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Diffstat (limited to 'go.html.markdown')
| -rw-r--r-- | go.html.markdown | 501 |
1 files changed, 291 insertions, 210 deletions
diff --git a/go.html.markdown b/go.html.markdown index fa4c8d0b..17f10bd9 100644 --- a/go.html.markdown +++ b/go.html.markdown @@ -8,9 +8,11 @@ contributors: - ["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 +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. @@ -24,7 +26,7 @@ Go comes with a great standard library and an enthusiastic community. ```go // Single line comment /* Multi- - line comment */ + line comment */ // A package clause starts every source file. // Main is a special name declaring an executable rather than a library. @@ -32,286 +34,362 @@ 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. - m "math" // Math library with local alias m. + "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. +// 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!") + // Println outputs a line to stdout. + // Qualify it with the package name, fmt. + fmt.Println("Hello world!") - // Call another function within this package. - beyondHello() + // 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! + 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. + 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. + // Short declaration usually gives you what you want. + str := "Learn Go!" // string type. - s2 := `A "raw" string literal + 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 float64's. - - // 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) + // 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 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 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. +} - 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. +// 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. + // 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) + 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". - case 43: - // Unreached. - } - // 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. - } - // 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. - - // When you need it, you'll love it. - goto love + // 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". + case 43: + // Unreached. + } + // 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: - learnDefer() // A quick detour to an important keyword. - learnInterfaces() // Good stuff coming up! + 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 + // 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 + String() string } // Define pair as a struct with two fields, ints named x and y. type pair struct { - x, y int + x, y int } -// Define a method on type pair. Pair now implements Stringer. +// 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) + // 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!") + // 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. - for _, param := range myStrings { - fmt.Println("param:", param) - } + // 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...)) + // Pass variadic value as a variadic parameter. + fmt.Println("params:", fmt.Sprintln(myStrings...)) - learnErrorHandling() + 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() + // ", 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. + 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. + // 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. - err := http.ListenAndServe(":8080", pair{}) - fmt.Println(err) // don't ignore errors + + // 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!")) + // 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)) } ``` @@ -320,12 +398,15 @@ func (p pair) ServeHTTP(w http.ResponseWriter, r *http.Request) { 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 +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 +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 +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/). |