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@melvinmt melvinmt/learngo.go
Created Sep 4, 2013

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Code from http://learnxinyminutes.com/docs/go/ formatted with gofmt
// Single line comment
/* Multi-
line comment */
// A package clause starts every source file.
// Main is a special name declaring an executable rather than a library.
package main
// Import declaration declares library packages referenced in this file.
import (
"fmt" // A package in the Go standard library
"net/http" // Yes, a web server!
"strconv" // String conversions
)
// A function definition. Main is special. It is the entry point for the
// executable program. Love it or hate it, Go uses brace brackets.
func main() {
// Println outputs a line to stdout.
// Qualify it with the package name, fmt.
fmt.Println("Hello world!")
// Call another function within this package.
beyondHello()
}
// Functions have parameters in parentheses.
// If there are no parameters, empty parentheses are still required.
func beyondHello() {
var x int // Variable declaration. Variables must be declared before use.
x = 3 // Variable assignment.
// "Short" declarations use := to infer the type, declare, and assign.
y := 4
sum, prod := learnMultiple(x, y) // function returns two values
fmt.Println("sum:", sum, "prod:", prod) // simple output
learnTypes() // < y minutes, learn more!
}
// Functions can have parameters and (multiple!) return values.
func learnMultiple(x, y int) (sum, prod int) {
return x + y, x * y // return two values
}
// Some built-in types and literals.
func learnTypes() {
// Short declaration usually gives you what you want.
s := "Learn Go!" // string type
s2 := `A "raw" string literal
can include line breaks.` // same string type
// non-ASCII literal. Go source is UTF-8.
g := 'Σ' // rune type, an alias for uint32, holds a unicode code point
f := 3.14195 // float64, an IEEE-754 64-bit floating point number
c := 3 + 4i // complex128, represented internally with two float64s
// Var syntax with an initializers.
var u uint = 7 // unsigned, but implementation dependent size as with int
var pi float32 = 22. / 7
// Conversion syntax with a short declaration.
n := byte('\n') // byte is an alias for uint8
// Arrays have size fixed at compile time.
var a4 [4]int // an array of 4 ints, initialized to all 0
a3 := [...]int{3, 1, 5} // an array of 3 ints, initialized as shown
// Slices have dynamic size. Arrays and slices each have advantages
// but use cases for slices are much more common.
s3 := []int{4, 5, 9} // compare to a3. no ellipsis here
s4 := make([]int, 4) // allocates slice of 4 ints, initialized to all 0
var d2 [][]float64 // declaration only, nothing allocated here
bs := []byte("a slice") // type conversion syntax
p, q := learnMemory() // declares p, q to be type pointer to int.
fmt.Println(*p, *q) // * follows a pointer. This prints two ints.
// Maps are a dynamically growable associative array type, like the
// hash or dictionary types of some other languages.
m := map[string]int{"three": 3, "four": 4}
m["one"] = 1
// Unused variables are an error in Go.
// The underbar lets you "use" a variable but discard its value.
_, _, _, _, _, _, _, _, _ = s2, g, f, u, pi, n, a3, s4, bs
// Output of course counts as using a variable.
fmt.Println(s, c, a4, s3, d2, m)
learnFlowControl() // back in the flow
}
// Go is fully garbage collected. It has pointers but no pointer arithmetic.
// You can make a mistake with a nil pointer, but not by incrementing a pointer.
func learnMemory() (p, q *int) {
// Named return values p and q have type pointer to int.
p = new(int) // built-in function new allocates memory.
// The allocated int is initialized to 0, p is no longer nil.
s := make([]int, 20) // allocate 20 ints as a single block of memory
s[3] = 7 // assign one of them
r := -2 // declare another local variable
return &s[3], &r // & takes the address of an object.
}
func expensiveComputation() int {
return 1e6
}
func learnFlowControl() {
// If statements require brace brackets, and do not require parens.
if true {
fmt.Println("told ya")
}
// Formatting is standardized by the command line command "go fmt."
if false {
// pout
} else {
// gloat
}
// Use switch in preference to chained if statements.
x := 1
switch x {
case 0:
case 1:
// cases don't "fall through"
case 2:
// unreached
}
// Like if, for doesn't use parens either.
for x := 0; x < 3; x++ { // ++ is a statement
fmt.Println("iteration", x)
}
// x == 1 here.
