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diff --git a/go.html.markdown b/go.html.markdown new file mode 100644 index 00000000..34b855e3 --- /dev/null +++ b/go.html.markdown @@ -0,0 +1,422 @@ +--- +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/). |