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<h2 class="slidenum" id="num"></h2>
<h1>A Tour of Go</h1>
<div class="slides">

<div class="toc">Introduction</div>

<div class="slide">
	<h2>Hello, 世界</h2>
	<p>
	Welcome to a tour of the Go programming language.
	<p>
	To run this program, click the Compile button (or type Shift-Enter).
	<p>
	The programs in the tour are meant to be starting points for your own experimentation.
	Edit the program and run it again.
<div>
package main

import "fmt"

func main() {
	fmt.Println("Hello, 世界")
}
</div>
</div>

<div class="slide">
	<h2>Hey</h2>
	<p>
	Now that the formalities are over, we can look at what's going on.
	When you type Shift-Enter, your computer compiles and runs the
	program and displays the result below.
	<p>
	Example programs demonstrate different aspects of Go.
	Play with them to try new things and explore the language.
	<p>
	Whenever you're ready to move on, click the Next button or type the PageDown key.
<div>
package main

import "fmt"

func main() {
	fmt.Println("Hey")
}
</div>
</div>

<div class="slide">
	<h2>Imports</h2>
	<p>
	This program shows a factored import statement.
	You can also write multiple import statements, like:
	<pre>
	import "fmt"
	import "math"
	</pre>
	but it's common to use the factored form to eliminate clutter.
<div>
package main

import (
	"fmt"
	"math"
)

func main() {
	fmt.Printf("Now you have %g problems.",
		math.Nextafter(2, 3))
}
</div>
</div>

<div class="slide">
	<h2>Packages</h2>
	<p>
	Every Go program is made up of packages.
	<p>
	Programs start running in package <code>main</code>.
	<p>
	This program is using the packages with import paths
	<code>"fmt"</code> and <code>"math"</code>.
	<p>
	By convention, the package name is the same as the
	last element of the import path.
<div>
package main

import (
	"fmt"
	"math"
)

func main() {
	fmt.Println("Happy", math.Pi, "Day")
}
</div>
</div>

<div class="slide">
	<h2>Exported names</h2>
	<p>
	After importing a package, you can refer to the names it exports.
	<p>
	In Go, a name is exported if it begins with a capital letter.
	<p>
	<code>Pi</code> is an exported name; so is <code>PI</code>; not <code>pi</code>.
<div>
package main

import (
	"fmt"
	"math"
)

func main() {
	fmt.Println(math.pi)
}
</div>
</div>

<div class="slide">
	<h2>Functions</h2>
	<p>
	A function can take zero or more arguments.
	<p>
	In this example, <code>add</code> takes two parameters of type <code>int</code>.
	<p>
	Notice that the type comes <i>after</i> the variable name.
<div>
package main

import "fmt"

func add(x int, y int) int {
	return x + y
}

func main() {
	fmt.Println(add(42, 13))
}
</div>
</div>

<div class="slide">
	<h2>Functions</h2>
	<p>
	When two or more consecutive function parameters share a type,
	you can omit the type from all but the last.
	<p>
	In this example, we shortened <code>x int, y int</code> to <code>x, y int</code>.
<div>
package main

import "fmt"

func add(x, y int) int {
	return x + y
}

func main() {
	fmt.Println(add(42, 13))
}
</div>
</div>

<div class="slide">
	<h2>Functions</h2>
	<p>
	A function can return zero or more results.
	<p>
	This function returns two strings.
<div>
package main

import "fmt"

func swap(x, y string) (string, string) {
	return y, x
}

func main() {
	fmt.Println(swap("hello", "world"))
}
</div>
</div>

<div class="slide">
	<h2>Functions</h2>
	<p>
	Function results can be named, in which case a
	<code>return</code> statement without arguments
	returns the current values of the results.
<div>
package main

import "fmt"

func split(sum int) (x, y int) {
	x = sum * 4/9
	y = sum - x
	return
}

func main() {
	fmt.Println(split(17))
}
</div>
</div>

<div class="slide">
	<h2>Variables</h2>
	<p>
	The <code>var</code> statement declares a list of variables;
	like in function argument lists, the type is last.
	<p>
	(For more about why types look the way they do, see <a target="_blank" href="http://blog.golang.org/2010/07/gos-declaration-syntax.html">this blog post</a>.)

