Go Details 101

A Go source file can imports the same package multiple times, but the import names must be different. These same-package imports reference the same package instance.

For example:

package main

import "fmt"
import "io"
import inout "io"

func main() {
	fmt.Println(&inout.EOF == &io.EOF) // true
}

For example:

	switch n := rand.Intn(3); n {
	case 0: fmt.Println("n == 0")
	case 1: fmt.Println("n == 1")
	default: fmt.Println("n == 2")
	}

	switch n := rand.Intn(3); n {
	default: fmt.Println("n == 2")
	case 0: fmt.Println("n == 0")
	case 1: fmt.Println("n == 1")
	}

	switch n := rand.Intn(3); n {
	case 0: fmt.Println("n == 0")
	default: fmt.Println("n == 2")
	case 1: fmt.Println("n == 1")
	}

	var x, y chan int

	select {
	case <-x:
	case y <- 1:
	default:
	}

	select {
	case <-x:
	default:
	case y <- 1:
	}

	select {
	default:
	case <-x:
	case y <- 1:
	}

For example, the following program fails to compile.

package main

func main() {
	switch 123 {
	case 123:
	case 123: // error: duplicate case
	}
}

But the following program compiles okay.

package main

func main() {
	switch false {
	case false:
	case false:
	}
}

For reasons, please read this issue. The behavior is compiler dependent. In fact, the standard Go compiler also doesn't allow duplicate string case expressions, but gccgo allows.

For example, the switch expression 123 in the following switch block is viewed as a value of int instead of an untyped integer. So the following program fails to compile.

package main

func main() {
	switch 123 {
	case int64(123):  // error: mismatched types
	case uint32(789): // error: mismatched types
	}
}

For example, the following program will print true.

package main

import "fmt"

func main() {
	switch { // <=> switch true {
	case true:  fmt.Println("true")
	case false: fmt.Println("false")
	}
}

For example:

package main

func main() {
	var i = 0
Outer:
	for
	{ // okay on the next line
		switch
		{ // okay on the next line
		case i == 5:
			break Outer
		default:
			i++
		}
	}
}

What result will the following program print? true or false? The answer is true. Please read line break rules in Go for reasons.

package main

import "fmt"

func False() bool {
	return false
}

func main() {
	switch False()
	{
	case true:  fmt.Println("true")
	case false: fmt.Println("false")
	}
}

For example, the following program fails to compile.

func demo(n, m int) (r int) {
	switch n {
	case 123:
		if m > 0 {
			goto End
		}
		r++

		End: // syntax error: missing statement after label
	default:
		r = 1
	}
	return
}

To make it compile okay, the case branch code block should be explicit:

func demo(n, m int) (r int) {
	switch n {
	case 123: {
		if m > 0 {
			goto End
		}
		r++

		End:
	}
	default:
		r = 1
	}
	return
}

Alternatively, we can let a semicolon follow the label declaration End::

func demo(n, m int) (r int) {
	switch n {
	case 123:
		if m > 0 {
			goto End
		}
		r++

		End:;
	default:
		r = 1
	}
	return
}

Please read line break rules in Go for reasons.

For example:

package main

import "fmt"

func F() (r int) {
	defer func() {
		r = 789
	}()

	return 123 // <=> r = 123; return
}

func main() {
	fmt.Println(F()) // 789
}

We can exit a program from any function by calling the os.Exit function. An os.Exit function call takes an int code as argument and returns the code to operating system.

An example:

// exit-example.go
package main

import "os"
import "time"

func main() {
	go func() {
		time.Sleep(time.Second)
		os.Exit(1)
	}()
	select{}
}

Run it:

$ go run a.go
exit status 1
$ echo $?
1

We can make a goroutine exit by calling the runtime.Goexit function. The runtime.Goexit function has no parameters.

