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[#]: collector: (lujun9972)
[#]: translator: (LazyWolfLin)
[#]: reviewer: ( )
[#]: publisher: ( )
[#]: url: ( )
[#]: subject: (An Introduction to Go)
[#]: via: (https://blog.jak-linux.org/2018/12/24/introduction-to-go/)
[#]: author: (Julian Andres Klode https://blog.jak-linux.org/)
An Introduction to Go
======
(What follows is an excerpt from my master’s thesis, almost all of section 2.1, quickly introducing Go to people familiar with CS)
Go is an imperative programming language for concurrent programming created at and mainly developed by Google, initially mostly by Robert Griesemer, Rob Pike, and Ken Thompson. Design of the language started in 2007, and an initial version was released in 2009; with the first stable version, 1.0 released in 2012 .
Go has a C-like syntax (without a preprocessor), garbage collection, and, like its predecessors devloped at Bell Labs – Newsqueak (Rob Pike), Alef (Phil Winterbottom), and Inferno (Pike, Ritchie, et al.) – provides built-in support for concurrency using so-called goroutines and channels, a form of co-routines, based on the idea of Hoare’s ‘Communicating Sequential Processes’ .
Go programs are organised in packages. A package is essentially a directory containing Go files. All files in a package share the same namespace, and there are two visibilities for symbols in a package: Symbols starting with an upper case character are visible to other packages, others are private to the package:
```
func PublicFunction() {
fmt.Println("Hello world")
}
func privateFunction() {
fmt.Println("Hello package")
}
```
### Types
Go has a fairly simple type system: There is no subtyping (but there are conversions), no generics, no polymorphic functions, and there are only a few basic categories of types:
1. base types: `int`, `int64`, `int8`, `uint`, `float32`, `float64`, etc.
2. `struct`
3. `interface` \- a set of methods
4. `map[K, V]` \- a map from a key type to a value type
5. `[number]Type` \- an array of some element type
6. `[]Type` \- a slice (pointer to array with length and capability) of some type
7. `chan Type` \- a thread-safe queue
8. pointer `*T` to some other type
9. functions
10. named type - aliases for other types that may have associated methods:
```
type T struct { foo int }
type T *T
type T OtherNamedType
```
Named types are mostly distinct from their underlying types, so you cannot assign them to each other, but some operators like `+` do work on objects of named types with an underlying numerical type (so you could add two `T` in the example above).
Maps, slices, and channels are reference-like types - they essentially are structs containing pointers. Other types are passed by value (copied), including arrays (which have a fixed length and are copied).
#### Conversions
Conversions are the similar to casts in C and other languages. They are written like this:
```
TypeName(value)
```
#### Constants
Go has “untyped” literals and constants.
```
1 // untyped integer literal
const foo = 1 // untyped integer constant
const foo int = 1 // int constant
```
Untyped values are classified into the following categories: `UntypedBool`, `UntypedInt`, `UntypedRune`, `UntypedFloat`, `UntypedComplex`, `UntypedString`, and `UntypedNil` (Go calls them basic kinds, other basic kinds are available for the concrete types like `uint8`). An untyped value can be assigned to a named type derived from a base type; for example:
```
type someType int
const untyped = 2 // UntypedInt
const bar someType = untyped // OK: untyped can be assigned to someType
const typed int = 2 // int
const bar2 someType = typed // error: int cannot be assigned to someType
```
### Interfaces and ‘objects’
As mentioned before, interfaces are a set of methods. Go is not an object-oriented language per se, but it has some support for associating methods with named types: When declaring a function, a receiver can be provided - a receiver is an additional function argument that is passed before the function and involved in the function lookup, like this:
```
type SomeType struct { ... }
func (s *SomeType) MyMethod() {
}
func main() {
var s SomeType
s.MyMethod()
}
```
An object implements an interface if it implements all methods; for example, the following interface `MyMethoder` is implemented by `*SomeType` (note the pointer), and values of `*SomeType` can thus be used as values of `MyMethoder`. The most basic interface is `interface{}`, that is an interface with an empty method set - any object satisfies that interface.
```
type MyMethoder interface {
MyMethod()
}
```
There are some restrictions on valid receiver types; for example, while a named type could be a pointer (for example, `type MyIntPointer *int`), such a type is not a valid receiver type.
### Control flow
Go provides three primary statements for control flow: `if`, `switch`, and `for`. The statements are fairly similar to their equivalent in other C-like languages, with some exceptions:
* There are no parentheses around conditions, so it is `if a == b {}`, not `if (a == b) {}`. The braces are mandatory.
