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|
// this is a remix of multiple documentations / tutorials / blogs.
// multlicensed under:
// SPDX-License-Identifier: EUPL-1.2
// SPDX-License-Identifier: CC-BY-3.0
// SPDX-License-Identifier: BSD-3-Clause
// code from https://gobyexample.com
// Copyright Mark McGranaghan
// Copyright Renée French
// code from https://go.dev/tour/list
// Copyright 2011 The Go Authors.
// code / notes from https://go.dev/blog/slices-intro
// Copyright Andrew Gerrand
package main
import (
"errors"
"fmt"
"iter"
"maps"
"math"
"reflect"
"slices"
"sync"
"time"
"unicode/utf8"
)
// 3 - constants
const const_s string = "constant hello world"
func main() {
fmt.Println("hello world")
// values, variables and constants ///////////////////////////////////////
fmt.Println("go" + "lang") // golang
fmt.Println("1+1=", 1+1) // 2
fmt.Println("7.0/3.0=", 7.0/3.0)
fmt.Println(true && false) // false
fmt.Println(true || false) // true
fmt.Println(!true) // false
var a = "initial"
var b, c int = 1, 2 // unlike c, type (class) name comes after var name
var d = true
var e int // is this uninitialized?
f := "apple" // shorthand for declaring + initialization
fmt.Println(a)
fmt.Println(b, c)
fmt.Println(d)
fmt.Println(e)
fmt.Println(f)
fmt.Println(const_s)
const n = 500000000
const n2 = 3e20 / n
fmt.Println(n2) // 6e+11
fmt.Println(int64(n2))
fmt.Println(math.Sin(n))
// control flows /////////////////////////////////////////////////////////
fmt.Println("------- - control flows---")
i := 1
// while style
for i <= 3 {
fmt.Println(i)
i = i + 1
}
// c style
for j := 0; j < 3; j++ {
fmt.Println(j)
}
// iterator style
for i := range 3 {
fmt.Println("range", i)
}
// continue
for n := range 6 {
if n%2 == 0 {
continue
}
fmt.Println(n)
}
// forever and break
for {
fmt.Println("loop")
break
}
// statements can precede if conditionals.
// variables declared here are visible in all branches
if num := 9; num < 0 {
fmt.Println(num, "is negative")
} else if num < 10 {
fmt.Println(num, "has 1 digit")
} else {
fmt.Println(num, "has multiple digits")
}
// switch-case; unlike C, only one arm is taken therefore no need for break.
something := 2
switch something {
case 1:
fmt.Println("one")
case 2:
fmt.Println("two")
case 3:
fmt.Println("three")
}
// can use switch-case as a tidier if-if-else: case can take conditions
t := time.Now()
switch {
case t.Hour() < 12:
fmt.Println("it's before noon")
default:
fmt.Println("it's after noon")
}
// likewise for `if` we can bind a value here
switch day := time.Now().Weekday(); day {
// can merge multiple arms
case time.Saturday, time.Sunday:
fmt.Println(day, "is weekend")
default:
fmt.Println(day, "is weekday")
}
// type switch : switch on type instead of values whatAmI is a function
// object, see later note: this can also be done with reflect.TypeOf() an
// empty interface means pretty much "anything"
whatAmI := func(i interface{}) {
switch t := i.(type) {
case bool:
fmt.Println("I'm bool")
case int:
fmt.Println("I'm int")
default:
fmt.Printf("Don't know type %T\n", t)
}
}
whatAmI(true)
whatAmI(1)
whatAmI("hello")
whatAmI(whatAmI)
// arrays ////////////////////////////////////////////////////////////////
fmt.Println("--------- arrays ---------")
// array is zero valued by default
var arr [5]int
fmt.Println("init array a[5]int:", arr)
// accessing by index:
arr[4] = 100
fmt.Println("set", arr)
fmt.Println("get @4", arr[4])
whatAmI(arr)
// length
fmt.Println("length of arr is", len(arr))
// const initializer:
arr_2 := [5]int{1, 2, 3, 4, 5}
fmt.Println("arr_2", arr_2)
// auto length
arr_3 := [...]int{1, 2, 3, 4, 5}
fmt.Println("arr_3", arr_3)
// syntax suggar, specify index: {100, 0, 0, 400, 500}
// unspecified elements are default to zero
arr_4 := [...]int{100, 3: 400, 500}
fmt.Println("arr_4", arr_4)
// multi-dimensional
var arr_2d [2][3]int
for i := 0; i < 2; i++ {
for j := 0; j < 3; j++ {
arr_2d[i][j] = i + j
}
}
fmt.Println("2d arr: ", arr_2d)
// 2d array const init
arr_2d = [2][3]int{
{1, 2, 3},
{1, 2, 3},
}
fmt.Println("2d arr: ", arr_2d)
// slices ////////////////////////////////////////////////////////////////
// https://go.dev/blog/slices-intro
// slices ar edynamically-sized, flexible view into elements of an array
// (pretty much the same as rust slice?)