<div>
package main

import "fmt"

var x, y, z int
var c, python, java bool

func main() {
	fmt.Println(x, y, z, c, python, java)
}
</div>
</div>

<div class="slide">
	<h2>Variables</h2>
	<p>
	A var declaration can include initializers, one per variable.
	<p>
	If an initializer is present, the type can be omitted;
	the variable will take the type of the initializer.
<div>
package main

import "fmt"

var x, y, z int = 1, 2, 3
var c, python, java = true, false, "no way!"

func main() {
	fmt.Println(x, y, z, c, python, java)
}
</div>
</div>

<div class="slide">
	<h2>Variables</h2>
	<p>
	Inside a function, the <code>:=</code> short assignment statement
	can be used in place of the short <code>var</code> declaration.
<div>
package main

import "fmt"

func main() {
	var x, y, z int = 1, 2, 3
	c, python, java := true, false, "certainly not"

	fmt.Println(x, y, z, c, python, java)
}
</div>
</div>

<div class="slide">
	<h2>Constants</h2>
	<p>
	Constants are declared like variables, but with the <code>const</code> keyword.
<div>
package main

import "fmt"

const Beta = 'β'

const (
	Pi = 3.14
	World = "世界"
)

func main() {
	fmt.Println("Hello", World)
	fmt.Println("Happy", Pi, "Day")
}
</div>
</div>

<div class="slide">
	<h2>Constants</h2>
	<p>
	Constants are high-precision <i>values</i>: numbers and strings.
	<p>
	An untyped constant takes the type needed by its context.
	<p>
	Try printing <code>needInt(Big)</code> too.

<div>
package main

import "fmt"

const (
	Big = 1<<100
	Small = Big>>99
)

func needInt(x int) int { return x*10 + 1 }
func needFloat(x float64) float64 { return x*0.1 }

func main() {
	fmt.Println(needInt(Small),
		needFloat(Small), needFloat(Big))
}
</div>
</div>

<div class="slide">
	<h2>For</h2>
	<p>
	Go has a single looping construct, the <code>for</code> loop.
	<p>
	The basic <code>for</code> loop looks like it does in C or Java,
	except that the <code>( )</code> are gone and the <code>{ }</code> are required.
<div>
package main

import "fmt"

func main() {
	sum := 0
	for i := 0; i < 10; i++ {
		sum += i
	}
	fmt.Println(sum)
}
</div>
</div>

<div class="slide">
	<h2>For</h2>
	<p>
	Like in C or Java, you can leave the pre and post statements empty.
<div>
package main

import "fmt"

func main() {
	sum := 1
	for ; sum < 1000; {
		sum += sum
	}
	fmt.Println(sum)
}
</div>
</div>

<div class="slide">
	<h2>For</h2>
	<p>
	At that point you can drop the semicolons:
	C's <code>while</code> is spelled <code>for</code> in Go.
<div>
package main

import "fmt"

func main() {
	sum := 1
	for sum < 1000 {
		sum += sum
	}
	fmt.Println(sum)
}
</div>
</div>

<div class="slide">
	<h2>For</h2>
	<p>
	If you omit the loop condition, it loops forever.
<div>
package main

func main() {
	for ; ; {
	}
}
</div>
</div>

<div class="slide">
	<h2>For</h2>
	<p>
	Like before, you can drop the semicolons.
<div>
package main

func main() {
	for {
	}
}
</div>
</div>

<div class="slide">
	<h2>If</h2>
	<p>
	The <code>if</code> statement looks like it does in C or Java,
	except that the <code>( )</code> are gone and the <code>{ }</code> are required.
	(Sound familiar?)
<div>
package main

import (
	"fmt"
	"math"
)

func sqrt(x float64) string {
	if x < 0 {
		return sqrt(-x) + "i"
	}
	return fmt.Sprint(math.Sqrt(x))
}

func main() {
	fmt.Println(sqrt(2), sqrt(-4))
}
</div>
</div>

<div class="slide">
	<h2>If</h2>
	<p>
	Like <code>for</code>, the <code>if</code> statement can start with a short
	statement to execute before the condition.
	<p>
	Variables declared by the statement are only in scope until the end of the <code>if</code>.
	(Try adding <code>neg</code> to the <code>fmt.Sprint</code> call.)
<div>
package main

import (
	"fmt"
	"math"
)

func sqrt(x float64) string {
	if neg := math.Signbit(x); neg {
		return sqrt(-x) + "i"
	}
	return fmt.Sprint(math.Sqrt(x))
}