* All of them can have initialisers, like this
`if result, err := someFunction(); err == nil { // use result }`
* The `switch` statement can use arbitrary expressions in cases
* The `switch` statement can switch over nothing (equals switching over true)
* Cases do not fall through by default (no `break` needed), use `fallthrough` at the end of a block to fall through.
* The `for` loop can loop over ranges: `for key, val := range map { do something }`
### Goroutines
The keyword `go` spawns a new goroutine, a concurrently executed function. It can be used with any function call, even a function literal:
```
func main() {
...
go func() {
...
}()
go some_function(some_argument)
}
```
### Channels
Goroutines are often combined with channels to provide an extended form of Communicating Sequential Processes . A channel is a concurrent-safe queue, and can be buffered or unbuffered:
```
var unbuffered = make(chan int) // sending blocks until value has been read
var buffered = make(chan int, 5) // may have up to 5 unread values queued
```
The `<-` operator is used to communicate with a single channel.
```
valueReadFromChannel := <- channel
otherChannel <- valueToSend
```
The `select` statement allows communication with multiple channels:
```
select {
case incoming := <- inboundChannel:
// A new message for me
case outgoingChannel <- outgoing:
// Could send a message, yay!
}
```
### The `defer` statement
Go provides a `defer` statement that allows a function call to be scheduled for execution when the function exits. It can be used for resource clean-up, for example:
```
func myFunc(someFile io.ReadCloser) {
defer someFile.close()
/bin /boot /dev /etc /home /lib /lib64 /lost+found /media /mnt /opt /proc /root /run /sbin /srv /sys /tmp /usr /var Do stuff with file */
}
```
It is of course possible to use function literals as the function to call, and any variables can be used as usual when writing the call.
### Error handling
Go does not provide exceptions or structured error handling. Instead, it handles errors by returning them in a second or later return value:
```
func Read(p []byte) (n int, err error)
// Built-in type:
type error interface {
Error() string
}
```
Errors have to be checked in the code, or can be assigned to `_`:
```
n0, _ := Read(Buffer) // ignore error
n, err := Read(buffer)
if err != nil {
return err
}
```
There are two functions to quickly unwind and recover the call stack, though: `panic()` and `recover()`. When `panic()` is called, the call stack is unwound, and any deferred functions are run as usual. When a deferred function invokes `recover()`, the unwinding stops, and the value given to `panic()` is returned. If we are unwinding normally and not due to a panic, `recover()` simply returns `nil`. In the example below, a function is deferred and any `error` value that is given to `panic()` will be recovered and stored in an error return value. Libraries sometimes use that approach to make highly recursive code like parsers more readable, while still maintaining the usual error return value for public functions.
```
func Function() (err error) {
defer func() {
s := recover()
switch s := s.(type) { // type switch
case error:
err = s // s has type error now
default:
panic(s)
}
}
}
```
### Arrays and slices
As mentioned before, an array is a value type and a slice is a pointer into an array, created either by slicing an existing array or by using `make()` to create a slice, which will create an anonymous array to hold the elements.
```
slice1 := make([]int, 2, 5) // 5 elements allocated, 2 initialized to 0
slice2 := array[:] // sliced entire array
slice3 := array[1:] // slice of array without first element
```
There are some more possible combinations for the slicing operator than mentioned above, but this should give a good first impression.
A slice can be used as a dynamically growing array, using the `append()` function.
```
slice = append(slice, value1, value2)
slice = append(slice, arrayOrSlice...)
```
Slices are also used internally to represent variable parameters in variable length functions.
### Maps
Maps are simple key-value stores and support indexing and assigning. They are not thread-safe.