fmt.Println("--------- slices ---------")
// slice has type []T (unlike array, no length specified)
arr_base := [6]int{1, 2, 3, 4, 5, 6}
// create slice from array with arr_base[low : high]
// left inclusive, right exclusive
var arr_slice []int = arr_base[1:4]
// or omit high/low bounds to use the defaults (0 for low, len for high)
var arr_slice2 []int = arr_base[:3]
var arr_slice3 []int = arr_base[:]
fmt.Println("array_base: ", arr_base, " has type ", reflect.TypeOf(arr_base))
fmt.Println("array_slice: ", arr_slice, " has type ", reflect.TypeOf(arr_slice))
// slice is mutable reference to the underlying array
arr_slice[1] = 42
fmt.Println("arr_base after mod: ", arr_base)
// all slices to the array base sees the modification
fmt.Println("arr_slice2 after mod: ", arr_slice2)
fmt.Println("arr_slice3 after mod: ", arr_slice3)
// slice has length and capacity
// read with len(s) and cap(s)
// cap is length of underlying array, _counting from slice low bound_
fmt.Println("arr_slice has length ", len(arr_slice), "and cap ", cap(arr_slice))
// slices can be trimmed or extended (given slice has enough capacity)
arr_slice = arr_slice[:0]
sliceInfo(arr_slice) // len = 0, cap=5 []
// extend the slice
arr_slice = arr_slice[:4] // looks like out-of-bound but no
sliceInfo(arr_slice) // len = 4, cap=5 [2 42 4 5]
// drop first to values
arr_slice = arr_slice[2:]
// slice cap changes when its low bound shifts
sliceInfo(arr_slice) // len = 2 cap=3 [4 5]
// there is no way (?) to extend the slice on the lower bound
// nil slice
var s_nil []int
sliceInfo(s_nil) // s_nil == nil
// append to slice
// The resulting value of append is a slice containing all the elements of
// the original slice plus the provided values. If the backing array of s is
// too small to fit all the given values a bigger array will be allocated.
// The returned slice will point to the newly allocated array.
s_nil = append(s_nil, 1)
sliceInfo(s_nil) // len 1, cap 1, [1]
s_nil = append(s_nil, 2, 3, 4) // can take VA
sliceInfo(s_nil) // len 4, cap 4, [1 2 3 4]
// make builtin fuction: make([]T, len, cap)
// create new, initialized slice value for a given element type T
// a slice created with make always allocates a new, hidden array to whicvh
// the the returned slice value refers.