func main() {
	fmt.Println(sqrt(2), sqrt(-4))
}
</div>
</div>

<div class="slide">
	<h2>Basic types</h2>
	<p>
	Go's basic types are
	<pre>
	bool
	string
	int       int8      int16     int32     int64
	uint      uint8     uint16    uint32    uint64
	float32   float64
	complex64 complex128
	</pre>
<div>
package main

import (
	"cmath"
	"fmt"
)

var (
	ToBe bool = false
	MaxInt uint64 = 1<<64 - 1
	z complex128 = cmath.Sqrt(-5+12i)
)

func main() {
	fmt.Printf("%T(%v) %T(%v) %T(%v)\n",
		ToBe, ToBe,
		MaxInt, MaxInt,
		z, z,
	)
}
</div>
</div>

<div class="slide">
	<h2>Structs</h2>
	<p>
	A <code>struct</code> is a collection of fields.
	<p>
	(And a <code>type</code> declaration does what you'd expect.)
<div>
package main

import "fmt"

type Point struct {
	X int
	Y int
}

func main() {
	fmt.Println(Point{1, 2})
}
</div>
</div>

<div class="slide">
	<h2>Pointers</h2>
	<p>
	Go has pointers, but no pointer arithmetic.
<div>
package main

import "fmt"

type Point struct {
	X int
	Y int
}

func main() {
	p := Point{1, 2}
	q := &p
	q.X = 1e9
	fmt.Println(p)
}
</div>
</div>

<div class="slide">
	<h2>Struct Literals</h2>
	<p>
	A struct literal denotes a struct value by listing the values
	of its fields.
	<p>
	You can list just a few fields by using the <code>Name:</code> syntax.
	<p>
	The special prefix <code>&</code> constructs a pointer to a struct literal.
<div>
package main

import "fmt"

type Point struct {
	X, Y int
}

var (
	p = Point{1, 2}   // has type Point
	q = &Point{1, 2}  // has type *Point
	r = Point{X: 1}   // Y:0 is implicit
	s = Point{}       // X:0 and Y:0 are implicit
)

func main() {
	fmt.Println(p, q, r, s)
}
</div>
</div>

<div class="slide">
	<h2>Maps</h2>
	<p>
	A map maps keys to values.
	<p>
	<!-- TODO: empty part not true in compilers yet -->
	Maps must be created with <code>make</code> before use; the <code>nil</code> map is empty
	and cannot be assigned to.
<div>
package main

import "fmt"

type Point struct {
	Lat, Long float64
}

var m map[string]Point

func main() {
	m = make(map[string]Point)
	m["Bell Labs"] = Point{40.68433, 74.39967}
	fmt.Println(m["Bell Labs"])
}
</div>
</div>

<div class="slide">
	<h2>Maps</h2>
	<p>
	Map literals are like struct literals, but the keys are required.
<div>
package main

import "fmt"

type Point struct {
	Lat, Long float64
}

var m = map[string]Point{
	"Bell Labs": Point{40.68433, -74.39967},
	"Google":    Point{37.42202, -122.08408},
}

func main() {
	fmt.Println(m)
}
</div>
</div>

<div class="slide">
	<h2>Slices</h2>
	<p>
	A slice points at an array of values and also includes a length.
<div>
package main

import "fmt"

type Point struct {
	Lat, Long float64
}

var places = []Point{
	Point{40.68433, -74.39967},
	Point{37.42202, -122.08408},
}

var empty []Point

var five = make([]Point, 5)

func main() {
	fmt.Println("Empty: ", len(empty), empty)
	fmt.Println("Five:  ", len(five), five)
	fmt.Println("Places:", len(places), places)

	primes := []int{2, 3, 5, 7, 11, 13}
	fmt.Println("Primes:", primes)
}
</div>
</div>

<div class="slide">
	<h2>Functions</h2>
	<p>
	Functions are values too.
<div>
package main

import (
	"fmt"
	"math"
)

func main() {
	hypot := func(x, y float64) float64 {
		return math.Sqrt(x*x + y*y)
	}

	fmt.Println(hypot(3, 4))
}
</div>
</div>

<div class="slide">
	<h2>Functions</h2>
	<p>
	And functions are full closures.
<div>
package main

import "fmt"

func adder() func(int) int {
	sum := 0
	return func(x int) int {
		sum += x
		return sum
	}
}

func main() {
	pos, neg := adder(), adder()

	for i := 0; i < 10; i++ {
		fmt.Println(i, pos(i), neg(-2*i))
	}
}
</div>
</div>

<div class="slide">
	<h2>Range</h2>
	<p>
	The <code>range</code> form of the <code>for</code>
	loop iterates over a slice or map.
<div>
package main

import "fmt"

var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}

func main() {
	for key, value := range pow {
		fmt.Printf("2^%d = %d\n", key, value)
	}
}
</div>
</div>