```
someValue := someMap[someKey]
someValue, ok := someMap[someKey] // ok is false if key not in someMap
someMap[someKey] = someValue
```
--------------------------------------------------------------------------------
via: https://blog.jak-linux.org/2018/12/24/introduction-to-go/
作者：[Julian Andres Klode][a]
选题：[lujun9972][b]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://blog.jak-linux.org/
[b]: https://github.com/lujun9972

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@ -0,0 +1,274 @@
[#]: collector: (lujun9972)
[#]: translator: (LazyWolfLin)
[#]: reviewer: ( )
[#]: publisher: ( )
[#]: url: ( )
[#]: subject: (An Introduction to Go)
[#]: via: (https://blog.jak-linux.org/2018/12/24/introduction-to-go/)
[#]: author: (Julian Andres Klode https://blog.jak-linux.org/)
Go 简介
======
（以下内容是我的硕士论文的摘录，几乎整个 2.1 章节，向具有 CS 背景的人快速介绍 Go）
Go 是一门用于并发编程的命令式编程语言，它主要由创造者 Google 进行开发，最初主要由 Robert Griesemer、Rob Pike 和 Ken Thompson开发。这门语言的设计起始于 2017 年，并在 2019 年推出最初版本；而第一个稳定版本是 2012 年发布的 1.0 版本。
Go 有 C 风格的语法（没有预处理器），垃圾回收机制，而且类似它在贝尔实验室里被开发出来的前辈们：Newsqueak (Rob Pike)、Alef (Phil Winterbottom) 和 Inferno (Pike, Ritchie, et al.)，使用所谓的 goroutines 和信道（一种基于 Hoare 的“通信顺序进程”理论的协程）提供内建的并发支持。
Go 程序以包的形式组织。包本质是一个包含 Go 文件的文件夹。包内的所有文件共享相同的命名空间，而包内的符号有两种可见性：以大写字母开头的符号对于其他包是可见，而其他符号则是该包私有的：
```
func PublicFunction() {
fmt.Println("Hello world")
}
func privateFunction() {
fmt.Println("Hello package")
}
```
### 类型
Go 有一个相当简单的类型系统：没有子类型（但有类型转换），没有泛型，没有多态函数，只有一些基本的类型：
1. 基本类型：`int`、`int64`、`int8`、`uint`、`float32`、`float64` 等。
2. `struct`
3. `interface` \- 一组方法的集合
4. `map[K, V]` \- 一个从键类型到值类型的映射
5. `[number]Type` \- 一些 Type 类型的元素组成的数组
6. `[]Type` \- 某种类型的切片（指向具有长度和功能的数组）
7. `chan Type` \- 一个线程安全的队列
8. 指针 `*T` 指向其他类型
9. 函数
10. 具名类型 - 可能具有关联方法的其他类型的别名（译者注：这里的别名并非指 Go 1.9 中的新特性“类型别名”）：
```
type T struct { foo int }
type T *T
type T OtherNamedType
```
具名类型完全不同于他们的底层类型，所以你不能让他们互相赋值，但一些运输符，例如 `+`，能够处理同一底层数值类型的具名类型对象们（所以你可以在上面的示例中把两个 `T` 加起来）。
Maps、slices 和 channels 是类似于引用的类型——他们实际上是包含指针的结构。包括数组（具有固定长度并可被拷贝）在内的其他类型则是值（拷贝）传递。
#### 类型转换
类型转换类似于 C 或其他语言中的类型转换。它们写成这样子：
```
TypeName(value)
```
#### 常量
Go 有“无类型”字面量和常量。
```
1 // 无类型整数字面量
const foo = 1 // 无类型整数常量
const foo int = 1 // int 类型常量
```
无类型值可以分为以下几类：`UntypedBool`、`UntypedInt`、`UntypedRune`、`UntypedFloat`、`UntypedComplex`、`UntypedString` 以及 `UntypedNil`（Go 称它们为基础类型，其他基础种类可用于具体类型，如 `uint8`）。一个无类型值可以赋值给一个从基础类型中派生的具名类型；例如：
```
type someType int
const untyped = 2 // UntypedInt
const bar someType = untyped // OK: untyped 可以被赋值给 someType
const typed int = 2 // int
const bar2 someType = typed // error: int 不能被赋值给 someType
```
### 接口和对象
正如上面所说的，接口是一组方法的集合。Go 本身不是一种面向对象的语言，但它支持将方法关联到命名类型上：当声明一个函数时，可以提供一个接收者。接收者是函数的一个额外参数，可以在函数之前传递并参与函数查找，就像这样：
```
type SomeType struct { ... }
func (s *SomeType) MyMethod() {
}
func main() {
var s SomeType
s.MyMethod()
}
```
如果对象实现了所有方法，那么它就实现了接口；例如，`*SomeType`（注意指针）实现了下面的接口 `MyMethoder`，因此 `*SomeType` 类型的值就能作为 `MyMethoder` 类型的值使用。最基本的接口类型是 `interface{}`，它是一个带空方法集的接口——任何对象都满足该接口。
```
type MyMethoder interface {
MyMethod()
}
```
合法的接收者类型是有些限制的；例如，命名类型可以是指针类型（例如，`type MyIntPointer *int`），但这种类型不是合法的接收者类型。
### 控制流
Go 提供了三个主要的控制了语句：`if`、`switch` 和 `for`。这些语句同其他 C 风格语言内的语句非常类似，但有一些不同：
* 条件语句没有括号，所以条件语句是 `if a == b {}` 而不是 `if (a == b) {}`。大括号是必须的。
* 所有的语句都可以有初始化，比如这个
`if result, err := someFunction(); err == nil { // use result }`
* `switch` 语句在 cases 里可以使用任何表达式
* `switch` 语句可以处理空的表达式（等于 true）
* 默认情况下，Go 不会从一个 case 进入下一个 case（不需要 `break`语句），在程序块的末尾使用 `fallthrough` 则会进入下一个 case。
* 循环语句 `for` 不仅能循环值域：`for key, val := range map { do something }`
### Goroutines
关键词 `go` 会产生一个新的 goroutine，一个可以并行执行的函数。它可以用于任何函数调用，甚至一个匿名函数：
```
func main() {
...