// For is the only loop statement in Go, but it has alternate forms.
for { // infinite loop
break // just kidding
continue // unreached
}
// As with for, := in an if statement means to declare and assign y first,
// then test y > x.
if y := expensiveComputation(); y > x {
x = y
}
// Function literals are closures.
xBig := func() bool {
return x > 100 // references x declared above switch statement.
}
fmt.Println("xBig:", xBig()) // true (we last assigned 1e6 to x)
x /= 1e5 // this makes it == 10
fmt.Println("xBig:", xBig()) // false now
// When you need it, you'll love it.
goto love
love:
learnInterfaces() // Good stuff coming up!
}
// Define Stringer as an interface type with one method, String.
type Stringer interface {
String() string
}
// Define pair as a struct with two fields, ints named x and y.
type pair struct {
x, y int
}
// Define a method on type pair. Pair now implements Stringer.
func (p pair) String() string { // p is called the "receiver"
// Sprintf is another public function in package fmt.
// Dot syntax references fields of p.
return fmt.Sprintf("(%d, %d)", p.x, p.y)
}
func learnInterfaces() {
// Brace syntax is a "struct literal." It evaluates to an initialized
// struct. The := syntax declares and initializes p to this struct.
p := pair{3, 4}
fmt.Println(p.String()) // call String method of p, of type pair.
var i Stringer // declare i of interface type Stringer.
i = p // valid because pair implements Stringer
// Call String method of i, of type Stringer. Output same as above.
fmt.Println(i.String())
// Functions in the fmt package call the String method to ask an object
// for a printable representation of itself.
fmt.Println(p) // output same as above. Println calls String method.
fmt.Println(i) // output same as above
learnErrorHandling()
}
func learnErrorHandling() {
// ", ok" idiom used to tell if something worked or not.
m := map[int]string{3: "three", 4: "four"}
if x, ok := m[1]; !ok { // ok will be false because 1 is not in the map.
fmt.Println("no one there")
} else {
fmt.Print(x) // x would be the value, if it were in the map.
}
// An error value communicates not just "ok" but more about the problem.
if _, err := strconv.Atoi("non-int"); err != nil { // _ discards value
// prints "strconv.ParseInt: parsing "non-int": invalid syntax"
fmt.Println(err)
}
// We'll revisit interfaces a little later. Meanwhile,
learnConcurrency()
}
// c is a channel, a concurrency-safe communication object.
func inc(i int, c chan int) {
c <- i + 1 // <- is the "send" operator when a channel appears on the left.
}
// We'll use inc to increment some numbers concurrently.
func learnConcurrency() {
// Same make function used earlier to make a slice. Make allocates and
// initializes slices, maps, and channels.
c := make(chan int)
// Start three concurrent goroutines. Numbers will be incremented
// concurrently, perhaps in parallel if the machine is capable and
// properly configured. All three send to the same channel.
go inc(0, c) // go is a statement that starts a new goroutine.
go inc(10, c)
go inc(-805, c)
// Read three results from the channel and print them out.
// There is no telling in what order the results will arrive!
fmt.Println(<-c, <-c, <-c) // channel on right, <- is "receive" operator.
cs := make(chan string) // another channel, this one handles strings.
cc := make(chan chan string) // a channel of string channels.
go func() { c <- 84 }() // start a new goroutine just to send a value
go func() { cs <- "wordy" }() // again, for cs this time
// Select has syntax like a switch statement but each case involves
// a channel operation. It selects a case at random out of the cases
// that are ready to communicate.
select {
case i := <-c: // the value received can be assigned to a variable
fmt.Printf("it's a %T", i)
case <-cs: // or the value received can be discarded
fmt.Println("it's a string")
case <-cc: // empty channel, not ready for communication.
fmt.Println("didn't happen.")
}
// At this point a value was taken from either c or cs. One of the two
// goroutines started above has completed, the other will remain blocked.
learnWebProgramming() // Go does it. You want to do it too.
}
// A single function from package http starts a web server.
func learnWebProgramming() {
// ListenAndServe first parameter is TCP address to listen at.
// Second parameter is an interface, specifically http.Handler.
err := http.ListenAndServe(":8080", pair{})
fmt.Println(err) // don't ignore errors
}
// Make pair an http.Handler by implementing its only method, ServeHTTP.
func (p pair) ServeHTTP(w http.ResponseWriter, r *http.Request) {
// Serve data with a method of http.ResponseWriter
w.Write([]byte("You learned Go in Y minutes!"))
}
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