<div class="slide">
	<h2>Range</h2>
	<p>
	You can skip the key or value by assigning to <code>_</code>.
	<p>
	You can also drop the &ldquo;<code>, value</code>&rdquo; entirely.
<div>
package main

import "fmt"

func main() {
	pow := make([]int, 10)
	for i := range pow {
		pow[i] = 1&lt;&lt;uint(i)
	}
	for _, value := range pow {
		fmt.Printf("%d\n", value)
	}
}
</div>
</div>

<div class="slide">
	<h2>Switch</h2>
	<p>
	You probably knew what <code>switch</code> was going to look like.
	<p>
	A case body breaks automatically, unless it ends with a <code>fallthrough</code> statement.

<div>
package main

import (
	"fmt"
	"runtime"
)

func main() {
	switch os := runtime.GOOS; os {
	case "darwin":
		fmt.Printf("Go runs on OS X.\n")
	case "freebsd":
		fmt.Printf("Go runs on FreeBSD.\n")
	case "linux":
		fmt.Printf("Go runs on Linux.\n")
	case "plan9":
		fmt.Printf("Go runs on Plan 9.\n")
	case "windows":
		fmt.Printf("Go runs on Windows.\n")
	default:
		fmt.Printf("I forgot %s.\n", os)
	}
}
</div>
</div>

<div class="slide">
	<h2>Switch</h2>
	<p>
	Switch cases don't have to be constants; the first match wins.

<div>
package main

import (
	"fmt"
	"time"
)

func main() {
	today := time.LocalTime().Weekday
	switch 6 {
	case today+0:
		fmt.Println("It's Saturday.")
	case today+1:
		fmt.Println("Tomorrow's Saturday.")
	case today+2:
		fmt.Println("Two days to Saturday.")
	default:
		fmt.Println("Saturday is too far away.")
	}
}
</div>
</div>

<div class="slide">
	<h2>Switch</h2>
	<p>
	Switch without a condition is the same as <code>switch true</code>.

<div>
package main

import (
	"fmt"
	"time"
)

func main() {
	t := time.LocalTime()
	switch {
	case t.Hour < 12:
		fmt.Println("Good morning!")
	case t.Hour < 17:
		fmt.Println("Good afternoon.")
	default:
		fmt.Println("Good evening.")
	}
}
</div>
</div>

<div class="slide">
	<h2>Exercise: Loops and Functions</h2>
	<p>
	As a simple way to play with functions and loops, implement
	the square root function using Newton's method.
	<p>
	In this case, Newton's method is to approximate <code>Sqrt(x)</code>
	by picking a starting point <i>z</i> and then repeating:
	<center>
	<img src="https://chart.googleapis.com/chart?cht=tx&chl=z=z-\frac{z^2-x}{2x}">
	</center>
	<p>
	To begin with, just repeat that calculation 10 times and see how close you
	get to the answer for various values (1, 2, 3, ...).
	<p>
	Next, change the loop condition to stop once the value has stopped changing.
	See if that's more or fewer iterations.
	How close are you to the <a target="_blank" href="http://golang.org/pkg/math/#Sqrt">math.Sqrt</a>?
	<p>
	Hint: to declare and initialize a floating point value, give it
	floating point syntax or use a conversion:
	<pre>
	z := float64(0)
	z := 0.0
	</pre>

<div>
package main

import (
	"fmt"
)

func Sqrt(x float64) float64 {
}

func main() {
	fmt.Println(Sqrt(2))
}
</div>
</div>

<div class="slide">
	<h2>Exercise: Maps</h2>
	<p>
	Implement <code>WordCount</code>.  It should return a map of the
	counts of each &ldquo;word&rdquo; in the string <code>s</code>.
	With the binary running, visit <a target="_blank" href="http://localhost:4000/">this page</a>
	to count words interactively.
	<p>
	You might find <a target="_blank" href="http://golang.org/pkg/strings/#Fields">strings.Fields</a> helpful.

