in Go:</h2>
	<ul>
	<li>Web servers</li>
	<li>Web browsers</li>
	<li>Web crawlers</li>
	<li>Search indexers</li>
	<li>Databases</li>
	<li>Word processors</li>
	<li>Integrated Development Environments (IDEs)</li>
	<li>Operating systems</li>
	<li>...</li>
	</ul>
	</div>

	<div class="slide">
	<h1>Systems Language</h1>

	<h2>Garbage collection has a reputation for being "slower"</h2>
	<h2>We're expecting Go to be slightly slower than optimized C, but faster than Java, depending on the task. Nonetheless:</h2>
	<ul>
	<li>Fast and buggy is worse than almost-as-fast and correct</li>
	<li>It is easier to optimize a correct program than to correct an optimized program</li>
	<li>Fundamentally, it's simply a trade-off we're willing to make</li>
	</ul>

	<h2>Memory layout can drastically affect performance. These two designs are equivalent in Go, but significantly different in Java:</h2>
	<pre>
	type Point struct { X, Y int }
	type Rect struct { P0, P1 Point }

	// or ...

	type Rect struct { X0, Y0, X1, Y1 int }
	</pre>
	</div>

	<div class="slide">
	<h1>Systems Language</h1>

	<h2>Quote from http://loadcode.blogspot.com/2009/12/go-vs-java.html</h2>

	<h2>
	"[Git] is known to be very fast. It is written in C. A Java version
	JGit was made. It was considerably slower. Handling of memory and lack
	of unsigned types was some of the important reasons.
	</h2>

	<h2>Shawn O. Pearce wrote on the git mailinglist:</h2>
	<ul><li>"JGit struggles with not
	having an efficient way to represent a SHA-1. C can just say "unsigned
	char[20]" and have it inline into the container's memory allocation. A
	byte[20] in Java will cost an additional 16 bytes of memory, and be
	slower to access because the bytes themselves are in a different area
	of memory from the container object. We try to work around it by
	converting from a byte[20] to 5 ints, but that costs us machine
	instructions"
	</li></ul>

	<h2>
	Like C, Go does allow unsigned types and defining data structures
	containing other data structures as continuous blocks of memory."
	</h2>
	</div>

	<div class="slide">
	<h1>Go</h1>

	<h2>New</h2>
	<h2>Experimental</h2>
	<h2>Concurrent</h2>
	<h2>Garbage Collected</h2>
	<h2>Systems Language</h2>

	<h2>And more:</h2>
	<ul>
	<li>I haven't talked about the type system, interfaces, slices, closures, selects, ...</li>
	<li>Tutorial, documentation, mailing list, source code all online</li>
	</ul>
	</div>

	<div class="slide titlepage">
	<h1>Questions?</h1>
	<br><br>
	<center>
	<img src="../gordon/bumper640x360.png" width="640px" height="360px">
	</center>
	</div>

	</body></html>