Golang

What is Go?
Go is:
• open source
• concurrent
• garbage-collected
• efficient
• scalable
• simple
• fun
• boring (to some)
http://golang.org

History
Design began in late 2007.
Became open source in November 2009.
Today's version is 1.2.2.
Key Players:
Robert Griesemer
(Hotspot JVM)
Rob Pike (58)
(Unix/UTF-8)
Kenneth Thompson (71)
(B and C lang/Unix/UTF-8)
Ian Lance Taylor
(GCC)
Russ Cox
(Plan9/CSP)

Google has big problems
Go is a programming language designed by Google
to help solve Google’s problems.
What problems?
• C++ for servers, plus lots of Java and Python
• thousand of engineers
• gazillions of lines of code
• distributed build system
• zillions of machines, which are treated as a modest
number of compute clusters

The reason for Go
Goals:
– eliminate slowness
– eliminate clumsiness
– improve effectiveness
– maintain (even improve) scale
Go was designed by and for people who write – and
read and debug and maintain – large software
systems.
Go’s purpose is not research programming language
design.
Go’s purpose is to make its designers’ programming
lives better.

Important stuff
• Object oriented without inheritance
• Syntax sugar declaration
• Strong and Static types
• Interfaces, but no declaration (duck typing)
• Functions and Methods (with receivers)
• No exceptions (error interface)
• Unused imports and variables occurs compile
error
• Excellent and complete standard library

Go is a tool
• Go is a tool for managing Go source code.
build compile packages and dependencies
clean remove object files
env print Go environment information
fix run go tool fix on packages
fmt run gofmt on package sources
get download and install packages and dependencies
install compile and install packages and dependencies
list list packages
run compile and run Go program
test test packages
tool run specified go tool
version print Go version
vet run go tool vet on packages

Cross compilation
Needs execute $GOROT/src/make
$GOOS $GOARCH
darwin 386 -- 32 bit MacOSX
darwin amd64 -- 64 bit MacOSX
freebsd 386
freebsd amd64
linux 386 -- 32 bit Linux
linux amd64 -- 64 bit Linux
linux arm -- RISC Linux
netbsd 386
netbsd amd64
openbsd 386
openbsd amd64
plan9 386
windows 386 -- 32 bit Windows
windows amd64 -- 64 bit Windows

C? Go? Cgo!
• Cgo lets Go packages call C code. Given a Go source file written with
some special features, cgo outputs Go and C files that can be combined
into a single Go package.
• Go’s use SWIG.
package cgoexample
/*
#include <stdio.h>
#include <stdlib.h>
void myprint(char* s) {
printf("%s", s);
}
*/
import "C"
import "unsafe"
func main() {
cs := C.CString("Hello from stdion")
C.myprint(cs)
C.free(unsafe.Pointer(cs))
}

How to Write Go Code
• Go code must be kept inside a workspace. A
workspace is a directory hierarchy with three
directories at its root:
– src contains Go source files organized into packages
(one package per directory)
– pkg contains package objects
– bin contains executable commands
• Environment variables
– GOROOT, GOPATH, GOOS, GOARCH...

Godoc
• The Go project takes documentation seriously.
Documentation is a huge part of making software
accessible and maintainable.
• The convention is simple: to document a type, variable,
constant, function, or even a package, write a regular
comment directly preceding its declaration, with no
intervening blank line.
$ godoc [flag] package [name ...]
// Package ftd provides content for learning Go
package ftd
// Greet greets ftd's participant
func Greet() { ... }

Testing
• Package testing provides support for automated testing
of Go packages. It is intended to be used in concert
with the “go test” command, which automates execution
of any function of the form.
func TestXxx(*testing.T)
import "testing"
func TestAverage(t *testing.T) {
var v float64
v = Average([]float64{1,2})
if v != 1.5 {
t.Error("Expected 1.5, got ", v)
}
}

Benchmark
• The benchmark function must run the target
code b.N times. The benchmark package will
vary b.N until the benchmark function lasts
long enough to be timed reliably.
func BenchmarkXxx(*testing.B)
func BenchmarkHello(b *testing.B) {
for i := 0; i < b.N; i++ {
fmt.Sprintf("hello")
}
}

Talk is cheap. Show me the ‘Go’ code!
$ go run hello.go // returns Hello World
package main
import "fmt"
func main() {
fmt.Println("Hello World")
}

Packages
• Package main;
• Unique name for paths, not name;
• init() function
• Packages are not per file
package ftd
package ftd/hello

Imports
• An import declaration states that the source file
containing the declaration depends on
functionality of the imported package and enables
access to exported identifiers of that package.
Import declaration Local name of Greet
import "ftd/hello" ftd.Greet
import gr "ftd/hello" gr.Greet
import . "ftd/hello" Greet
import _ "ftd/hello" initialization
import "github.com/user/project" Remote import paths