<div>
package main

import (
	"go-tour.googlecode.com/hg/wc"
)

func WordCount(s string) map[string]int {
	return map[string]int{"x": 1}
}

func main() {
	wc.Serve(WordCount)
}
</div>
</div>

<div class="slide">
	<h2>Exercise: Slices</h2>
	<p>
	Implement <code>Pic</code>.  It should return a slice of length <code>dy</code>,
	each element of which is a slice of <code>dx</code> 8-bit unsigned integers.
	When you run the program, it will display your picture, interpreting the
	integers as grayscale (well, bluescale) values.
	<p>
	The choice of image is up to you.
	Interesting functions include <code>x^y</code>, <code>(x+y)/2</code>, and <code>x*y</code>.

<div>
package main

import "go-tour.googlecode.com/hg/pic"

func Pic(dx, dy int) [][]uint8 {
}

func main() {
	pic.Show(Pic)
}
</div>
</div>

<div class="slide">
	<h2>Advanced Exercise: Complex cube roots</h2>
	<p>
	Let's explore Go's built-in support for complex numbers via the
	<code>complex64</code> and <code>complex128</code> types.
	For cube roots, Newton's method amounts to repeating:
	<center>
	<img src="https://chart.googleapis.com/chart?cht=tx&chl=z=z-\frac{z^3-x}{3x^2}">
	</center>
	<p>
	Find the cube root of 2, just to make sure the algorithm works.
	There is a <a target="_blank" href="http://golang.org/pkg/cmath/#Pow">cmath.Pow</a> function.

<div>
package main

import "fmt"

func Cbrt(x complex128) complex128 {
}

func main() {
	fmt.Println(Cbrt(2))
}
</div>
</div>


<div class="slide">
	<h2>Advanced Exercise: High Precison</h2>
	<p>
	<a target="_blank" href="http://golang.org/pkg/big/">Package big</a>
	implements arbitrary precision integer arithmetic.
	Convert your square root program to work in fixed point arithmetic
	for some number of decimal places <code>P</code> and then compute
	50 digits of the square of 2.  Double check them against the
	constant listed in the <a target="_blank" href="http://golang.org/pkg/math/">package math documentation</a>.

<div>
package main

func main() {
}
</div>
</div>

<div class="toc">Methods and Interfaces</div>

<div class="slide nocode">
<h2>Methods and Interfaces</h2>
</div>

<div class="slide">
	<h2>Methods</h2>
	<p>
	Methods can be associated with any named type,
	but only the package that defines a type can
	define methods on it.

	<p>
	The <i>method receiver</i> appears in its own argument list
	between the <code>func</code> keyword and the method name.

<div>
package main

import (
	"fmt"
	"math"
)

type Point struct {
	X, Y float64
}

func (p *Point) Abs() float64 {
	return math.Sqrt(p.X*p.X + p.Y*p.Y)
}

func main() {
	p := &Point{3, 4}
	fmt.Println(p.Abs())
}
</div>
</div>

<div class="slide">
	<h2>Methods</h2>
	<p>
	Methods can be associated with <i>any</i> named type.

<div>
package main

import (
	"fmt"
	"math"
)

type MyFloat float64

func (f MyFloat) Abs() float64 {
	if f < 0 {
		return float64(-f)
	}
	return float64(f)
}

func main() {
	f := MyFloat(-math.Sqrt2)
	fmt.Println(f.Abs())
}
</div>
</div>

<div class="slide">
	<h2>Interfaces</h2>
	<p>
	An interface type is defined by a set of methods.
	<p>
	A value of interface type can hold any value that
	implements those methods.

<div>
package main

import (
	"fmt"
	"math"
)

type Abser interface {
	Abs() float64
}

func main() {
	var (
		a Abser
		f = MyFloat(-math.Sqrt2)
		p = Point{3, 4}
	)

	a = f     // f, a MyFloat, implements Abser
	a = &p    // &p, a *Point, implements Abser
	a = p     // p, a Point, does NOT implement Abser

	fmt.Println(a.Abs())
}











type MyFloat float64

func (f MyFloat) Abs() float64 {
	if f < 0 {
		return float64(-f)
	}
	return float64(f)
}

type Point struct {
	X, Y float64
}

func (p *Point) Abs() float64 {
	return math.Sqrt(p.X*p.X + p.Y*p.Y)
}
</div>
</div>

<div class="slide">
	<h2>Interfaces</h2>
	<p>
	A type implements an interface by implementing the methods.
	<p>
	<i>There is no explicit declaration of intent.</i>
	<p>
	This decouples implementation packages from the packages that define the interfaces:
	neither depends on the other.
	<p>
	It also encourages the definition of precise interfaces,
	because you don't have to find every implementation and tag it with the new interface name.
	<p>
	<a href="http://golang.org/pkg/io/">Package io</a> defines <code>Reader</code> and <code>Writer</code>; you don't have to.