// sm1 and sm2 are equivalent
var sm1 = make([]int, 50, 100)
var sm2 = new([100]int)[0:50]
fmt.Println("sm1 has type ", reflect.TypeOf(sm1))
fmt.Println("sm2 has type ", reflect.TypeOf(sm2))
// slice literals : q is a reference to a (anonymous) constant array
p := [...]int{1, 2, 3, 4, 5, 6, 7}
q := []int{1, 2, 3, 4, 5, 6, 7}
fmt.Println("p has type ", reflect.TypeOf(p))
fmt.Println("q has type ", reflect.TypeOf(q))
var s []string
fmt.Println("uninit:", s)
s = make([]string, 3, 3)
// shorthand : can use make([]T, len)
s[0] = "apple"
s[1] = "banana"
s[2] = "carrot"
s = append(s, "e", "f", "g")
fmt.Println(s, "len:", len(s), "cap:", cap(s))
// 2d slices (like 2d array)
slice_2d := make([][]int, 3)
for i := 0; i < 3; i++ {
innerLen := i + 1
slice_2d[i] = make([]int, innerLen)
for j := 0; j < innerLen; j++ {
slice_2d[i][j] = i + j
}
}
fmt.Println("2d slice ", slice_2d)
// slices package functions
// slices. {Max, Min, Equal, Grow ....}
// maps //////////////////////////////////////////////////////////////////
// https://go.dev/blog/slices-intro
fmt.Println("--------- maps -----------")
// create map
// make(map[key_type]val_type) // TODO: what's under the hood?
m := make(map[string]int)
m["k1"] = 7 // if key doesn't exist a new pair is created
m["k2"] = 13
m["k3"] = 0
fmt.Println("map: ", m) // [k1:7 k2:13 k3:0]
fmt.Println("len of map:", len(m))
fmt.Println("v1: ", m["k1"]) // 7
fmt.Println("v2: ", m["k2"]) // 13
fmt.Println("v3: ", m["k3"]) // 0
// if key doesn't exist, returns default value (0)
fmt.Println("v_na: ", m["kna"])
// disambiguate between missing keys and zero-valued keys
// optionally return a presence bool from map
v, present := m["kna"]
fmt.Println("presence of kna ", present, " value is ", v) // false, 0
delete(m, "k2") // remove a kv pair
fmt.Println("map: ", m) // [k1:7 k3:0]
delete(m, "kna") // no error reported when deleting non-exist
fmt.Println("map: ", m) // [k1:7 k3:0]
clear(m) // remove everything
fmt.Println("map: ", m) // []
// inline declaration
map1 := map[string]int{"foo": 1, "bar": 2}
map2 := map[string]int{"foo": 1, "bar": 2}
// map equal?
if maps.Equal(map1, map2) {
fmt.Println("n1 == n2: ", map1)
}
// range /////////////////////////////////////////////////////////////////
// iterate over array (slice) with auto index (from 0)
for i, n := range arr_base {
fmt.Printf("%d:%d ", i, n)
}
fmt.Println()
// range index always starts from 0
for i, n := range arr_base[2:] {
fmt.Printf("%d:%d ", i, n)
}
fmt.Println()
// range over map:
kvs := map[string]string{"a": "apple", "b": "banana"}
for k, v := range kvs {
fmt.Printf("%s -> %s \n", k, v)
}
// range over string: (by unicode rune)
for i, c := range "go" {
fmt.Println(i, c)
}
// with widechars the indexing will be quite different (TODO)
// https://gobyexample.com/strings-and-runes
// for i, c := range "测试" {
// fmt.Println(i,c)
// }
// pointers //////////////////////////////////////////////////////////////
// just like c.