In the following example, the Java word will not be printed.

package main

import "fmt"
import "runtime"

func main() {
	c := make(chan int)
	go func() {
		defer func() {c <- 1}()
		defer fmt.Println("Go")
		func() {
			defer fmt.Println("C")
			runtime.Goexit()
		}()
		fmt.Println("Java")
	}()
	<-c
}

For example:

package main

import "fmt"

type T struct {
	x int
	y *int
}

func main() {
	var t T
	p := &t.x // <=> p := &(t.x)
	fmt.Printf("%T\n", p) // *int

	*p++ // <=> (*p)++
	*p-- // <=> (*p)--

	t.y = p
	a := *t.y // <=> *(t.y)
	fmt.Printf("%T\n", a) // int
}

package main

func main() {
}

const M  = 2
// Compiles okay. 1.0 is deduced as an int value.
var _ = 1.0 << M

var N = 2
// Fails to compile. 1.0 is deduced as a float64 value.
var _ = 1.0 << N

Please read this article for reasons.

An example:

package main

type MyInt int64
type Ta    *int64
type Tb    *MyInt

func main() {
	var a Ta
	var b Tb

	// Direct conversion is not allowed.
	//a = Ta(b) // error

	// But indirect conversion is possible.
	y := (*MyInt)(b)
	x := (*int64)(y)
	a = x           // <=> the next line
	a = (*int64)(y) // <=> the next line
	a = (*int64)((*MyInt)(b))
	_ = a
}

Whether or not the addresses of two zero-sized values are equal is compiler and compiler version dependent.

package main

import "fmt"

func main() {
	a := struct{}{}
	b := struct{}{}
	x := struct{}{}
	y := struct{}{}
	m := [10]struct{}{}
	n := [10]struct{}{}
	o := [10]struct{}{}
	p := [10]struct{}{}

	fmt.Println(&x, &y, &o, &p)

	// For the standard Go compiler (1.16),
	// x, y, o and p escape to heap, but
	// a, b, m and n are allocated on stack.

	fmt.Println(&a == &b) // false
	fmt.Println(&x == &y) // true
	fmt.Println(&a == &x) // false

	fmt.Println(&m == &n) // false
	fmt.Println(&o == &p) // true
	fmt.Println(&n == &p) // false
}

The outputs indicated in the above code are for the standard Go compiler 1.16.

An example:

package main

func main() {
	type P *P
	var p P
	p = &p
	p = **************p
}

Similarly,

• the element type of a slice type can be the slice type itself,
• the element type of a map type can be the map type itself,
• the element type of a channel type can be the channel type itself,
• and the argument and result types of a function type can be the function type itself.

package main

func main() {
	type S []S
	type M map[string]M
	type C chan C
	type F func(F) F

	s := S{0:nil}
	s[0] = s
	m := M{"Go": nil}
	m["Go"] = m
	c := make(C, 3)
	c <- c; c <- c; c <- c
	var f F
	f = func(F)F {return f}

	_ = s[0][0][0][0][0][0][0][0]
	_ = m["Go"]["Go"]["Go"]["Go"]
	<-<-<-c
	f(f(f(f(f))))
}

For a pointer value, which type is either defined or not, if the base type of its (pointer) type is a struct type, then we can select the fields of the struct value referenced by the pointer value through the pointer value. However, if the type of the pointer value is a defined type, then we can't select the methods of the struct value referenced by the pointer value through the pointer value.

package main

type T struct {
	x int
}
func (T) m(){} // T has one method.

type P *T  // a defined one-level pointer type.
type PP *P // a defined two-level pointer type.

func main() {
	var t T
	var tp = &t
	var tpp = &tp
	var p P = tp
	var pp PP = &p
	tp.x = 12  // okay
	p.x = 34   // okay
	pp.x = 56  // error: type PP has no field or method x
	tpp.x = 78 // error: type **T has no field or method x

	tp.m()  // okay. Type *T also has a "m" method.
	p.m()   // error: type P has no field or method m
	pp.m()  // error: type PP has no field or method m
	tpp.m() // error: type **T has no field or method m
}

For example, the Foo1 and the Foo2 functions are equivalent, but the function Foo2 is much tidier than the function Foo1.

func Foo1(m map[string]int) int {
	if m != nil {
		return m["foo"]
	}
	return 0
}

func Foo2(m map[string]int) int {
	return m["foo"]
}

For example, the following program will not panic.

package main

func main() {
	var m map[string]int // nil
	delete(m, "foo")
}

For example,

package main

import "fmt"

func main() {
	s := make([]int, 3, 9)
	fmt.Println(len(s)) // 3
	s2 := s[2:7]
	fmt.Println(len(s2)) // 5
}

Please read derive slices from arrays and slices for details.