<div>
package main

import (
	"fmt"
	"os"
)

type Reader interface {
	Read(b []byte) (n int, err os.Error)
}

type Writer interface {
	Write(b []byte) (n int, err os.Error)
}

type ReadWriter interface {
	Reader
	Writer
}

func main() {
	var w Writer
	w = os.Stdout  // os.Stdout implements our interface
	fmt.Fprintf(w, "hello, writer\n")
}
</div>
</div>

<div class="slide">
	<h2>Errors</h2>
	<p>
	An error is anything that can describe itself:
	<pre>
	package os

	type Error interface {
		String() string
	}
	</pre>

<div>
package main

import (
	"fmt"
	"time"
	"os"
)

type MyError struct {
	When *time.Time
	What string
}

func (e *MyError) String() string {
	return "at " + e.When.String() + ", " + e.What
}

func run() os.Error {
	return &MyError{time.LocalTime(), "it didn't work"}
}

func main() {
	if err := run(); err != nil {
		fmt.Println(err)
	}
}
</div>
</div>

<div class="slide">
	<h2>Web servers</h2>
	<p>
	<a href="http://golang.org/pkg/http/">Package http</a> serves HTTP requests using any value
	that implements <code>http.Handler</code>:
	<pre>
	package http

	type Handler interface {
		ServeHTTP(w ResponseWriter, req *Request)
	}
	</pre>
	<p>
	In this example, the type <code>MyHandler</code> implements <code>http.Handler</code>.
	<p>
	Visit <a href="http://localhost:4000/" target="_blank">http://localhost:4000/</a> to see the greeting.
<div>
package main

import (
	"fmt"
	"http"
	"log"
)

type MyHandler struct{}

func (h MyHandler) ServeHTTP(w http.ResponseWriter,
                             r *http.Request) {
	fmt.Fprint(w, "Hello!")
}

func main() {
	var h MyHandler
	err := http.ListenAndServe("localhost:4000", h)
	if err != nil {
		log.Fatal(err)
	}
}
</div>
</div>

<div class="slide">
	<h2>Images</h2>
	<p>
	<a href="http://golang.org/pkg/image/#Image">Package image</a> defines the <code>Image</code>
	interface:
	<pre>
package image

type Image interface {
	<span>// ColorModel returns the Image's ColorModel.</span>
	ColorModel() ColorModel

	<span>// Bounds returns the domain for which At can return non-zero color.</span>
	<span>// The bounds do not necessarily contain the point (0, 0).</span>
	Bounds() Rectangle

	<span>// At returns the color of the pixel at (x, y).</span>
	<span>// At(Bounds().Min.X, Bounds().Min.Y) returns the upper-left pixel of the grid.</span>
	<span>// At(Bounds().Max.X-1, Bounds().Max.Y-1) returns the lower-right one.</span>
	At(x, y int) Color
}</pre>
	<p>
	<code>Color</code> and <code>ColorModel</code> are interfaces too,
	but we'll ignore that by using the predefined implementations
	<code>image.RGBAColor</code> and <code>image.RGBAColorModel</code>.

<div>
package main

import (
	"fmt"
	"image"
)

func main() {
	m := image.NewRGBA(100, 100)
	fmt.Println(m.Bounds())
	fmt.Println(m.At(0, 0).RGBA())
}
</div>
</div>

<div class="slide">
	<h2>Exercise: Hello, world 2.0</h2>
	<p>
	Implement an <code>http.Handler</code> that responds to every request by saying
<pre>	Hello, <i>name</i></pre>
	<p>
	where <i>name</i> is the value of the request parameter <code>name</code>.
	<p>
	For example, <a target="_blank"
	href="http://localhost:4000/?name=Joe">http://localhost:4000/?name=Joe</a>
	should display
<pre>	Hello, Joe</pre>
	<p>
	Hint: use the <code><a href="http://golang.org/pkg/http/#Request.FormValue"
	target="_blank">FormValue</a></code> method of
	<code>*http.Request</code> to access the request parameters.
<div>
package main

import (
	"http"
)

func main() {
	var h http.Handler = // your value here
	http.ListenAndServe("localhost:4000", h)
}
</div>
</div>