f_val := func(ival int) {
ival = 0
}
f_ptr := func(iptr *int) {
*iptr = 0
}
test_vi := 1
fmt.Println(test_vi) // 1
f_val(test_vi)
fmt.Println(test_vi) // 1
f_ptr(&test_vi)
fmt.Println(test_vi) // 0
fmt.Println("ptr addr:", &test_vi)
// strings and runes (TODO) //////////////////////////////////////////////
const test_s = "测试"
fmt.Println("string: ", test_s, " len: ", len(test_s), " runes: ", utf8.RuneCountInString(test_s))
// strings are equivalent to []byte, iterating results in raw bytes
for i := 0; i < len(s); i++ {
fmt.Printf("%x ", s[i])
}
fmt.Println()
// range over string gets runes. The idx is byte idx where the rune starts
for idx, runeValue := range test_s {
// use %U to print a rune
fmt.Printf("%#U starts a at %d\n", runeValue, idx)
}
// type rune and literals
examineRune := func(r rune) {
if r == '试' {
fmt.Println("found \"试\"")
}
}
// manually decode runes from raw byte array
for i, w := 0, 0; i < len(test_s); i += w {
runeValue, width := utf8.DecodeRuneInString(test_s[i:])
fmt.Printf("%#U starts at %d\n", runeValue, i)
w = width
examineRune(runeValue)
}
// structs / embedded structs ////////////////////////////////////////////
// TODO: https://gobyexample.com/struct-embedding
// create a type "person" from a struct
type person struct {
name string
age int
}
test_p := person{name: "john"} // age will take default value
test_p.age = 42
test_p2 := person{name: "alice", age: 24}
fmt.Println(test_p, test_p2)
// automatic dereference
fmt.Println(test_p.name)
test_p_ptr := &test_p
fmt.Println(test_p_ptr.name)
// anonymous struct:
dog := struct {
name string
isGood bool
}{
"Rex",
true,
}
fmt.Println(dog)
// goroutines ////////////////////////////////////////////////////////////
// add keyword `go` before function calls to run the function concurrently
test_func := func(str string) {
for i := 0; i < 3; i++ {
fmt.Println(str, ":", i)
}
}
go test_func("hello")
// anonymous
go func(msg string) {
fmt.Println(msg)
}("world")
// wait group
var wg sync.WaitGroup
for i := 1; i <= 5; i++ {
wg.Add(1)
go func() { // a wrapper func with to notify finish
defer wg.Done()
func(id int) { // the actual worker function
fmt.Printf("Worker %d starting\n", id)
time.Sleep(50 * time.Millisecond)
fmt.Printf("Worker %d done\n", id)
}(i)
}()
}
wg.Wait()
// TODO for error propagation from workders, check errgroup package
// https://pkg.go.dev/golang.org/x/sync/errgroup
// channels //////////////////////////////////////////////////////////////
// create a new channel with make(chan valType)
messages := make(chan string)
// send a value to channel with `channel <- thing`
go func() { messages <- "ping" }()
// receive message from channel with `<- channel`
msg := <-messages
fmt.Println(msg)
// channel with buffering
messages_2 := make(chan string, 2)
messages_2 <- "hello"
messages_2 <- "world"
// messages_2 <- "foo"
// exceeding buffer limit
// fatal error: all goroutines are asleep - deadlock!
fmt.Println(<-messages_2)
fmt.Println(<-messages_2)
// channel and goroutine sync:
test_chan_func := func(done chan bool) {
fmt.Println("working")
time.Sleep(50 * time.Millisecond)
done <- true
}
done := make(chan bool)
go test_chan_func(done)
<-done // this will block until the goroutine sends to channel
// specify channel direction in function parameter
ping := func(pings chan<- string, msg string) {
pings <- msg
}
pong := func(pings <-chan string, pongs chan<- string) {
msg := <-pings
pongs <- msg
}
pings := make(chan string, 1)
pongs := make(chan string, 1)
go ping(pings, "hello")
go pong(pings, pongs)
fmt.Println("received from pongs: ", <-pongs)
// select : wait on multiple channels (like unix epoll)
c1 := make(chan string, 1) // buffered, non-blocking send
c2 := make(chan string, 1)
go func() {
time.Sleep(1 * time.Second)
c1 <- "one"
}()
go func() {
time.Sleep(2 * time.Second)
c1 <- "two"
}()
for i := 0; i < 2; i++ {
select {
case msg1 := <-c1:
fmt.Println("received", msg1)
case msg2 := <-c2:
fmt.Println("received", msg2)
case <-time.After(3 * time.Second):
// implements a timenot, note that channels are buffered and sends
// are non-blocking
fmt.Println("timeout 3")
}
}
// non-blocking channels
messages_3 := make(chan string)
messages_4 := make(chan string)
// messages_3 is unbuffered; send/receive typically blocks
// however this select will not block with a default select case
// multi-way nonblocking select:
select {
case msg := <-messages_3:
fmt.Println("received message 3", msg)
case msg := <-messages_4:
fmt.Println("received message 4", msg)
default:
fmt.Println("no message received")
}
// TODO closing a channel
close(messages_3)
close(messages_4)
// TODO worker pools
// https://gobyexample.com/worker-pools
// TODO rate limiting
// https://gobyexample.com/rate-limiting
// iterate / range over channels
messages_5 := make(chan string, 2)
messages_5 <- "one"
messages_5 <- "two"
close(messages_5) // need to close before iterating
for elem := range messages_5 {
fmt.Println(elem) // one two
}
// nonblocking send
select {
case messages <- "hello":
fmt.Println("sent message hello")
default:
fmt.Println("no message sent")
}
// timers and tickers ////////////////////////////////////////////////////
// TODO https://gobyexample.com/timers
// TODO https://gobyexample.com/tickers
// atomic types ; mutex //////////////////////////////////////////////////
// TODO https://gobyexample.com/atomic-counters
// TODO stateful goroutines (BOOKMARK!!!!)
