// Single line comment
/* Multi-
line comment */
/* A build tag is a line comment starting with //go: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. */
//go: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" // Implements some I/O utility functions.
m "math" // Math library with local alias m.
"net/http" // Yes, a web server!
_ "net/http/pprof" // Profiling library imported only for side effects
"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 := 3.14159 // 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 slice 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
// Looking up a missing key returns the zero value,
// which is 0 in this case, since it's a map[string]int
m["key not present"] // 0
// Check if a key is present in the map like this:
if val, ok := m["one"]; ok {
// Do something
}
// 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 ...any) { // any is an alias for interface{}
// Iterate each value of the variadic.
// The underscore 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 := io.ReadAll(resp.Body)
fmt.Printf("\nWebserver said: `%s`", string(body))
}