go func() {
...
}()
go some_function(some_argument)
}
```
### 信道
Goroutines 通常和信道结合，用来提供一种通信顺序进程的扩展。信道是一个并发安全的队列，而且可以选择是否缓冲数据：
```
var unbuffered = make(chan int) // 直到数据被读取时完成数据块发送
var buffered = make(chan int, 5) // 最多有 5 个未读取的数据块
```
运算符 `<-` 用于和单个信道进行通信。
```
valueReadFromChannel := <- channel
otherChannel <- valueToSend
```
语句 `select` 允许多个信道进行通信：
```
select {
case incoming := <- inboundChannel:
// 一条新消息
case outgoingChannel <- outgoing:
// 可以发送消息
}
```
### `defer` 声明
Go 提供语句 `defer` 允许函数退出时调用执行预定的函数。它可以用于进行资源释放操作，例如：
```
func myFunc(someFile io.ReadCloser) {
defer someFile.close()
/* 文件相关操作 */
}
```
当然，它允许使用匿名函数作为被调函数，而且编写被调函数时可以像平常一样使用任何变量。
### 错误处理
Go 没有提供异常类或者结构化的错误处理。然而，它通过第二个及后续的返回值来返回错误从而处理错误：
```
func Read(p []byte) (n int, err error)
// 内建类型：
type error interface {
Error() string
}
```
必须在代码中检查错误或者赋值给 `_`：
```
n0, _ := Read(Buffer) // 忽略错误
n, err := Read(buffer)
if err != nil {
return err
}
```
有两个函数可以快速跳出和恢复调用栈：`panic()` 和 `recover()`。当 `panic()` 被调用时，调用栈开始弹出，同时每个 `defer` 函数都会正常运行。当一个 `defer` 函数调用 `recover()`时，调用栈停止弹出，同时返回函数 `panic()` 给出的值。如果我们让调用栈正常弹出而不是由于调用 `panic()` 函数，`recover()` 将只返回 `nil`。在下面的例子中，`defer` 函数将捕获 `panic()` 抛出的任何 `error` 类型的值并储存在错误返回值中。第三方库中有时会使用这个方法增强递归代码的可读性，如解析器，同时保持公有函数仍使用普通错误返回值。
```
func Function() (err error) {
defer func() {
s := recover()
switch s := s.(type) { // type switch
case error:
err = s // s has type error now
default:
panic(s)
}
}
}
```
### 数组和切片
正如前边说的，数组是值类型而切片是指向数组的指针。切片可以由现有的数组切片产生，也可以使用 `make()` 创建切片，这会创建一个匿名数组以保存元素。
```
slice1 := make([]int, 2, 5) // 分配 5 个元素，其中 2 个初始化为0
slice2 := array[:] // 整个数组的切片
slice3 := array[1:] // 除了首元素的切片
```
除了上述例子，还有更多可行的切片运算组合，但需要明了直观。
使用 `append()` 函数，切片可以作为一个变长数组使用。
```
slice = append(slice, value1, value2)
slice = append(slice, arrayOrSlice...)
```
切片也可以用于函数的变长参数。
### Maps
Maps 是简单的键值对储存容器并支持索引和分配。但它们不是线程安全的。
```
someValue := someMap[someKey]
someValue, ok := someMap[someKey] // 如果键值不在 someMap 中，变量 ok 会赋值为 `false`
someMap[someKey] = someValue
```
--------------------------------------------------------------------------------
via: https://blog.jak-linux.org/2018/12/24/introduction-to-go/
作者：[Julian Andres Klode][a]
选题：[lujun9972][b]
译者：[LazyWolfLin](https://github.com/LazyWolfLin)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://blog.jak-linux.org/
[b]: https://github.com/lujun9972