// https://gobyexample.com/stateful-goroutines
// sugars ..//////////////////////////////////////////////////////////////
// 不是，你们go玩家吃的都那么好的吗
// inlined declaration (anonymous type)
arr_struct := []struct {
i int
b bool
}{
{1, true},
{2, false},
{3, true},
}
fmt.Println(arr_struct)
// turn slice into multiple arguments:
nums := []int{1, 2, 3, 4}
fmt.Println(sums(nums...))
// x - keywords and builtin functions ///
fmt.Println("--- [x] - keywords -------")
// make, new
// other tests ///////////////////////////////////////////////////////////
fmt.Println(sums(1, 2, 3, 4))
// use ... to expand slice into variadic attrs
fmt.Println(sums(nums...))
// closure:
nextInt := intSeq()
fmt.Println(nextInt()) // 1
fmt.Println(nextInt()) // 2
fmt.Println(nextInt()) // 3
fmt.Println(nextInt()) // 4
newInt := intSeq()
fmt.Println(newInt()) // 1
// return function local reference safely
sth := new_something("hello", 42)
fmt.Println(*sth)
// test methods + auto value/ptr conversion
r := rect{width: 10, height: 5}
rp := &r
fmt.Println("area: ", r.area()) // 50
fmt.Println("perim:", r.perim2()) // 30
fmt.Println("area: ", rp.area()) // 50
fmt.Println("perim:", rp.perim2()) // 30
// test interfaces
cir := circle{radius: 5}
measure(cir)
measure(r)
// test enums
ns := transition(StateIdle)
fmt.Println(ns)
ns2 := transition(ns)
fmt.Println(ns2)
// test generics
var test_strarr = []string{"foo", "bar", "zoo"}
// can explicitly specify the types but the compiler can infer the types
// automatically : below is warning "unnecessary type arguments"
// _ = SliceIndex[[]string, string](test_strarr, "zoo")
fmt.Println("index of zoo:", SliceIndex(test_strarr, "zoo")) // 2
// test generics (linked list)
lst := List[int]{}
lst.Push(1)
lst.Push(2)
lst.Push(3)
fmt.Println("list is : ", lst.AllElements())
// test iterators
for e := range lst.All() {
fmt.Println(e)
}
// slices.Collect:
fmt.Println(slices.Collect(lst.All()))
// "infinite" iterator
for n := range genFib() {
if n >= 10 {
break
}
fmt.Println(n)
}
}
// functions /////////////////////////////////////////////////////////////////
//
func sliceInfo(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}
// with return type
func plusplus(a, b, c int) int {
return a + b + c
}
// with multiple returns, can use blank identifier to ignore a ret value
// a, b := vals()
// _, b := vals()
func vals() (int, int) {
return 3, 7
}
// variadic functions
func sums(nums ...int) int {
// nums is slice type: []int
total := 0
for _, num := range nums {
total += num
}
return total
}
// closures (anonymous/lambda functions)
// this function returns a function
func intSeq() func() int {
i := 0
// i is basically a "static" variable to the returned function.
// each returned closure instance has its own copy of the variable
return func() int {
i++
return i
}
}
// recursion
func fact(n int) int {
if n == 0 {
return 1
}
return n * fact(n-1)
}
// go is GC language, can safely return reference to a function local variable
type something struct {
name string
val int
}
func new_something(name string, val int) *something {
p := something{name: name}
p.val = val
return &p // ok ....