<div class="slide">
	<h2>Exercise: HTTP Handlers</h2>
	<p>
	Implement the following types and define ServeHTTP methods on them.
	Register them to handle specific paths in your web server.
<pre>type String string
	
type Struct struct {
	Greeting string
	Punct    string
	Who      string
}</pre>
	<p>
	For example, you should be able to register handlers using:
<pre>http.Handle("/string", String("I'm a frayed knot."))
http.Handle("/struct", &Struct{"Hello", ":", "Gophers!"})</pre>
<div>
package main

import (
	"http"
)

func main() {
	// your http.Handle calls here
	http.ListenAndServe("localhost:4000", nil)
}
</div>
</div>

<div class="slide">
	<h2>Exercise: Images</h2>
	<p>
	Remember the picture generator you wrote earlier?
	Let's write another one, but this time it will return
	an implementation of <code>image.Image</code> instead of a slice of data.
	<p>
	Define your own <code>Image</code> type, implement
	<a href="http://golang.org/pkg/image/#Image" target="_blank">the necessary methods</a>,
	and call <code>pic.ShowImage</code>.
	<p>
	<code>Bounds</code> should return a <code>image.Rectangle</code>, like
	<code>image.Rect(0, 0, w, h)</code>.
	<p>
	<code>ColorModel</code> should return <code>image.RGBAColorModel</code>.
	<p>
	<code>At</code> should return a color;
	the value <code>v</code> in the last picture generator corresponds to
	<code>image.RGBAColor{v, v, 255, 255}</code> in this one.

<div>
package main

import (
	"image"
	"go-tour.googlcode.com/hg/pic"
)

func main() {
	m := ... my image
	pic.ShowImage(m)
}
</div>
</div>

<div class="slide">
	<h2>Exercise: Images</h2>
	<p>
	Let's eliminate some more scaffolding. 
<div>
</div>
</div>


<div class="toc">Concurrency</div>

<div class="slide nocode">
<h2>Concurrency</h2>
</div>

<div class="slide">
	<h2>Goroutines</h2>

        <p><code>go f(x, y, z)</code> starts a new goroutine running <code>f(x,
        y, z)</code>.  The evaluation
        of <code>f</code>, <code>x</code>, <code>y</code>, and <code>z</code>
        happens in the current goroutine.  After they are evaluated, a new goroutine
        is created and it calls <code>f</code> with those arguments.

        <p>Goroutines share memory, so access to shared memory must be
        synchronized.  The <code>sync</code> package provides useful primitives.
<div>
package main

import (
       "fmt"
       "time"
)

func say(s string) {
       for i := 0; i < 5; i++ {
               time.Sleep(100e6)
               fmt.Println(s)
       }
}

func main() {
       go say("world")
       say("hello")
}
</div>
</div>

<div class="slide">
	<h2>Channels</h2>

<p>
Channels are typed message buffers with send and receive.

<p>Like maps and slices, channels must be created before use:
<pre>
ch := make(chan int, 100)
</pre>

<p>This channel has room for 100 ints in its buffer.  Sends block when the
buffer is full.  Receives block when there's nothing available.  This allows
goroutines to synchronize without explicit locks or condition variables.

<p>If the second arg to <code>make</code> is omitted or zero, the channel is
  unbuffered.  Sends to an unbuffered channel block until another goroutine
  receives from that channel, and receives block until a value is sent.

<p>The built-in function <code>cap(ch)</code> returns the size
of <code>ch</code>'s buffer; <code>len(ch)</code> returns the number of elements
in the buffer ready to be received.

<div>
package main

import (
	"fmt"
)

func sum(a []int, c chan int) {
	res := 0
	for _, v := range a {
		res += v
	}
	c <- res
}

func main() {
	a := []int{7, 2, 8, -9, 4, 0}

        c := make(chan int)
	go sum(a[:len(a)/2], c)
	go sum(a[len(a)/2:], c)
        x, y := <-c, <-c

	fmt.Println(x, y, x + y)
}
</div>
</div>


<div class="slide">
	<h2>Range and Close</h2>

<p>A sender can <code>close</code> a channel to indicate that no more values
will be sent.  Receivers can test whether a channel has been closed by assigning
  a second parameter to the receive expression: in <code>v, ok :=
    &lt;-ch</code>, <code>ok</code> is <code>false</code> if <code>ch</code> is closed.