Types
• A type determines the set of values and operations specific to
values of that type. Types may be named or unnamed.
Predeclared identifiers:
Types
bool byte complex64 complex128 error float32 float64 int
int8 int16 int32 int64 rune string uint uint8 uint16 uint32
uint64 uintptr
Constants
true false iota
Zero value
nil
Functions
append cap close complex copy delete imag len make new
panic print println real recover

Type Conversions
• Basically, you can convert between a named
typed and its underlying type.
type Mystring string
var myStr Mystring = Mystring("awesome")
var str string = string(myStr)
• There are few rules to keep in mind when it
comes to type conversions.

Variables
• Declaration one or more variables
• You can declare multiple variables at once.
• Infer the type of initialized variables.
• Zero-valued initialization
• Go supports constants of character, string,
boolean, and numeric values (arbitrary precision)
var number = 12
var event string = "ftd"
isFtd := true // syntax sugar with type inference
const n = 500000000

Iota
• Go's iota identifier is used in in const declarations
to simplify definitions of incrementing numbers.
Because it can be used in expressions, it provides a
generality beyond that of simple enumerations.
const (
Sunday = iota // 0
Monday // 1
Tuesday // 2
Wednesday // 3
Thursday // 4
Friday // 5
Saturday // 6
)

Arrays
• An array is a numbered sequence of elements
of a single type, called the element type.
The number of elements is called the length
and is never negative. (Arrays are values)
[32]byte
[2] struct { x, y int32 }
[1000]*float64
array := [...]float64{7.0, 8.5, 9.1}
names := [2]string{"FTD", "Golang"}

Slices
• Slices are a key data type in Go, giving a
more powerful interface to sequences than
arrays.
s := make([]string, 3)
s[0] = "a"
s[1] = "b"
s[2] = "c"
s1 := s[0:2] // [a b]
s2 := append(s1, "d", "e", "f") // [a b d e f]

Maps
• Maps are Go’s built-in associative data type
(sometimes called hashes or dicts in other
languages).
m := make(map[string]int)
m["one"] = 1
m["nine"] = 9
delete(m, "one")
n := map[string]int{"foo": 1, "bar": 2}

Range
• range iterates over of elements in a variety of data
structures.
nums := []int{2, 3, 4}
for idx, num := range nums {
fmt.Println("Index:", idx, "Value:", num)
}
kvs := map[string]string{"a": "apple", "b": "banana"}
for k, v := range kvs {
fmt.Printf("%s -> %sn", k, v)
}
for i, c := range "go" {
fmt.Println(i, c)
}

Control structures
• If/Else statement without parentheses and
initialization statement
• There is no ternary if in Go
• For only in three forms
• For range clause (array, slice, string, or
map, or channel read)
• Switch statements express conditionals across
many branches.

Functions
• A function declaration binds an identifier,
the function name, to a function.
• Multiple return values
• Named result parameters
func plus(a int, b int) int {
return a + b
}

Multiple return values
• One of Go's unusual features is that
functions and methods can return multiple
values.
func (file *File) Write(b []byte) (n int, err error)

Named result parameters
• When named, they are initialized to the zero
values for their types when the function begins;
if the function executes a return statement with
no arguments.
func Connect(host string) (result int, err error) {
if host == nil {
result = 0
error = errors.New("No internet connection")
return
}
...
}

Variadic Functions
• Variadic functions can be called with any number
of trailing arguments. (aka varargs).
func sum(nums ...int) {
total := 0
for _, num := range nums {
total += num
}
fmt.Println("Result:", total)
}
nums := []int{2,4,6,8,10}
sum(s...) // Result: 30

Logging
• Package log implements a simple logging
package. It defines a type, Logger, with
methods for formatting output.
import "log"
func Fatal(v ...interface{})
func Println(v ...interface{})
func Printf(format string, v ...interface{})
func New(out io.Writer, prefix string, flag int) *Logger

Closures
• Go supports anonymous functions, which can
form closures. Anonymous functions are useful
when you want to define a function inline
without having to name it.
plus := func(a, b int) int {
return a + b
}
fmt.Println(plus(2, 2))

Pointers
• Go supports pointers, allowing you to pass
references to values and records within your
program.
• No pointer arithmetic
i := 1
var ip *int = &i
fmt.Println("Value:", i)
fmt.Println("Pointer:", &i)
fmt.Println("Pointer:", ip)

Structs
• Go’s structs are typed collections of fields. They’re
useful for grouping data together to form records.
type Person struct {
Name string
Age int
}
Person{"Costinha", 68}
Person{Name:"Costinha", Age:68}
&Person{"Costinha", 68} // struct pointer
&Person{Age:11} // struct pointer