For example, the following program will not panic at run time.

package main

import "fmt"

func main() {
	var x []int // nil
	a := x[:]
	b := x[0:0]
	c := x[:0:0]
	// Print three "true".
	fmt.Println(a == nil, b == nil, c == nil)
}

Please read derive slices from arrays and slices for details.

For example, the following program compiles okay.

package main

func main() {
	var s []int // nil
	for range s {
	}

	var m map[string]int // nil
	for range m {
	}
}

For example, the following program will print 01234.

package main

import "fmt"

func main() {
	var a *[5]int // nil
	for i, _ := range a {
		fmt.Print(i)
	}
}

For example, the following code fails to compile.

var k = 1
// error: index must be non-negative integer constant
var x = [2]int{k: 1}
// error: index must be non-negative integer constant
var y = []int{k: 1}

Note, the keys in map composite literals are not required to be constants.

For example, the following code fails to compile.

// error: duplicate index in array literal: 1
var a = []bool{0: false, 1: true, 1: true}
// error: duplicate index in array literal: 0
var b = [...]string{0: "foo", 1: "bar", 0: "foo"}
// error: duplicate key "foo" in map literal
var c = map[string]int{"foo": 1, "foo": 2}

This feature can be used to assert some conditions at compile time.

The reason is the elements of an array value and the array will be stored in the same memory block when the array is stored in memory. But the situation is different for slices.

An example:

package main

func main() {
	// Container composite literals are unaddressable.

	// It is ok to take slice literal element addresses.
	_ = &[]int{1}[0] // ok
	// Cannot take addresses of array literal elements.
	_ = &[5]int{}[0] // error

	// It is ok to modify slice literal elements.
	[]int{1,2,3}[1] = 9  // ok
	// Cannot modify array literal elements.
	[3]int{1,2,3}[1] = 9 // error
}

The reason is the same as the last detail.

An example:

package main

func main() {
	// Map elements are unaddressable in Go.

	// The following lines compile okay. Deriving
	// slices from unaddressable slices is allowed.
	_ = []int{6, 7, 8, 9}[1:3]
	var ms = map[string][]int{"abc": {0, 1, 2, 3}}
	_ = ms["abc"][1:3]

	// The following lines fail to compile. Deriving
	// slices from unaddressable arrays is not allowed.
	/*
	_ = [...]int{6, 7, 8, 9}[1:3] // error
	var ma = map[string][4]int{"abc": {0, 1, 2, 3}}
	_ = ma["abc"][1:3]  // error
	*/
}

This reason is NaN != NaN, which is another detail will be described below. Before Go 1.12, the elements with NaN as keys can only be found out in a for-range loop, Since Go 1.12, the elements with NaN as keys can also be printed out by fmt.Print alike functions.

package main

import "fmt"
import "math"

func main() {
	var a = math.NaN()
	fmt.Println(a) // NaN

	var m = map[float64]int{}
	m[a] = 123
	v, present := m[a]
	fmt.Println(v, present) // 0 false
	m[a] = 789
	v, present = m[a]
	fmt.Println(v, present) // 0 false

	fmt.Println(m) // map[NaN:789 NaN:123]
	delete(m, a)   // no-op
	fmt.Println(m) // map[NaN:789 NaN:123]

	for k, v := range m {
		fmt.Println(k, v)
	}
	// the above loop outputs:
	// NaN 123
	// NaN 789
}

Please note, before Go 1.12, the two fmt.Println(m) calls both printed map[NaN:<nil> NaN:<nil>].

We should not assume the length and the capacity of the result slice are always equal.

