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424 lines
18 KiB
Markdown
424 lines
18 KiB
Markdown
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---
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name: Go
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category: language
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language: Go
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filename: learngo.go
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contributors:
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- ["Sonia Keys", "https://github.com/soniakeys"]
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---
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Go was created out of the need to get work done. It's not the latest trend
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in computer science, but it is the newest fastest way to solve real-world
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problems.
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It has familiar concepts of imperative languages with static typing.
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It's fast to compile and fast to execute, it adds easy-to-understand
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concurrency to leverage today's multi-core CPUs, and has features to
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help with large-scale programming.
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Go comes with a great standard library and an enthusiastic community.
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```Go
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// Single line comment
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/* Multi-
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line comment */
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// A package clause starts every source file.
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// Main is a special name declaring an executable rather than a library.
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package main
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// An import declaration comes next. It declares library packages referenced
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// in this file. The list must be exactly correct! Missing or unused packages
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// are errors, not warnings.
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import (
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"fmt" // A package in the Go standard library
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"net/http" // Yes, a web server!
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"strconv" // String conversions
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)
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// A function definition. Main is special. It is the entry point for the
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// executable program. Love it or hate it, Go uses brace brackets.
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func main() {
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// Println is a function that outputs a line to stdout. It can be
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// called here because fmt has been imported and the function name
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// "Println" is upper case. Symbols starting with an upper case letter
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// are publicly visible. No other special syntax is needed to export
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// something from a package.
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// To call Println, qualify it with the package name, fmt.
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fmt.Println("Hello world!")
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// Call another function within this package.
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beyondHello()
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}
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// Idiomatic Go uses camel case. Functions have parameters in parentheses.
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// If there are no parameters, empty parens are still required.
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func beyondHello() {
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var x int // Variable declaration. Variables must be declared before use.
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x = 3 // Variable assignment.
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// "Short" declarations use := syntax to declare and assign, infering the
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// type from the right hand side as much as possible and using some
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// defaults where the rhs could be interpreted different ways.
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// Idiomatic Go uses short declarations in preference to var keyword.
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y := 4
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sum, prod := learnMultiple(x, y) // function returns two values
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fmt.Println("sum:", sum, "prod:", prod) // simple output
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learnTypes() // < y minutes, learn more!
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}
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// Functions can have parameters and (multiple!) return values.
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// In declarations, the symbol precedes the type, and the type does not have
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// to be repeated if it is the same for multiple symbols in a row.
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func learnMultiple(x, y int) (sum, prod int) {
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return x + y, x * y // return two values
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}
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// Some built-in types and literals.
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func learnTypes() {
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// Short declaration usually gives you what you want.
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s := "Learn Go!" // string type
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s2 := `A "raw" string literal
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can include line breaks.` // same string type
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// non-ASCII literal. Go source is UTF-8.
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g := 'Σ' // rune type, an alias for uint32, holds a UTF-8 code point
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f := 3.14195 // float64, an IEEE-754 64-bit floating point number
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c := 3 + 4i // complex128, represented internally with two float64s
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// You can use var syntax with an initializer if you want
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// something other than the default that a short declaration gives you.
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var u uint = 7 // unsigned, but implementation dependent size as with int
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var pi float32 = 22. / 7
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// Or more idiomatically, use conversion syntax with a short declaration.
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n := byte('\n') // byte is an alias for uint8
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// Arrays have size fixed at compile time.
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var a4 [4]int // an array of 4 ints, initialized to all 0
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a3 := [...]int{3, 1, 5} // an array of 3 ints, initialized as shown
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// Slices have dynamic size. Arrays and slices each have advantages
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// but use cases for slices are much more common.
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s3 := []int{4, 5, 9} // compare to a3. no ellipsis here
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s4 := make([]int, 4) // allocates slice of 4 ints, initialized to all 0
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var d2 [][]float64 // declaration only, nothing allocated here
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bs := []byte("a slice") // type conversion syntax
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p, q := learnMemory() // A little side bar.
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// Did you read it? This short declaration declares p and q to be of
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// type pointer to int. P is now pointing into a block of of 20 ints, but
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// the only one accessible is the one that p is pointing at. There is
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// no p++ to get at the next one.