Allocation
• Go has two allocation primitives, the built-in
functions:
– new(T): it returns a pointer to a newly allocated zero
value of type T.
– make(T, len, cap): it creates slices, maps, and channels
only, and it returns an initialized (notzeroed) value of
type T (not *T)
type Person struct {
Name string
}
p := new(Person) // *Person
p.Name = "Golang"
nums := make([]int, 10) // slice []int length 10

Methods
• Go supports methods defined on struct types. Called
receiver method.
type rect struct {
width, height int
}
func (r *rect) area() int {
return r.width * r.height
}
func (r rect) perim() int {
return 2*r.width + 2*r.height
}

Interfaces
• Interfaces are named collections of method signatures.
• No implements keyword
• Duck typing support
type Animal interface {
Name() string
}
type Dog struct{}
func (dog Dog) Name() string {
return "AuAu"
}

Errors
• In Go it’s idiomatic to communicate errors
via an explicit, separate return value.
• error interface must be implemented
func (e InternetError) Error() string {
return "No internet connection"
}

Defer
• Defer is used to ensure that a function call
is performed later in a program’s execution,
usually for purposes of cleanup.
• Recover from the run-time panic
• Deferred functions are executed in LIFO order
defer func()

Files
• Package io provides basic interfaces to I/O
primitives and package ioutil implements some
I/O utility functions.
bs, err := ioutil.ReadFile("test.txt")
if err != nil {
// handle the error here
}
fmt.Println(string(bs))

Files (cont.)
• Write a file
file, err := os.Create("test.txt")
if err != nil {
// handle the error here return
}
defer file.Close()
file.WriteString("test")

Sorting
• Go’s sort package implements sorting for builtins
and user-defined types. We’ll look at sorting for
builtins first.
• Sorting by functions support
strs := []string{"f", "t", "d"}
sort.Strings(strs)
fmt.Println("Strings:", strs) // [d f t]
ints := []int{7, 2, 4}
sort.Ints(ints)
fmt.Println("Ints:", ints) // [2 4 7]

Containers
• Go has several more collections available in the container
package. (heap, list and ring).
import "container/list"
l := list.New() // returns an initialized list.
e4 := l.PushBack(4) // inserts element at the back of list and returns element
e1 := l.PushFront(1) // inserts element at the front of list and returns element
l.InsertBefore(3, e4) // inserts element before mark and returns element
l.InsertAfter(2, e1) // inserts element after mark and returns element
for e := l.Front(); e != nil; e = e.Next() {
fmt.Println(e.Value)
}

Goroutines
• A lightweight thread of execution.
• To invoke this function in a goroutine, use
go f(s). This new goroutine will execute
concurrently with the calling one.
• M:N threading model
• Environment variable GOMAXPROCS, define
runtime native threads

Goroutines (cont.)
• Examples
go f("Goroutine")
go func(msg string) {
fmt.Println(msg)
}("Goroutine")

Channels
• A channel provides a mechanism for two
concurrently executing functions to
synchronize execution and communicate by
passing a value of a specified element type.
The value of an uninitialized channel is nil.
messages := make(chan string) //unbuffered chan
go func() { messages <- "ping" }()
msg := <-messages
fmt.Println(msg)

Channels (cont.)
• By default channels are unbuffered, meaning that they
will only accept sends (chan <-) if there is a
corresponding receive (<- chan) ready to receive the
sent value. Buffered channels accept a limited number
of values without a corresponding receiver for those
values.
messages := make(chan string, 2) //buffered chan
messages <- "ftd"
messages <- "Golang"
fmt.Println(<-messages)
fmt.Println(<-messages)

Handling panics
• Two built-in functions, panic and recover, assist in
reporting and handling run-time panics and program-
defined error conditions.
func panic(interface{})
panic(42)
panic("unreachable")
panic(Error("cannot parse")) // runtime.Error
func recover() interface{}
defer func() {
if x := recover(); x != nil {
log.Printf("run time panic: %v", x)
}
}()

JSON
• Go offers built-in support for JSON encoding
and decoding, including to and from built-in
and custom data types.
package "ecoding/json"
func Marshal(interface{}) ([]byte, error)
func Unmarshal(data []byte, v interface{}) error

References
• The Go Programming Language Specification
http://golang.org/ref/spec
• Effective Go
http://golang.org/doc/effective_go.html
• Go by example
https://gobyexample.com/

Get involved!
• Go Nuts
https://groups.google.com/forum/#!forum/golang-nuts
• GopherCon
http://gophercon.com/
• Go user groups
https://code.google.com/p/go-wiki/wiki/GoUserGroups

Golang

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