$ curl cheat.sh/
// Single line comment
/* Multi-
 line comment */

 /* A build tag is a line comment starting with // +build
  and can be executed by go build -tags="foo bar" command.
  Build tags are placed before the package clause near or at the top of the file
  followed by a blank line or other line comments. */
// +build prod, dev, test

// 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.
	"math"    // Math library with local alias m.
	"net/http"  // Yes, a web server!
	"os"        // OS functions like working with the file system
	"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.
	// It comes from the package 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.
	a5 := [...]int{3, 1, 5, 10, 100} // An array initialized with a fixed size of five
	// elements, with values 3, 1, 5, 10, and 100.

	// Arrays have value semantics.
	a4_cpy := a4            // a4_cpy is a copy of a4, two separate instances.
	a4_cpy[0] = 25          // Only a4_cpy is changed, a4 stays the same.
	fmt.Println(a4_cpy[0] == a4[0]) // false

	// 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 a5. 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.

	// Slices (as well as maps and channels) have reference semantics.
	s3_cpy := s3            // Both variables point to the same instance.
	s3_cpy[0] = 0           // Which means both are updated.
	fmt.Println(s3_cpy[0] == s3[0]) // true	

	// Because they are dynamic, slices can be appended to on-demand.
	// To append elements to a slice, the 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 ellipsis, 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 underscore lets you "use" a variable but discard its value.
	_, _, _, _, _, _, _, _, _, _ = str, s2, g, f, u, pi, n, a5, s4, bs
	// Usually you use it to ignore one of the return values of a function
	// For example, in a quick and dirty script you might ignore the
	// error value returned from os.Create, and expect that the file
	// will always be created.
	file, _ := os.Create("output.txt")
	fmt.Fprint(file, "This is how you write to a file, by the way")
	file.Close()
	
	// 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.
// Unlike in C/Cpp taking and returning an address of a local variable is also safe. 
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 slice 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.
}

// Use the aliased math library (see imports, above) 
func expensiveComputation() float64 {
	return m.Exp(10)
}

func learnFlowControl() {
	// If statements require brace brackets, and do not require parentheses.
	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, 2: // Can have multiple matches on one case
	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.
	}

	// Type switch allows switching on the type of something instead of value
	var data interface{}
	data = ""
	switch c := data.(type) {
	case string:
		fmt.Println(c, "is a string")
	case int64:
		fmt.Printf("%d is an int64\n", c)
	default:
		// all other cases
	}

	// 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)
	}
	// If you only need the value, use the underscore as the key
	for _, name := range []string{"Bob", "Bill", "Joe"} {
		fmt.Printf("Hello, %s\n", name)
	}

	// 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.
	}
	x = 99999
	fmt.Println("xBig:", xBig()) // true
	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) {
	// A defer statement pushes a function call onto a list. The list of saved
	// calls is executed AFTER the surrounding 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 because Pair has defined all the methods in the interface.
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))
}

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