In the following example, if the last fmt.Println line is removed, the outputs of the two lines before it print the same value 32, otherwise, one print 32 and one print 8 (for the standard Go compiler 1.16).

package main

import "fmt"

func main() {
	s := "a"
	x := []byte(s)              // len(s) == 1
	fmt.Println(cap([]byte(s))) // 32
	fmt.Println(cap(x))         // 8
	fmt.Println(x)
}

Some buggy code will be written if we assume the length and the capacity of the result slice are always equal.

The respective final values of the iteration variable i may be different for the two loops.

package main

import "fmt"

var i int

func fa(s []int, n int) int {
	i = n
	for i = 0; i < len(s); i++ {}
	return i
}

func fb(s []int, n int) int {
	i = n
	for i = range s {}
	return i
}

func main() {
	s := []int{2, 3, 5, 7, 11, 13}
	fmt.Println(fa(s, -1), fb(s, -1)) // 6 5
	s = nil
	fmt.Println(fa(s, -1), fb(s, -1)) // 0 -1
}

For example, the following program will not run infinitely:

package main

import "fmt"

func f(m map[byte]byte) string {
	bs := make([]byte, 0, 2*len(m))
	for k, v := range m {
		bs = append(bs, k, v)
	}
	return string(bs)
}

func main() {
	m := map[byte]byte{'a':'A', 'b':'B', 'c':'C'}
	s0 := f(m)
	for i := 1; ; i++{
		if s := f(m); s != s0 {
			fmt.Println(s0)
			fmt.Println(s)
			fmt.Println(i)
			return
		}
	}
}

Please note, the entries in the JSON marshal result on maps are sorted by their keys. And since Go 1.12, printing maps (with the print functions in the standard fmt package) also results sorted entries.

A proof:

package main

import "fmt"

func main() {
	m := map[int]int{0: 0, 1: 100, 2: 200}
	r, n, i:= len(m), len(m), 0
	for range m {
		m[n] = n*100
		n++
		i++
	}
	fmt.Printf("%d new entries, iterate %d and skip %d\n",
		i, i - r, n - i,
	)
}

Thanks to Valentin Deleplace for the above two detail suggestions.

For example:

package main

func main() {
	var x interface{} = []int{}
	_ = x == x // panic
}

For example:

package main

type Foo interface {
	foo()
}

type T int
func (T) foo() {}

func main() {
	var x interface{} = T(123)
	// The following two lines fails to compile, for the
	// same reason: interface{} does not implement Foo.
	/*
	var _ Foo = x   // error
	var _ = Foo(x)  // error
	*/
	// But the following line compiles and runs okay.
	var _ = x.(Foo) // okay
}

If the second optional result presents in a failed type assertion, the type assertion will not produce a panic. Otherwise, a panic will occur. For example:

package main

func main() {
	var x interface{} = true

	// Assertion fails, but doesn't cause a panic.
	_, _ = x.(int)

	// Assertion fails, which causes a panic.
	_ = x.(int)
}

At compile time, some to-interface assertions can be deducted as impossible to succeed. For example, the assertion shown in the following code:

package main

type Ia interface {
	m()
}

type Ib interface {
	m() int
}

type T struct{}

func (T) m() {}

func main() {
	var x Ia = T{}
	_ = x.(Ib) // panic: main.T is not main.Ib
}

Such assertions will not make code compilations fail (but the program will panic at run time). Since Go Toolchain 1.15, the go vet command warns on such assertions.

The reason is the errors.New function will copy the input string argument and use a pointer to the copied string as the dynamic value of the returned error value. Two different calls will produce two different pointers.

package main

import "fmt"
import "errors"

func main() {
	notfound := "not found"
	a, b := errors.New(notfound), errors.New(notfound)
	fmt.Println(a == b) // false
}

For example, the following code fails to compile.

package main

func main() {
}

func foo(c <-chan int) {
	close(c) // error: cannot close receive-only channel
}

For example, in the following program, when the second case branch gets selected, it will produce a panic at run time.

package main

func main() {
	var c = make(chan bool)
	close(c)
	select {
	case <-c:
	case c <- true: // panic: send on closed channel
	default:
	}
}