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fmt.Println(*p, *q) // * follows a pointer. This prints two ints.
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// Maps are a dynamically growable associative array type, like the
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// hash or dictionary types of some other languages.
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m := map[string]int{"three": 3, "four": 4}
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m["one"] = 1
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// Unused variables are an error in Go.
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// The underbar lets you "use" a variable but discard its value.
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_, _, _, _, _, _, _, _, _ = s2, g, f, u, pi, n, a3, s4, bs
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// Output of course counts as using a variable.
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fmt.Println(s, c, a4, s3, d2, m)
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learnFlowControl() // back in the flow
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}
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// Go is fully garbage collected. It has pointers but no pointer arithmetic.
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// You can make a mistake with a nil pointer, but not by incrementing a pointer.
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func learnMemory() (p, q *int) {
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// Named return values p and q have type pointer to int. They are
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// initialized to nil at this point. Evaluating *p or *q here would cause
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// a panic--a run time error.
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p = new(int) // built-in function new allocates memory.
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// The allocated int is initialized to 0, p is no longer nil.
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s := make([]int, 20) // allocate 20 ints as a single block of memory
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s[3] = 7 // assign one of them
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r := -2 // declare another local variable
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return &s[3], &r // Oh my.
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// The line above returns two values, yes, and both of the expressions
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// are valid. & takes the address of an object. Elements of a slice are
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// addressable, and so are local variables. Built-in functions new and
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// make explicitly allocate memory, but local objects can be allocated
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// as needed. Here memory for r will be still be referenced after the
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// function returns so it will be allocated as well. The int allocated
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// with new on the other hand will no longer be referenced and can be
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// garbage collected as needed by the Go runtime. The memory allocated
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// with make will still be referenced at that one element, and so it
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// cannot be garbage collected. All 20 ints remain in memory because
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// one of them is still referenced.
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}
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func expensiveComputation() int {
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return 1e6
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}
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func learnFlowControl() {
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// If statements require brace brackets, and do not require parens.
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if true {
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fmt.Println("told ya")
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}
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// This is how we format the brace brackets. Formatting is standardized
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// by the command line command "go fmt." Everybody does it. You will
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// suffer endless disparaging remarks until you conform as well.
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if false {
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// pout
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} else {
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// gloat
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}
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// If statements can be chained of course, but it's idiomatic to use
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// the handy switch statement instead.
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x := 1
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switch x {
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case 0:
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case 1:
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// cases don't "fall through"
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case 2:
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// unreached
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}
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// Like if, for doesn't use parens either. The scope of a variable
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// declared in the first clause of the for statement is the statement
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// and block. This x shadows the x declared above, but goes out of
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// scope after the for block.
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for x := 0; x < 3; x++ { // ++ is a statement
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fmt.Println("iteration", x)
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}
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// x == 1 here.
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// For is the only loop statement in Go, but it has alternate forms.
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for { // infinite loop
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break // just kidding
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continue // unreached
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}
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// The initial assignment of the for statement is handy enough that Go
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// if statements can have one as well. Just like in the for statement,
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// the := here means to declare and assign y first, then test y > x.
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// The scope of y is limited to the if statement and block.
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if y := expensiveComputation(); y > x {
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x = y
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}
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// Functions are first class objects and function literals are handy.
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// Function literals are closures.
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xBig := func() bool {
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return x > 100 // references x declared above switch statement.
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}
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fmt.Println("xBig:", xBig()) // true (we last assigned 1e6 to x)
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x /= 1e5 // this makes it == 10
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fmt.Println("xBig:", xBig()) // false now
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// When you need it, you'll love it. Actually Go's goto has been reformed
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// a bit to avoid indeterminate states. You can't jump around variable
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// declarations and you can't jump into blocks.
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goto love
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love:
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learnInterfaces() // Good stuff coming up!
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}
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// An interface is a list of functionality that a type supports. Notably
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// missing from an interface definition is any declaration of which types
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// implement the interface. Types simply implement an interface or they don't.