<p>The loop <code>for i := range c</code> receives values from the channel
repeatedly until it is closed.

<p>Note: Only the sender should close a channel, never the receiver.
<div>
package main

import (
	"fmt"
)

func fibonacci(n int, c chan int) {
        x, y := 1, 1
        for i := 0; i < n; i++ {
                c <- x
                x, y = y, x + y
        }
        close(c)
}

func main() {
        c := make(chan int, 10)
	go fibonacci(cap(c), c)
        for i := range c {
                fmt.Println(i)
        }
}
</div>
</div>

<div class="slide">
	<h2>Select</h2>

<p>The <code>select</code> statement lets a goroutine wait on multiple send or
receive operations.

<p>A <code>select</code> blocks until one of its cases can run, then it executes
  that case.  It chooses one at random if multiple are ready.

<div>
package main

import (
	"fmt"
)

func fibonacci(c, quit chan int) {
        x, y := 1, 1
        for {
                select {
                case c <- x:
                          x, y = y, x + y
                case <-quit:
                        return
                }
        }
}

func main() {
        c, quit := make(chan int), make(chan int)
	go fibonacci(c, quit)
        for i := 0; i < 10; i++ {
                fmt.Println(<-c)
        }
        quit <- 0
}
</div>
</div>

<div class="slide">
	<h2>Default Selection</h2>

<p>A <code>default</code> case in a <code>select</code> can always run.

<p>Use a <code>default</code> case to try a send or receive without blocking:
<pre>
select {
case i := <-c:
	// use i
default:
	// receiving from c would block
}
</pre>

<div>
package main

import (
	"fmt"
	"time"
)

func main() {
        tick := time.Tick(1e8)
        boom := time.After(5e8)
        for {
                select {
                case <-tick:
                        fmt.Println("tick.")
                case <-boom:
                        fmt.Println("BOOM!")
                        return
                default:
                        fmt.Println("    .")
                        time.Sleep(5e7)
                }
        }
}
</div>
</div>

<div class="slide">
	<h2>Exercise: Web Crawler</h2>
	<p>

<p>Change <code>Crawl</code> to run each call to <code>fetcher.Fetch</code> in
its own goroutine and avoid fetching the same URL twice.
<div>
package main

import (
        "os"
        "fmt"
)

type Fetcher interface {
        // Fetch returns the contents of url and a slice
        // of URLs on that page that may be crawled.
        // It returns an error if url was not found or the fetch failed.
        Fetch(url string) (contents string, urls []string, err os.Error)
}

// Crawl fetches pages recursively starting from url to depth using fetcher.
func Crawl(url string, depth int, fetcher Fetcher) {
        // TODO(you): Run each call to fetcher.Fetch in its own goroutine.
        // TODO(you): Don't fetch the same URL twice.
        // This implementation doesn't do either:
        if depth <= 0 {
                return
        }
        contents, urls, err := fetcher.Fetch(url)
        if err != nil {
                fmt.Println(err)
                return
        }
        fmt.Println(contents)
        for _, u := range urls {
                Crawl(u, depth-1, fetcher)
        }
        return
}

// Fake web of URLs for testing.
type fakeResult struct {
	contents string
	urls []string
}

type fakeFetcher map[string]*fakeResult

func (f *fakeFetcher) Fetch(url string) (contents string, urls []string, err os.Error) {
        if res, ok := (*f)[url]; ok {
                return res.contents, res.urls, nil
        }
        return "", nil, fmt.Errorf("not found: %s", url)
}

func main() {
        fetcher := &fakeFetcher{
                "http://golang.org": &fakeResult{
                        "The Go Programming Language",
                        []string{
                                "http://golang.org/pkg",
                                "http://golang.org/cmd",
                        },
                },
                "http://golang.org/pkg": &fakeResult{
                        "Packages",
                        []string{
                                "http://golang.org",
                                "http://golang.org/cmd",
                                "http://golang.org/pkg/fmt",
                                "http://golang.org/pkg/os",
                        },
                },
                "http://golang.org/pkg/fmt": &fakeResult{
                        "Package fmt",
                        []string{},
                },
                "http://golang.org/pkg/os": &fakeResult{
                        "Package os",
                        []string{},
                },
        }
	Crawl("http://golang.org", 3, fetcher)
}
</div>
</div>

<div class="slide nocode">
<h2>More here...</h2>
</div>

</div>