Types can be declared in function bodies. For example,

package main

func main() {
	type T struct{}
	type S = []int
}

This follows IEEE-754 standard and is consistent with most other programming languages:

package main

import "fmt"
import "math"

func main() {
	var a = math.Sqrt(-1.0)
	fmt.Println(a)      // NaN
	fmt.Println(a == a) // false

	var x = 0.0
	var y = 1.0 / x
	var z = 2.0 * y
	fmt.Println(y, z, y == z) // +Inf +Inf true
}

For example, if the following types are declared in package foo:

package foo

type I = interface {
	about() string
}

type S struct {
	a string
}

func (s S) about() string {
	return s.a
}

and the following types are declared in package bar:

package bar

type I = interface {
	about() string
}

type S struct {
	a string
}

func (s S) about() string {
	return s.a
}

then,

• values of the two respective types S from the two packages can't be converted to each other.
• the two respective interface types S from the two packages denote two distinct method sets.
• type foo.S doesn't implement the interface type bar.I.
• type bar.S doesn't implement the interface type foo.I.

package main

import "包2/foo"
import "包2/bar"

func main() {
	var x foo.S
	var y bar.S
	var _ foo.I = x
	var _ bar.I = y

	// The following lines fail to compile.
	x = foo.S(y)
	y = bar.S(x)
	var _ foo.I = y
	var _ bar.I = x
}

Blank fields are those fields whose name are the blank identifier _. The following program will print true.

package main

import "fmt"

type T struct {
	_ int
	_ bool
}

func main() {
	var t1 = T{123, true}
	var t2 = T{789, false}
	fmt.Println(t1 == t2) // true
}

For example:

package main

type T struct{x, y int}

func main() {
	// Each of the following three lines makes code
	// fail to compile. Some "{}"s confuse compilers.
	/*
	if T{} == T{123, 789} {}
	if T{} == (T{123, 789}) {}
	if (T{}) == T{123, 789} {}
	var _ = func()(nil) // nil is viewed as a type
	*/

	// We must add parentheses like the following
	// two lines to make code compile okay.
	if (T{} == T{123, 789}) {}
	if (T{}) == (T{123, 789}) {}
	var _ = (func())(nil) // nil is viewed as a value
}

For the current main stream Go compilers, stack overflows are fatal errors. Once a stack overflow happens, the whole program will crash without recovery ways.

package main

func f() {
	f()
}

func main() {
	defer func() {
		recover() // helpless to avoid program crashing
	}()
	f()
}

the running result:

runtime: goroutine stack exceeds 1000000000-byte limit
fatal error: stack overflow

runtime stack:
...

About more crash cases, please read this wiki article.

The function call reflect.DeepEqual(x, y) will always return false if the types of its two arguments are different, whereas x == y may return true even if the types of the two operands are different.

The second difference is a DeepEqual call with two pointer argument values of the same type returns whether or not the two respective values referenced by the two pointers are deep equal. So the call might return true even if the two pointers are not equal.

The third difference is the result of a DeepEqual call may be not correct if the compared two arguments are in the same cyclic reference chain.

The fourth difference is, the function call reflect.DeepEqual(x, y) is not expected to panic generally, whereas x == y will panic if the two operands are both interface values and their dynamic types are identical and incomparable.

An example showing these differences:

package main

import (
	"fmt"
	"reflect"
)

func main() {
	type Book struct {page int}
	x := struct {page int}{123}
	y := Book{123}
	fmt.Println(reflect.DeepEqual(x, y)) // false
	fmt.Println(x == y)                  // true

	z := Book{123}
	fmt.Println(reflect.DeepEqual(&z, &y)) // true
	fmt.Println(&z == &y)                  // false

	type T struct{p *T}
	t := &T{&T{nil}}
	t.p.p = t // form a cyclic reference chain.
	fmt.Println(reflect.DeepEqual(t, t.p)) // true
	fmt.Println(t == t.p)                  // false

	var f1, f2 func() = nil, func(){}
	fmt.Println(reflect.DeepEqual(f1, f1)) // true
	fmt.Println(reflect.DeepEqual(f2, f2)) // false

	var a, b interface{} = []int{1, 2}, []int{1, 2}
	fmt.Println(reflect.DeepEqual(a, b)) // true
	fmt.Println(a == b)                  // panic
}

