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+---
+name: Go
+category: language
+language: Go
+filename: learngo.go
+contributors:
+ - ["Sonia Keys", "https://github.com/soniakeys"]
+ - ["Christopher Bess", "https://github.com/cbess"]
+ - ["Jesse Johnson", "https://github.com/holocronweaver"]
+ - ["Quint Guvernator", "https://github.com/qguv"]
+ - ["Jose Donizetti", "https://github.com/josedonizetti"]
+ - ["Alexej Friesen", "https://github.com/heyalexej"]
+---
+
+Go was created out of the need to get work done. It's not the latest trend
+in computer science, but it is the newest fastest way to solve real-world
+problems.
+
+It has familiar concepts of imperative languages with static typing.
+It's fast to compile and fast to execute, it adds easy-to-understand
+concurrency to leverage today's multi-core CPUs, and has features to
+help with large-scale programming.
+
+Go comes with a great standard library and an enthusiastic community.
+
+```go
+// 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.
+ "io/ioutil" // Implements some I/O utility functions.
+ m "math" // Math library with local alias m.
+ "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!
+}
+
+/* <- multiline comment
+Functions can have parameters and (multiple!) return values.
+Here `x`, `y` are the arguments and `sum`, `prod` is the signature (what's returned).
+Note that `x` and `sum` receive the type `int`.
+*/
+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.
+ str := "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 int32, 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 float64's.
+
+ // var syntax with 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 initialized with a fixed size of three
+ // elements, with values 3, 1, and 5.
+
+ // 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.
+
+ // Because they are dynamic, slices can be appended to on-demand.
+ // To append elements to a slice, built-in append() function is used.
+ // First argument is a slice to which we are appending. Commonly,
+ // the array variable is updated in place, as in example below.
+ s := []int{1, 2, 3} // Result is a slice of length 3.
+ s = append(s, 4, 5, 6) // Added 3 elements. Slice now has length of 6.
+ fmt.Println(s) // Updated slice is now [1 2 3 4 5 6]
+ // To append another slice, instead of list of atomic elements we can
+ // pass a reference to a slice or a slice literal like this, with a
+ // trailing elipsis, meaning take a slice and unpack its elements,
+ // appending them to slice s.
+ s = append(s, []int{7, 8, 9}...) // Second argument is a slice literal.
+ fmt.Println(s) // Updated slice is now [1 2 3 4 5 6 7 8 9]
+
+ 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.
+ _, _, _, _, _, _, _, _, _, _ = str, 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.
+}
+
+// It is possible, unlike in many other languages for functions in go
+// to have named return values.
+// Assigning a name to the type being returned in the function declaration line
+// allows us to easily return from multiple points in a function as well as to
+// only use the return keyword, without anything further.
+func learnNamedReturns(x, y int) (z int) {
+ z = x * y
+ return // z is implicit here, because we named it earlier.
+}
+
+// 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() float64 {
+ return m.Exp(10)
+}
+
+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 := 42.0
+ switch x {
+ case 0:
+ case 1:
+ case 42:
+ // Cases don't "fall through".
+ /*
+ There is a `fallthrough` keyword however, see:
+ https://github.com/golang/go/wiki/Switch#fall-through
+ */
+ case 43:
+ // Unreached.
+ default:
+ // Default case is optional.
+ }
+ // Like if, for doesn't use parens either.
+ // Variables declared in for and if are local to their scope.
+ for x := 0; x < 3; x++ { // ++ is a statement.
+ fmt.Println("iteration", x)
+ }
+ // x == 42 here.
+
+ // For is the only loop statement in Go, but it has alternate forms.
+ for { // Infinite loop.
+ break // Just kidding.
+ continue // Unreached.
+ }
+
+ // You can use range to iterate over an array, a slice, a string, a map, or a channel.
+ // range returns one (channel) or two values (array, slice, string and map).
+ for key, value := range map[string]int{"one": 1, "two": 2, "three": 3} {
+ // for each pair in the map, print key and value
+ fmt.Printf("key=%s, value=%d\n", key, value)
+ }
+
+ // 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 > 10000 // References x declared above switch statement.