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//
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// An interface can have any number of methods, but it's actually common
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// for an interface to have only single method. It is idiomatic in this
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// case for the single method to be named with some action, and for the
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// interface name to end in "er."
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//
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// An interface definition is one kind of a type definition. Interface is
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// a built in type. Stringer is defined here as an interface type with one
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// method, String.
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type Stringer interface {
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String() string
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}
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// Struct is another built in type. A struct aggregates "fields."
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// Pair here has two fields, ints named x and y.
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type pair struct {
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x, y int
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}
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// User defined types can have "methods." These are functions that operate
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// in the context of an instance of the user defined type. The instance
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// is called the "receiver" and is identified with a declaration just in front
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// of the method name. The receiver here is "p." In most ways the receiver
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// works just like a function parameter.
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//
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// This String method has the same name and return value as the String method
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// of the Stringer interface. Further, String is the only method of Stringer.
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// The pair type thus implements all methods of the Stringer interface and
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// we say simply that pair implements Stringer. No other syntax is needed.
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func (p pair) String() string {
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// Sprintf is another public function in package fmt.
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// Dot syntax references fields of p.
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return fmt.Sprintf("(%d, %d)", p.x, p.y)
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}
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func learnInterfaces() {
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// Brace syntax is a "struct literal." It evaluates to an initialized
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// struct. The := syntax declares and initializes p to this struct.
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p := pair{3, 4}
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fmt.Println(p.String()) // call String method of p, of type pair.
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var i Stringer // declare i of type Stringer.
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i = p // valid because pair implements Stringer
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// Call String method of i, of type Stringer. Output same as above.
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fmt.Println(i.String())
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// It gets more interesting now. We defined Stringer in this file,
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// but the same interface happens to be defined in package fmt.
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// Pair thus implements fmt.Stringer as well, and does so with no
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// declaration of the fact. The definition of pair doesn't mention
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// any interfaces at all, and of course the authors of fmt.Stringer
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// had no idea that we were going to define pair.
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//
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// Functions in the fmt package know how to print some standard built in
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// types, and beyond that, they see if a type implements fmt.Stringer.
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// If so, they simply call the String method to ask an object for a
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// printable representation of itself.
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fmt.Println(p) // output same as above. Println calls String method.
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fmt.Println(i) // output same as above
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learnErrorHandling()
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}
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func learnErrorHandling() {
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// Sometimes you just need to know if something worked or not. Go has
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// a ", ok" idiom for that. Something, a map expression here, but commonly
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// a function, can return a boolean value of ok or not ok as a second
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// return value.
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m := map[int]string{3: "three", 4: "four"}
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if x, ok := m[1]; !ok { // , ok is optional but see how useful it is.
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fmt.Println("no one there")
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} else {
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fmt.Print(x)
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}
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// An error value communicates not just "ok" but more about the problem.
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if _, err := strconv.Atoi("non-int"); err != nil { // _ discards value
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// prints "strconv.ParseInt: parsing "non-int": invalid syntax"
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fmt.Println(err)
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}
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// error is a built in type. It is an interface with a single method,
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// defined internally as,
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//
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// type error interface {
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// Error() string
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// }
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//
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// The string returned by the Error method is conventionally a printable
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// error message. You can define your own error types by simply adding
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// an Error method. Your type then automatically implements the error
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// interface. We've seen two interfaces now, fmt.Stringer and error.
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// We'll revisit interfaces a little later. Meanwhile,
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learnConcurrency()
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}
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// Go has concurrency support in the language definition. The element of
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// concurrent execution is called a "goroutine" and is similar to a thread
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// but "lighter." Goroutines are multiplexed to operating system threads
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// and a running Go program can have far more goroutines than available OS
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// threads. If a machine has multiple CPU cores, goroutines can run in
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// parallel.
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//
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// Go "Channels" allow communication between goroutines in a way that is
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// both powerful and easy to understand. Channel is a type in Go and objects
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// of type channel are first class objects--they can be assigned to variables,
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// passed around to functions, and so on. A channel works conceptually much
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// like a Unix pipe. You put data in at one end and it comes out the other.
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// Channel "send" and "receive" operations are goroutine-safe. No locks
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// or additional synchronization is needed.