Note, if the two arguments of a DeepEqual call are both function values, then the call returns true only if the two function arguments are both nil and their types are identical. It is similar to compare container values whose elements contain function values or compare struct values whose fields contain function values. But please also note that the result of comparing two slices (of the same type) is always true if the two slices exactly share the same elements (in other words, they have the same length and each pair of their corresponding elements have the same address). An example:

package main

import (
	"fmt"
	"reflect"
)

func main() {
	a := [1]func(){func(){}}
	b := a
	fmt.Println(reflect.DeepEqual(a, a))       // false
	fmt.Println(reflect.DeepEqual(a[:], a[:])) // true
	fmt.Println(reflect.DeepEqual(a[:], b[:])) // false
	a[0], b[0] = nil, nil
	fmt.Println(reflect.DeepEqual(a[:], b[:])) // true
}

Assume the underlying type of a defined type MyByte is the predeclared type byte, we know that Go type system forbids the conversions between []MyByte and []byte values. However, it looks the implementation of the method Bytes of the reflect.Value type partially violates this restriction unintentionally, by allowing converting a []MyByte value to []byte.

Example:

package main

import "bytes"
import "fmt"
import "reflect"

type MyByte byte

func main() {
	var mybs = []MyByte{'a', 'b', 'c'}
	var bs []byte

	// bs = []byte(mybs) // this line fails to compile

	v := reflect.ValueOf(mybs)
	bs = v.Bytes() // okay. Violating Go type system.
	fmt.Println(bytes.HasPrefix(bs, []byte{'a', 'b'})) // true

	bs[1], bs[2] = 'r', 't'
	fmt.Printf("%s \n", mybs) // art
}

But it looks the violation is not harmful. On the contrary, it makes some benefits. For example, with this violation, we can use the functions in the bytes standard package for the []MyByte values.

Note, the reflect.Value.Bytes() method might be removed later.

Using err == os.ErrNotExist may miss errors.

package main

import (
	"fmt"
	"os"
)

func main() {
	_, err := os.Stat("a-nonexistent-file.abcxyz")
	fmt.Println(os.IsNotExist(err))    // true
	fmt.Println(err == os.ErrNotExist) // false
}

For projects only supporting Go 1.13+, errors.Is(err, os.ErrNotExist) is more recommended to be used to check whether or not a file exists.

package main

import (
	"errors"
	"fmt"
	"os"
)

func main() {
	_, err := os.Stat("a-nonexistent-file.abcxyz")
	fmt.Println(errors.Is(err, os.ErrNotExist)) // true
}

There are three forms to pass flag options.

1. -flag, for boolean flags only.
2. -flag=x, for any flag.
3. -flag x, for non-boolean flags only.

And please note that, a boolean flag with the first form is viewed as the last flag, all items following it are viewed as arguments.

package main

import "fmt"
import "flag"

var b = flag.Bool("b", true, "a boolean flag")
var i = flag.Int("i", 123, "an integer flag")
var s = flag.String("s", "hi", "a string flag")

func main() {
	flag.Parse()
	fmt.Print("b=", *b, ", i=", *i, ", s=", *s, "\n")
	fmt.Println("arguments:", flag.Args())
}

If we run this program with the below shown flags and arguments

./exampleProgram -b false -i 789 -s bye arg0 arg1

the output will be

b=true, i=123, s=hi
arguments: [false -i 789 -s bye arg0 arg1]

This output is obviously not what we expect.

We should pass the flags and arguments like

./exampleProgram -b=false -i 789 -s bye arg0 arg1

or

./exampleProgram -i 789 -s bye -b arg0 arg1

to get the output we expect:

b=true, i=789, s=bye
arguments: [arg0 arg1]

The following program will print coco.

package main

import "fmt"

func main() {
	// The next line prints: coco
	fmt.Printf("%[2]v%[1]v%[2]v%[1]v", "o", "c")
}