+ }
+ fmt.Println("xBig:", xBig()) // true (we last assigned e^10 to x).
+ x = 1.3e3 // This makes x == 1300
+ fmt.Println("xBig:", xBig()) // false now.
+
+ // What's more is function literals may be defined and called inline,
+ // acting as an argument to function, as long as:
+ // a) function literal is called immediately (),
+ // b) result type matches expected type of argument.
+ fmt.Println("Add + double two numbers: ",
+ func(a, b int) int {
+ return (a + b) * 2
+ }(10, 2)) // Called with args 10 and 2
+ // => Add + double two numbers: 24
+
+ // When you need it, you'll love it.
+ goto love
+love:
+
+ learnFunctionFactory() // func returning func is fun(3)(3)
+ learnDefer() // A quick detour to an important keyword.
+ learnInterfaces() // Good stuff coming up!
+}
+
+func learnFunctionFactory() {
+ // Next two are equivalent, with second being more practical
+ fmt.Println(sentenceFactory("summer")("A beautiful", "day!"))
+
+ d := sentenceFactory("summer")
+ fmt.Println(d("A beautiful", "day!"))
+ fmt.Println(d("A lazy", "afternoon!"))
+}
+
+// Decorators are common in other languages. Same can be done in Go
+// with function literals that accept arguments.
+func sentenceFactory(mystring string) func(before, after string) string {
+ return func(before, after string) string {
+ return fmt.Sprintf("%s %s %s", before, mystring, after) // new string
+ }
+}
+
+func learnDefer() (ok bool) {
+ // Deferred statements are executed just before the function returns.
+ defer fmt.Println("deferred statements execute in reverse (LIFO) order.")
+ defer fmt.Println("\nThis line is being printed first because")
+ // Defer is commonly used to close a file, so the function closing the
+ // file stays close to the function opening the file.
+ return true
+}
+
+// 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.
+
+ learnVariadicParams("great", "learning", "here!")
+}
+
+// Functions can have variadic parameters.
+func learnVariadicParams(myStrings ...interface{}) {
+ // Iterate each value of the variadic.
+ // The underbar here is ignoring the index argument of the array.
+ for _, param := range myStrings {
+ fmt.Println("param:", param)
+ }
+
+ // Pass variadic value as a variadic parameter.
+ fmt.Println("params:", fmt.Sprintln(myStrings...))
+
+ 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.
+ ccs := 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 <-ccs: // 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() {
+
+ // First parameter of ListenAndServe is TCP address to listen to.
+ // Second parameter is an interface, specifically http.Handler.
+ go func() {
+ err := http.ListenAndServe(":8080", pair{})
+ fmt.Println(err) // don't ignore errors
+ }()
+
+ requestServer()
+}
+
+// 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!"))
+}
+
+func requestServer() {
+ resp, err := http.Get("http://localhost:8080")
+ fmt.Println(err)
+ defer resp.Body.Close()
+ body, err := ioutil.ReadAll(resp.Body)
+ fmt.Printf("\nWebserver said: `%s`", string(body))
+}
+```
+
+## Further Reading
+
+The root of all things Go is the [official Go web site](http://golang.org/).
+There you can follow the tutorial, play interactively, and read lots.
+
+The language definition itself is highly recommended. It's easy to read
+and amazingly short (as language definitions go these days.)
+
+You can play around with the code on [Go playground](https://play.golang.org/p/tnWMjr16Mm). Try to change it and run it from your browser! Note that you can use [https://play.golang.org](https://play.golang.org) as a [REPL](https://en.wikipedia.org/wiki/Read-eval-print_loop) to test things and code in your browser, without even installing Go.
+
+On the reading list for students of Go is the [source code to the standard
+library](http://golang.org/src/pkg/). Comprehensively documented, it
+demonstrates the best of readable and understandable Go, Go style, and Go
+idioms. Or you can click on a function name in [the
+documentation](http://golang.org/pkg/) and the source code comes up!
+
+Another great resource to learn Go is [Go by example](https://gobyexample.com/).