}
// methods ///////////////////////////////////////////////////////////////
type rect struct {
width, height float64
}
// methods have receiver
func (r rect) area() float64 {
return r.width * r.height
}
func (r rect) perim() float64 {
return r.width * r.height
}
// auto conversion between value and pointers
func (r *rect) perim2() float64 {
return 2 * (2*r.width + 2*r.height)
}
// interface /////////////////////////////////////////////////////////////
// interfaces are named collections of method signatures
type geometry interface {
area() float64
perim() float64
}
type circle struct {
radius float64
}
func (c circle) area() float64 {
return math.Pi * c.radius * c.radius
}
func (c circle) perim() float64 {
return 2 * math.Pi * c.radius
}
// take interface as parameter type: i.e. g implements interface geometry
func measure(g geometry) {
fmt.Println(g)
fmt.Println(g.area())
fmt.Println(g.perim())
}
// enums /////////////////////////////////////////////////////////////////
type State int
const (
// keyword iota generates successive constant values automatically
StateIdle State = iota // 0
StateConnected // 1
StateError // 2
StateRetrying // 3
)
var stateName = map[State]string{
StateIdle: "idle",
StateConnected: "connected",
StateError: "error",
StateRetrying: "retrying",
}
// .String() method is automatically called when e.g. printing as %s
func (ss State) String() string {
return stateName[ss]
}
// switch case w/ enums
func transition(s State) State {
switch s {
case StateIdle:
return StateConnected
case StateConnected, StateRetrying:
return StateIdle
case StateError:
return StateError
default:
panic(fmt.Errorf("unknown state: %s", s))
}
}
// generics //////////////////////////////////////////////////////////////
// function takes a slice of a comparable type E
// and finds the index of first occurrence of value v
// ~ means the type argument for S can be any type whose underlying type is a
// slice type
// https://stackoverflow.com/questions/70888240/whats-the-meaning-of-the-new-tilde-token-in-go
// the [] brackets here is just like <> for c++ and rust
func SliceIndex[S ~[]E, E comparable](s S, v E) int {
for i := range s {
if v == s[i] {
return i
}
}
return -1
}
type List[T any] struct {
head, tail *element[T] // tail is a pointer to an element with type parameter T
}
type element[T any] struct {
next *element[T]
val T
}
func (lst *List[T]) Push(v T) {
if lst.tail == nil {
lst.head = &element[T]{val: v} // empty list, create anew
lst.tail = lst.head
} else {
lst.tail.next = &element[T]{val: v} // append to tail
lst.tail = lst.tail.next
}
}
func (lst *List[T]) AllElements() []T {
var elems []T
for e := lst.head; e != nil; e = e.next {
elems = append(elems, e.val)
}
return elems
}
// (custom) iterators ////////////////////////////////////////////////////
// returns an iterator from a linked list
func (lst *List[T]) All() iter.Seq[T] {
// returns a function, that takes a `yield` function as parameter
// yield is called on every element we want to iterate over
// * need to check yield return value for early termination
return func(yield func(T) bool) {
for e := lst.head; e != nil; e = e.next {
if !yield(e.val) {
return
}
}
}
}
func genFib() iter.Seq[int] {
return func(yield func(int) bool) {
a, b := 1, 1
for {
if !yield(a) {
return
}
a = b
b = a + b
}
}
}
// errors ////////////////////////////////////////////////////////////////
// TODO https://gobyexample.com/errors
// TODO https://gobyexample.com/custom-errors
// TODO matching errors (along the chains)
// by convention, erros are the last return value and have type error
func f(arg int) (int, error) {
if arg == 42 {
return -1, errors.New("can't work with 42")
}
return arg + 3, nil
}
// custom error type
type argError struct {
arg int
message string
}
// implement method Error() string
// to comply with the error interface
func (e *argError) Error() string {
return fmt.Sprintf("%d - %s", e.arg, e.message)
}
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