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// Inc increments a number, and sends the result on a channel. The channel
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// operation makes this function useful to run concurrently with other
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// goroutines. There is no special declaration though that says this function
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// is concurrent. It is an ordinary function that happens to have a
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// parameter of channel type.
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func inc(i int, c chan int) {
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c <- i + 1 // <- is the "send" operator when a channel appears on the left.
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}
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// We'll use inc to increment some numbers concurrently.
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func learnConcurrency() {
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// Same make function used earlier to make a slice. Make allocates and
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// initializes slices, maps, and channels.
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c := make(chan int)
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// Start three concurrent goroutines. Numbers will be incremented
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// concurrently, perhaps in parallel if the machine is capable and
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// properly configured. All three send to the same channel.
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go inc(0, c) // go is a statement that starts a new goroutine.
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go inc(10, c)
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go inc(-805, c)
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// Read three results from the channel and print them out.
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|
// There is no telling in what order the results will arrive!
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fmt.Println(<-c, <-c, <-c) // channel on right, <- is "receive" operator.
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cs := make(chan string) // another channel, this one handles strings.
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cc := make(chan chan string) // a channel of channels.
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go func() { c <- 84 }() // start a new goroutine just to send a value
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go func() { cs <- "wordy" }() // again, for cs this time
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|
// Select has syntax like a switch statement but is doing something
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// pretty different. Each case involves a channel operation. In rough
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|
// terms, a case is selected at random out of the cases that are ready to
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// communicate. If none are ready, select waits for one to become ready.
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select {
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case i := <-c: // the value received can be assigned to a variable
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fmt.Println("it's a", i)
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case <-cs: // or the value received can be discarded
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fmt.Println("it's a string")
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case <-cc: // empty channel, not ready for communication.
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fmt.Println("didn't happen.")
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}
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// At this point a value was taken from either c or cs. One of the two
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// goroutines started above has completed, the other will remain blocked.
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learnWebProgramming() // Go does it. You want to do it too.
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}
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|
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// A simple web server can be created with a single function from the standard
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|
// library. ListenAndServe, in package net/http, listens at the specified
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|
// TCP address and uses an object that knows how to serve data. "Knows how"
|
||
|
// means "satisfies an interface." The second parameter is of type interface,
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|
// specifically http.Handler. http.Handler has a single method, ServeHTTP.
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func learnWebProgramming() {
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err := http.ListenAndServe(":8080", pair{})
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|
// Error returns are ubiquitous in Go. Always check error returns and
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||
|
// do something with them. Often it's enough to print it out as an
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||
|
// indication of what failed. Of course there are better things to do
|
||
|
// in production code: log it, try something else, shut everything down,
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||
|
// and so on.
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|
fmt.Println(err)
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}
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|
// You can make any type into an http.Hander by implementing ServeHTTP.
|
||
|
// Lets use the pair type we defined earlier, just because we have it
|
||
|
// sitting around. ServeHTTP has two parameters. The request parameter
|
||
|
// is a struct that we'll ignore here. http.ResponseWriter is yet another
|
||
|
// interface! Here it is an object supplied to us with the guarantee that
|
||
|
// it implements its interface, which includes a method Write.
|
||
|
// We call this Write method to serve data.
|
||
|
func (p pair) ServeHTTP(w http.ResponseWriter, r *http.Request) {
|
||
|
w.Write([]byte("You learned Go in Y minutes!"))
|
||
|
}
|
||
|
|
||
|
// And that's it for a proof-of-concept web server! If you run this program
|
||
|
// it will print out all the lines from the earlier parts of the lesson, then
|
||
|
// start this web server. To hit the web server, just point a browser at
|
||
|
// localhost:8080 and you'll see the message. (Then you can probably press
|
||
|
// ctrl-C to kill it.)
|
||
|
```
|
||
|
|
||
|
## 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.)
|
||
|
|
||
|
On the reading list for students of Go is the source code to the standard
|
||
|
library. Comprehensively documented, it demonstrates the best of readable
|
||
|
and understandable Go, Go style, and Go idioms. Click on a function name
|
||
|
in the documentation and the source code comes up!
|
||
|
|